The Reaction Between Furfuryl Alcohol and Model Compound of Protein

polymers

Article The Reaction between Furfuryl Alcohol and Model Compound of Protein

Jiankun Liang 1,2,†, Zhigang Wu 3,†, Hong Lei 1,*, Xuedong Xi 1, Taohong Li 1 ID and Guanben Du 1,* 1 Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China; [email protected] (J.L.); [email protected] (X.X.); [email protected] (T.L.) 2 Material Science and Technology College, Beijing Forestry University, Beijing 100083, China 3 College of Forestry, Guizhou University, Guiyang 550025, China; [email protected] * Correspondence: [email protected] (H.L.); [email protected] (G.D.); Tel.: +86-871-6386-2676 (H.L.) † These authors contributed equally to this work.

Received: 28 September 2017; Accepted: 6 December 2017; Published: 14 December 2017

Abstract: To guide the preparation of protein-based adhesive, especially the soy-based adhesive, the reaction between a simple dipeptide N-(2)-L-alanyl-L-glutamine (AG), being used as a model compound of protein, and its cross-linker furfuryl alcohol were studied in this paper. The products that were prepared with furfuryl alcohol and AG under different pHs were analyzed by ESI-MS, 13C NMR, and FT-IR. It was found that the medium environment had great effects on the competition of the co-condensation reaction between furfuryl alcohol and AG and self-condensation reaction of furfuryl alcohol molecules in the mixing system with furfuryl alcohol and AG. Under alkaline conditions, both co- and self-condensation were not obviously detected. Only when the value of pH was higher than 11, were a few co-condensation reaction products gotten. The reaction occurred mainly between furfuryl alcohol and the primary amido groups of AG. Under acid conditions, both co- and self-condensation were observed. The more acid the preparation conditions were, the easier to be observed the self-condensation of furfuryl alcohol molecules would be than the co-condensation between furfuryl alcohol and AG. When the value of pH was higher than 5, both co- and self-condensation were not outstanding. In this study, under pH 3, the co- and self-condensation found equilibrium. There was a great possibility for the primary amido and aliphatic amino groups of AG molecules to react with furfuryl alcohol molecules. No reaction was detected between the secondary amido groups of AG and furfuryl alcohol.

Keywords: furfuryl alcohol; protein-based adhesives; model compound; modiﬁcation mechanism

1. Introduction The application of soy protein-based adhesives in wood industry has been reported [1–3]. To meet with the requirements on mechanical performances and water resistance, soy protein-based adhesives normally has to be modiﬁed with some cross-linkers, such as formaldehyde and its derivatives, isocyanate, epoxy, etc. [4–9]. The modiﬁcation mechanism was presumed to be based on the reaction between the cross-linkers and the reactive groups from protein molecules, which including –NH2, –CO–NH–, –CO–NH2, –COOH, –OH, –SH, and –Ph–OH [10–12]. Because of the similar chemical structure of furfuryl alcohol as formaldehyde and its derivatives, furfuryl alcohol can be used as a cross-linker of protein-based adhesives, which has already been proved by the study of Kumar [13,14]. Furfuryl alcohol is the hydrogenation product of furfural, which can be gotten from some bio-based materials, such as corn, wheat, etc. Being different from petroleum-based formaldehyde and its derivatives, furfuryl alcohol is sustainable and environment-friendly, which are

Polymers 2017, 9, 711; doi:10.3390/polym9120711 www.mdpi.com/journal/polymers Polymers 2017, 9, 711 2 of 16 definitely favorable for being a cross-linker of protein-based adhesives [15,16]. After all, the toxicity caused by the addition of formaldehyde and its derivatives are not desirable for the usage of protein-based adhesives, although the addition amount of the cross-linker might be low. Under determined conditions, furfuryl alcohol molecules have a tendency of self-condensation to get furfuryl alcohol resin. With its good mechanical performances and heat and water resistance, the resin can be used as adhesives of wood, rubber, metal, and ceramic [17–19]. When using as a cross-linker of protein-based adhesives, the competition between the self-condensation of furfuryl alcohol and the co-condensation of furfuryl alcohol and protein has to be considered. Since the composition amino acids units of protein were very complex and the soy flour for the preparation of soy protein-based adhesives was a mixture with some ingredients, it seems almost impossible to study the modification mechamisms with a soy flour material. Based-on our previous study [20–22], N-(2)-L-alanyl-L-glutamine (AG) was chosen as a model compound to react with furfuryl alcohol to simplify the study. The objective of this study is to find suitable reaction condition when using furfuryl alcohol to cross-link the protein molecules, and then be guidance on the preparation of soy protein-based adhesives. The main reason for the choosing of AG is that it is an easily-gotten dipeptide. Although it cannot represent soy protein, its reaction with furfuryl alcohol can still give some useful information on the reaction pH and the possible reactive groups of amino acid, dipeptide or other compositions of protein molecules when using furfuryl alcohol as a cross-linker of protein-based adhesives. After all, AG owns some similar characteristic groups as those of soy protein, specifically, –NH2, –CO–NH–, –CO–NH2, and –COOH. The mechanism on the modification of protein-based adhesives is rather few till now. The study on the reaction with AG model compound and furfuryl alcohol will be just a beginning to pursuing the law for the modification of soy protein-based adhesives and even other protein-based ones.

2. Materials and Methods

2.1. Materials

N-(2)-L-alanyl-L-glutamine with a purity of 99%, furfuryl alcohol (FA, 98.5 wt %), toluene-p-sulfonic acid (99 wt %) and sodium hydroxide (96 wt %) in reagent grade were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China.

2.2. Preparation of Resins When considering the pH affect the reactivity of furfuryl alcohol and protein molecules, some samples with AG and furfuryl alcohol were prepared under different pH conditions. The objective was to see whether the co-condensation of AG and furfuryl alcohol exists besides the self-condensation of furfuryl alcohol in the mixing system and what were the favorable conditions for the reactions. The preparation procedure was as below: The AG and furfuryl alcohol with molar ratio n(FA)/n(AG) = 3:1 were charged to a ﬂask equipped with a condenser, thermometer and a magnetic stirrer bar. Some distilled water was added to get a mixture with solid content 30%. The mixture was then kept at 75–80 ◦C for 1 h under a desired pH. The pH was adjusted with toluene-p-sulfonic acid saturated solution or 20% sodium hydroxide solution and was kept as 1, 3, 5, 7, 9, 11, and 13, respectively, to get a series of samples with the name of FA/AG. To see the effects of pH on the self-condensation of furfuryl alcohol, two samples with furfuryl alcohol materials were prepared under two different pHs. The preparation procedure was as below: some furfuryl alcohol and distilled water were charged to a ﬂask equipped with a condenser, thermometer, and a magnetic stirrer bar to get a mixture with solid content 30%. Toluene-p-sulfonic acid saturated solution or 20% sodium hydroxide solution was used to adjust the pH. The mixture being kept at 75–80 ◦C for 1 h at pH 1 was named as FA1 and that at pH 11 was named as FA2. Polymers 2017, 9, 711 3 of 16 Polymers 2017, 9, 711 3 of 16

2.3. Electrospray Ionization Mass Spectrometry (ESI-MS) 2.3. Electrospray Ionization Mass Spectrometry (ESI-MS) The spectra were recorded on a Waters Xevo TQ-S instrument. The FA/AG and FA series samples The spectra were recorded on a Waters Xevo TQ-S instrument. The FA/AG and FA series were dissolved in chloroform, respectively, at a concentration of about 10 µL/mL and then were samples were dissolved in chloroform, respectively, at a concentration of about 10 μL/mL and then µ injectedwere into injected the ESIinto sourcethe ESI plussource ion plus trap ion mass trap mass spectrometer spectrometer via via a syringe a syringe at at a ﬂowa flow rate rate of of 55 g/s. Spectraμg/s. were Spectra recorded were recorded in a positive in a positive mode, withmode, ion with energy ion energy of 0.3 of eV 0.3 and eV and scan scan range range of 0–1000of 0–1000 Da. Da. 2.4. 13C NMR 2.4. 13C NMR The 300 µL liquid sample was directly mixed with 100 µL DMSO-d6 for 13C NMR determination. The spectraThe were 300 obtained μL liquid on sample a Bruker was AVANCE directly mixed 600 NMR with spectrometer 100 μL DMSO using-d6 12for µ13sC pulse NMR width (90◦).determination. The relaxation The delay spectra was were 6 s. obtained To achieve on a aBruker sufﬁcient AVANCE signal-to-noise 600 NMR spectrometer ratio, inverse-gated using 12 μs proton decouplingpulse width method (90°). was The applied. relaxation The spectra delay was were 6 taken s. To at achieve 150 MHz a sufficient with 800–1200 signal- scansto-noise accumulated. ratio, inverse-gated proton decoupling method was applied. The spectra were taken at 150 MHz with 2.5. FT-IR800–1200 Analysis scans accumulated.

The2.5. FT KBr-IR pillsAnalysis with AG material and FA/AG series samples were prepared for FT-IR analysis. The solidThe AG KBr material pills with was AG pre-dried material toand a constantFA/AG series weight. samples The were liquid prepared FA/AG for series FT-IR samples analysis. were freeze-driedThe solid in AG advance material to was remove pre-dried the to water, a constant too. Theweight. FT-IR The spectra liquid FA/AG were gottenseries samples on a Varian were 1000 infraredfreeze spectrophotometer.-dried in advance to remove the water, too. The FT-IR spectra were gotten on a Varian 1000 infrared spectrophotometer. 3. Results and Discussion 3. Results and Discussion 3.1. The ESI-MS Analysis 3.1. The ESI-MS Analysis The ESI-MS spectra of FA/AG series samples prepared at different pHs, speciﬁcally pH = 1, 3, 5, 7, 9, 11, andThe 13,ESI are-MS given spectra as of Figure FA/AG1. series The assignmentssamples prepared on the at different main ion pHs, peaks specifically gotten underpH = 1, acid3, 5, and alkaline7, 9, conditions11, and 13, are given given as in Figure Tables 1.1 Theand assignments2, seperately. on According the main ion to peaks the mechanism gotten under of acid ESI-MS and and alkaline conditions are given in Tables 1 and 2, seperately. According to the mechanism of ESI-MS the chemical structure of furfuryl alcohol and AG molecules, the ion peaks observed in the ESI-MS and the chemical structure of furfuryl alcohol and AG molecules, the ion peaks observed in the spectra mainly came from ions in three forms, namely [M + H]+, [M + Na]+, and [M + K]+. The former ESI-MS spectra mainly came from ions in three forms, namely [M + H]+, [M + Na]+, and [M + K]+. The + cameformer fromthe came combination from the combination of amino groupsof amino in groups AG molecules in AG molecules and H andand H the+ and latter the latter two cametwo came from the + + combinationfrom the ofcombination oxygen atom of oxygen from carbonylatom from groups carbonyl in AGgroups molecules in AG molecules and Na andor KNa.+Since or K+. thereSince were p–π conjugatedthere were p structure–π conjugated in furfuryl structure alcolhol in furfuryl molecules, alcolhol the molecules, furfural carbonium the furfural ions carbonium were possible ions to be detected.were possible to be detected.

BGFA1 13 (0.296) Cm (8:35-1:3x2.000) 1: MS2 ES+ 310 100 4.84e7 pH=1 428 435

328

% 252 354 447 533 293 414 396 534 470 474 390 277 496 545 638 661 218 631 683 736 236 687 781 799 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800

Figure 1. Cont. BGFA2 14 (0.320) Cm (8:33-1:3x2.000) 1: MS2 ES+ 310 533 9.21e7 100 354 pH=3

338

298 534

355 426 293 444

568 571 473 519 356 669 396 402 506 252 617 670 590 639 781 702 782 218 688 759 783 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800

BGFA3 14 (0.320) Cm (10:24-1:4x2.000) 1: MS2 ES+ 473 100 9.74e7 pH=5435

365

474 402 728 358 712 298 729 218 493 582 256 601 683 819 968 732799 900 945 994 0 m/z 200 300 400 500 600 700 800 900 1000

BGFA4 13 (0.296) Cm (10:21-2:4x2.000) 1: MS2 ES+ 457 100 6.49e7 435

pH=7

240

473

479

495 218 262 306 315 413 690696 914 535 585652 712 794 802 892 1000 0 m/z 200 300 400 500 600 700 800 900 1000 BGFA1 13 (0.296) Cm (8:35-1:3x2.000) 1: MS2 ES+ 310 4.84e7 BGFA1100 13 (0.296) Cm (8:35-1:3x2.000) 1: MS2 ES+ 310 100 4.84e7 pH=1 pH=1 428 428 435 435 328 328

% 252 354 252 354 447 533 447 533 293 414 293 414 396 534 396 534 470 474 390 474 277 390 277 496 545 545 638 661 218 631 683 736 236 687 781 799 Polymers 20170, 9, 711 m/z 4 of 16 200 250 300 350 400 450 500 550 600 650 700 750 800 Polymers 2017, 9, 711 4 of 16

BGFA2 14 (0.320) Cm (8:33-1:3x2.000) 1: MS2 ES+ 533 9.21e7 100 310 533 9.21e7 100 354 pH=3 338 338

298 298 534 534

% 355 426 293 444 355 426 293 444 568 568571 473 519 356 571 669 473 519 356396 402 506 669 252 617 396 402 506 670 590617639 781 252 702 782 218 670688 759 590 639 781783 702 782 0 218 688 759 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800783 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800 BGFA3 14 (0.320) Cm (10:24-1:4x2.000) 1: MS2 ES+ 473 100 9.74e7 BGFA3 14 (0.320) Cm (10:24-1:4x2.000) 1: MS2 ES+ 473 9.74e7 100 pH=5435 pH=5435

% 365

474 365402 728 358 474 712 298 402 729 218 493 582 728 256 601 683 819 968 358 732799 900 945 994 0 m/z 200 300 400 500 600 712700 800 900 1000 298 729 218 493 582 256 601 683 819 968 732799 900 945 994 0 BGFA4 13 (0.296) Cm (10:21-2:4x2.000) 1: MS2 ES+m/z 200 300 400 457500 600 700 800 900 1000 100 6.49e7 435

BGFA4 13 (0.296) Cm (10:21-2:4x2.000) 1: MS2 ES+ 457 100 6.49e7 pH=7435

pH=7

240

% 473

240 479

495 473 218 262 306 315 413 690696 914 535 585652 712 794 802 892 1000 0 m/z 200 300 400 500 600 700 800 900 1000 479 495 218 262 306 315 413 690696 914 535Figure585 1.652Cont. 712 794 802 892 1000 0 m/z 200 300 400 500 600 700 800 900 1000

BGFA4 13 (0.296) Cm (10:21-2:4x2.000) 1: MS2 ES+ 457 100 6.49e7 435

pH=7

240

473

479

495 218 262 306 315 413 690696 914 535 585652 712 794 802 892 1000 Polymers 2017, 90, 711 m/z 5 of 16 Polymers 2017200, 9, 711 300 400 500 600 700 800 900 1000 5 of 16

BGFA5 14 (0.320) Cm (10:28-1:4x2.000) 1: MS2 ES+ 457 100 7.23e7 pH=9 240

435

458 262 501

502 696 718 218 299 315 397 936 979 535 610 674 794 805 816 914 0 m/z 200 300 400 500 600 700 800 900 1000

BGFA6 15 (0.345) Cm (10:27-2:3x2.000) 1: MS2 ES+ 457 100 9.63e7 240 pH=11

501

262

458

% 740

435

479 502 741

299 318 397 413 621 264 503 535 610 622 718 742 218 340 384 696 805 763 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800

BGFA7 15 (0.345) Cm (10:29-1:3x2.000) 1: MS2 ES+ 501 100 1.10e8 457 pH=13

240 262

502

458

% 740

741 763 479 524 263 435 299 632 764 397 413 644 318 525 621 285 579 786 218 341 645 718 791 0 m/z 200 250 300 350 400 450 500 550 600 650 700 750 800

FigureFigure 1. ESI-MS 1. ESI spectra-MS spectra of the of samples the samples with furfuryl with furfuryl alcohol alcohol and N -(2)- and LN-alanyl--(2)-L-alanylL-glutamine-L-glutamine prepared underprepared different under pHs. different pHs.

In Figure1, more ion peaks could be detected for FA/AG series samples prepared under acid conditions than those prepared under alkaline conditions. 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Under acidIn theory, conditions, hydroxymethyl with a h igherfurfural H+ reactioncarboniumconditions between furfurylthan ions mightthose alcohol prepared be the and active AG.under While intermediates. alkaline under conditions. alkaline Under acidIn conditions, theory, conditions, hydroxymethyl the furfuralwith a h carbeniumigher furfural H+ carboniumconditions than ions mightthose prepared be the active under intermediates. alkaline conditions. Under acidIn theory, conditions, hydroxymethyl with a higher furfural H+ carboniumconcentration, ions the might concentration be the activeof the carbonium intermediates.intermedia tes.ions Underwould acidincrease conditions, and then with be helpful a hhigherigher for the H+ carboniumconcentration, ions the might concentration be the activeof the carboniumintermedia tes.ions Underwould acidincrease conditions, and then with be helpful a higher for theH + ions becomeconcentration,carbonium unstable ions the [ 23might concentration]. be the activeof the carboniumintermedia tes.ions Underwould acidincrease conditions, and then with be helpful a higher for theH+ concentration,reaction between the furfuryl concentration alcohol of and thethe AG. carboniumcarbonium While under ionsions wouldwouldalkaline increaseincrease conditions, andand thenthethen furfural bebe helpfulhelpful carbenium forfor thethe reactionconcentration, between the furfuryl concentration alcohol of and the AG. carbonium While under ions wouldalkaline increase conditions, and thethen furfural be helpful carbenium for the ionsreaction become between unstable furfuryl [23]. alcohol and AG. While under alkaline conditions, the furfural carbenium Tablereactionionsreaction 1. becomeThe betweenbetween main unstable ion furfurylfurfuryl peaks [23]. ofalcohol alcohol ESI-MS andand and AG.AG. their WhileWhile assignments underunder alkalinealkaline for the conditions,conditions, samples with thethe furfuralfurfural furfuryl carbeniumcarbenium alcohol ionsreaction become between unstable furfuryl [23]. alcohol and AG. While under alkaline conditions, the furfural carbenium ionsions becomebecome unstableunstable [23].[23]. andionsN-(2)- becomeTableL-alanyl- 1. TheunstableL main-glutamine [23].ion pe aks prepared of ESI-MS under and acidictheir assignments conditions for and the their samples assignments. with furfuryl alcohol Table 1. The main ion peaks of ESI-MS and their assignments for the samples with furfuryl alcohol Table 1. The main ion pepeaksaks of ESIESI-MS-MS and their assignments for the samples with furfuryl alcohol Tableand N -1.(2) The-L-alanyl main- Lion-glutamine peakspeaks of preparedESI-MSESI-MS and under their acidic assignments conditions for and the theirsamples assignments. with furfuryl alcohol andTable N -1.(2) The-L-alanyl main- Lion-glutamine peaks of prepared ESI-MS and under their acidic assignments conditions for and the their samples assignments. with furfuryl alcohol andTable N Experimental-(2)--1.(2) The-L-alanyl--alanyl main- Lion-glutamine-glutamine (Da) peaks of prepared ESI-MS and under their acidic assignments conditions for and the their samples assignments. with furfuryl alcohol and N-(2)--(2)-L-alanyl--alanyl-L-glutamine-glutamine prepared under acidic conditions and their assignments. and N-(2)-L-alanyl-ExperimentalL-glutamine (Da) prepared under acidicChemical conditions Species and their assignments. and N-(2)-L-alanyl-+ ExperimentalL-glutamine +(Da) prepared under acidicChemical conditions species and their assignments. [M + H] [MExperimental +[M H]+ + Na][M +(Da) Na] + Chemical species [MExperimental + H]+ [M +(Da) Na] + Chemical species [MExperimental + H]+ [M + (Da) Na] + Chemical species [MExperimental + H]+ [M + (Da) Na] + Chemical species [M + H]+ [M + Na]+ Chemical speciesO [M + H] + [M + Na] + ChemicalCH3 speciesO + + CH3 O [M + H] [M + Na] CH3 H O [M + H] [M + Na] CH3 H O CH3 N O CCHH3 H O 218 CH 3 HN NH 218 218 H N CH3 NH O NH2 218 H2N 3 HN O NH2 218 2 CH3 HN NH 2 218 H 2N NH NH2 218 H2N NH COOH NNHH2 218 H2N O N COOH NH 2 218 H 2N O N COOH NH2 218 H2 N O COOH NH2 H2N O CCOOHOOH 2 2 O COOH O COOH O CH3 O COOH O CH3 O O CH3 H O CH3 HN O CHCH3 NH O CH3 H O N CH 252 H N CH NH N CH2 252 252 H2N 3 NH O H 2 252 2 CH3 N O NH CH2 252 H2N CH3 HN H 2 252 H2N H COONa HN CH2 252 H2N O N COONa N CCHH2 252 H2N O HN H 2 252 H2N COONa NH CH2 H2N O N COONa N CH 252 H N O COONaCOONa H 2 252 H2N O COONa NH CH2 2 OH O COONa H OH O COONa OH O COONa O OSH O O OHSOH O O OSH O O O OHS O O O SOH O O SOH O O O S O O OH 293 O S O --- Na+--- O OH 293 O S O --- Na+--- OH --- +--- O OH 293 293 --- Na+--- O OHOH 293 --- Na+--- O OH 293 ------Na+------OH 293 --- Na+ OH 293 Na +--- OH 293 --- Na+--- 293 CH3 --- Na --- CH3 CH3 CH3 CH3 CH3 CH3 CH3 O CH3 CH3 O CH3 CH3 O O CH3 H O O CH3 H O O CHCH3 NH O O CH3 HN O 298 O CH NH O NH2 298 O HN CH3 NH O NH2 298 HN 3 N NH2 298 O HN CH3 HN NH2 298 298 O HN NH NH2 298 O HN H COOH NNHH2 298 HHNN O N COOH NH 2 298 HN N COOH NH2 HN O COOH NH2 298 HN O CCOOHOOH 2 O COOH O COOH O CH3 O COOH O CH3 O O O CH3 H O CH O CH3 HN O CH2 O CHCH3 H O 2 O CH3 HN O CH2 310 O CH 3 NH N CH2 310 O NH 3 NH O NH CH2 310 O NH CH3 HN O N CCHH2 310 O NH CH HN NH 2 310 O NH 3 H COOH HN CH2 310 310 NH O N COOH HN CH2 310 O NNHH O HN H CH 310 O N COOH NH 2 310 NH O COOH HN 310 NH O CCOOHOOH NH NH O H OO COOH CH3 O COOH CH3 OO COOH O CH3 H OO CH O CH3 HN O CH2 O CH3 H O 2 O CHCH3 HN N CH2 O OH 328 3 NH O CH2 O 328 H2N CH3 HN O NH CH --- O OH H2N H N CH2 --- O OH 328 CH3 N O H CH2 OH 328 H2N CH NHN NH CH --- O 328 H2N 3 H COOH NH 2 --- O OOHH 328 H2N O N COOH N CH2 --- OH 328 328 HH2NN O HN COOH H CH ------OH 328 H 2N O N COOH NH 2 --- 328 2 O COOH HN OH 328 H2N OO COOHCOOH NH --- OH H2N O COOH H --- O COOH O O CH3 O O O CH3 O COOH O O CH3 H O O CH3 HN O O OH CHCH3 NH O + O OH CH3 H O NH --- Na+--- OH 338 H N CH NH NH2 --- Na+--- O OH 338 H2N 3 NH O 2 --- Na+--- O OH 338 2 CH3 N O NH2 --- Na+--- O OH 338 H2N CH3 HN NH2 --- +--- OH 338 H2N H COOH NH2 --- NaNa+--- OH 338 H N O N COOH NH2 --- Na+--- 338 H2N HN NH2 --- OH 338 H2N O COOH NH Na +--- OH 338 H N O N COOH NH2 --- Na+--- 338 H2 N O COOH 2 --- Na --- 338 2 O CCOOHOOH NH2 H2N O O COOH O H O COOH O H O COOH O H O OS H O O O OHOSH O CH3 O O S O CH3 O O OHS O O CH3 H O O SOH O O CH3 H O O OHS O O CHCH3 NH O O S O O CH3 HN O --- + O S O 470 O CH3 NH O NH2 H+ --- O S O 470 O NH CH HN O NH2 --- H+ --- 470 NH 3 HN NH2 --- H+ --- 470 O NH CH3 N NH2 --- H+ --- 470 O NH NH COOH NHNH2 ------H+ ------470 O NHNH O HN COOH NH2 --- H+ --- 470 470 NH O N COOH NH2 --- H + --- 470 NH O COOH NH2 --- H+ --- 470 NH O COOHCOOH NH2 --- H --- NH O COOH CH3 O COOH CH3 O COOH CH3 O COOH CH3 O CH33 CH3 OH CH3 OH CH3 OH CH3 O OSH O O OOHSH O O OHS O O OHS O O O O O O S O O O O O OH + O O O O O S O --- Na+--- O O O O O S O --- Na --- O O O O OH 533 O S O --- Na+--- O O O O OH 533 --- Na+--- O O O O OH 533 ------NaNa+------O O O O OH 533 --- Na+--- O O O O OOHH 533 --- Na +--- OH 533 --- Na+--- OH 533 533 --- Na --- OH CH3 CH3 CH3 CH3 CH3 CH3 CH3 O CH3 O CH CH3 3 CH3 O O CH3 H O O CH3 H O O CCHH3 NH O O CH3 HN O 617 O 3 NH NH2 2 O HN CH3 HN O 2 617 O HN CH3 HN NH2 617 2 O HN NH NH2 617 2 O HN NH COOH NNHH2 617 2 HHNN O N COOH NH2 617 2 HN O N COOH NH2 617 2 HN O COOH NH2 617 2 HN O CCOOHOOH 2 O COOH O COOH O COOH O

Polymers 2017, 9, 711 7 of 16 PolymersPolymers 20172017,,, 99,,, 711711 77 ofof 1616 Polymers 2017, 9, 711 7 of 16 Polymers 2017, 9, 711 7 of 16 Polymers 20172017,, 99,, 711711 77 ofof 1616 Polymers 2017,, 9,, 711711 7 of 16 PolymersTable 2017 2. The, 9, main711 ion peaks of ESI-MS and their assignments for the samples with furfuryl alcohol 7 of 16 andTableTableN-(2)- 22L...-alanyl- TheThe mainmainL-glutamine ionion peakspeaks prepared ofof ESIESI--MSMS under andand alkalinetheirtheir assignmentsassignments conditions forfor and thethe their samplessamples assignments. withwith furfurylfurfuryl alcoholalcohol Table 22.. The main ion peaks of ESIESI-MS-MS and their assignments for the samples with furfuryl alcohol Table 2. The main ion peaks of ESI-MS and their assignments for the samples with furfuryl alcohol Tableandand NN --2.(2)(2) TheThe--LL--alanylalanyl mainmain-- LionionL--glutamineglutamine peakspeaks ofof preparedpreparedESI-MSESI-MS andand uundernder theirtheir alkalinealkaline assignmentsassignments conditionsconditions forfor thethe andand samplessamples theirtheir assignments.assignments. withwith furfurylfurfuryl alcoholalcohol andTable N --(2)-2.(2) The-L--alanyl-alanyl main- Lion--glutamineglutamine peaks of prepared ESI-MS and uundernder their alkaline assignments conditions for the and samples their assignments. with furfuryl alcohol andTable N -(2)-2. TheL-alanyl- main Lion-glutamine peaks of preparedESI-MS and under their alkaline assignments conditions for the and samples their assignments. with furfuryl alcohol and NExperimental-(2)--(2)-L-alanyl--alanyl-L (Da)-glutamine-glutamine preparedprepared underunder alkalinealkaline conditionsconditions andand theirtheir assignments.assignments. and N-(2)-L-alanyl-ExperimentalExperimentalL-glutamine (Da)(Da) prepared under alkaline conditionsChemical and Species their assignments. + Experimental+ (Da) + ChemicalChemical speciesspecies [M + H] [M[M ++[M H]H]Experimental++ + Na][M[M ++ Na]Na][M++ + (Da) +[M[M K] ++ K]K]+++ Chemical species [M + H]Experimental+ [M + Na] +(Da) [M + K]+ Chemical species [M + H]Experimental+ [M[M+Na] + Na]+ (Da)[M + K]+ Chemical species [M + H]Experimental++ [M+Na]++ (Da)[M + K]++ Chemical species [M + H]+ [M+Na]+ [M + K]+ Chemical speciesOO [M + H] + [M+Na]+ [M + K] + CCHHChemical33 species [M[M ++ H]H]+ [M+Na][M+Na]+ [M[M ++ K]K]+ CH33 O [M + H] [M+Na] [M + K] CH3 H O [M + H] [M+Na] [M + K] CH3 HH O CH3 HNN O CH3 NH O CH3 NH O NH 218218 240240 256 256 CH3 NH O NNHH222 218218 240240 256256 HH22NN CH 3 N O 2 218 240 256 H22N CH 3 HN NNHH2 218 240 256 H2N 3 H NH 2 218 240 256 H2N NH NH2 218 240 256 2 O HN CCOOOOHH 2 218 240 256 H2N OO N NH2 (AG)(AG) 218 240 256 H2N N CCOOHOOH NH2 (AG) 218 240 256 H2 N O COOH NH2 (AG)(AG) 218 240 256 H2N O COOH NH2 (AG) 218 240 256 H2N O (AG) O COOH (AG)(AG) O COOH (AG) O COOH OO (AG) CCHH3 O (AG) CH333 O CH3 H O CH3 HH CH3 HN O CH3 NHN O CH3 H O NH 262 262 CH3 NH O NNHH222 262262 HH22NN CH 3 N O 2 262 H22N CH 3 HN NNHH2 H2N 3 H 2 262 H2N NH NH2 262 H2N NH CCOOOONNaa NH2 262 H N OO N COONa NH2 262 H 2N O N COONa NH2 (AG(AG--Na)Na) 262 H2 N O COONa NH2 (AG-Na) 262 H2 N O COONa NH2 (AG(AG-Na)-Na) 262 H2N O COONa 2 (AG-Na) 2 O COONa (AG-Na) O COONa (AG-Na) O COONaO (AG-Na) OO O COONaOO (AG-Na)OO O O (AG-Na)O O O O O O O O OH O OO O ++ OOHH 299 299299 O O ------Na+ ------O 299 O O O ---NNaa+ --- O OOHH 299 O O O ------Na+--- O OH 299 O O O --- Na+ --- O OH 299 O O ---Na+ --- O OH 299299 O --- Na++------OH 299 O ---Na +--- OH 299 O ---Na+ --- OH 299 O --- Na --- OO OO Na OO O O O O O O O O O OH O OO O ++ OOHH 315315 O O ------K+ ------O 315 315 O O O ---KK+ --- O OOHH 315 O O O ------K+ --- O OH 315 O O O ---K+ --- O OH 315 O O ---K+ --- O OH 315315 O --- K++ ------OH 315 O ---K + --- OH 315 O ---K+ --- OH 315 OO ---K --- O O O OOHH 318318 (317)(317) 33 O OH 318 (317) 3 O OOHH 318 (317)318 (317) 3 O OH 318 (317) 3 O OH 318 (317) 33 OH 318318 (317)(317) 3 OH 318 (317) 3 OH 318 (317) O 3 O OO OO OO O O O O O O O ++ O OH O OO O ------Na+ ------OOHH 397397 O O ---NNaa+ --- 22 O 397 O O O --- + --- 2 O OOHH 397 O O O ---NNaa+ ---2 O OH 397 397 O O O ---Na+ --- 2 O OH 397 O O Na+ --- O OH 397 O ------Na++------22 OH 397 O ---Na+ ---2 OH 397 O --- Na+ --- 2 OH 397 O O 2 OO OO OO O O O O O OO O ++ O OH O O O ------K+ ------OOHH 413413 O O KK+ --- 22 O OH 413 O O O --- + --- 2 O OH 413 O O O ---K+ ---2 O OH 413 O O O ---K+ --- 2 O OH 413 413 O O K+ --- O OH 413 O ------K ++ ------22 OH 413 O ---K+ ---2 OH 413 O --- K+ --- 2 OH 413 K 2O CH OO CCHH333 3 O CH3 HH O CH 3 H 3 HNN O CH3 H O CH NH O NNHH222 435435 HH NN 3 N O NH2 435 22H222N CH3 NH O NH 435 2 H2N CH 3 H O NH2 435 2 H2 N CH 3 HN NH2 435 2 H2N 3 NH CCOOOOHH NH2 435 2 H2N OO NH COOH 2 435 435 2 2 N COOH NH2 435 H2N O N COOH NH2 435 2 H2N O COOH NH 2 435 2 H 2N O COOH NH2 435 2H2N O 2 2 2 COOH O COOHCOOH O OO O COOH OO CH O O CCHH333 CCHH333 O CH3 O 3 O CH3 O CCHH3 HH O CH3 HH O CH 3 H O CH3 O CH3 HNN O CH3 HNN O CH 3 NH O + CH3 NH O NH 457457 CH3 NH O NNHH ------++------CH3 HN O NNHH222 457 CH3 NH O NH222------H ------HH222NN CH3 HN 2 457 HH222NN CH3 HN 2 HH+ H2N CH3 HN NH2 457 2 CH3 HN NNHH2 ------+------H N H NH2 457 H2N NH NH2 ---H --- H22N NH 2 457 H2N NH NH2 H+ H2N HN CCOOOOHH NH2 2 N CCOOOONNaa 2 --- + --- H2N O N COOH NH2 457 457 H2N OO N COONa NH2 --- H+--- H N OO N NH2 457 H N O N COONa NH2 ---H +--- H2 N COOH NH2 457 H2 N COONa NH 2 ---H+--- H2 N O COOH NH2 457 H2 N O COONa NH2 ---H --- H2N O 2 H2N O COONa 2 H H2N O COOH 2 O COONa O COOH O COONa O COOH O COONa OO O COOH OO CHO O CCHH333O CCHH3O33 O CH3 O 3 O CH3 O CCHH3 HH O CH3 HH O CH 3 H O CH3 N CH3 HNN O CH3 HNN O CH 3 NH O ++ CH3 NH O NNHH 479479 CH3 NH O NNHH2 ------+------CH3 HN O NH222 479 H N CH3 NH O NH222---HH --- HH222NN CH3 HN 2 479 HH222NN CH3 HN NH H+ H2N CH3 HN NNHH2 479 2 CH3 HN NH2 ------+------H2N H NH2 479 H2N NH NH2 ---H --- H2N NH 2 479 H2N NH NH2 H+ H2N NH CCOOOONNaa NH2 2 HN CCOOOONNaa 2 --- +--- H2N OO N COONa NH2 479 H2N OO N COONa NH2 --- H+--- H N O N NH2 479 479 H N O N COONa NH2 ---H +--- H2 N CCOONaOONa NH2 479 H2 N O COONa NH 2 ---H+--- H2 N O COONa NH2 479 H2 N O COONa NH2 ---H --- H2N O 2 H2N O COONa 2 H H2N O COONa 2 O COONa O COONa O COONa O COONa O COONa OO O COONa OO CH O O CCHH333O CCHH33O3 O CH3 O 3 O CH3 O CCHH3 HH O CH3 HH O CH3 H O CH3 N CH3 HNN O CH3 HNN O CH 3 NH O ++ CH3 NH O NNHH 501501 CH3 N O NNHH2 ------+------CH3 HN O NH222 501 H N CH3 NH O NH222---NNaa--- HH222NN CH3 HN 2 501 HH222NN CH3 HN NH Na+ H2N CH3 HN NNHH2 501 2 CH3 HN NH2 ------+-- H2N H NH2 501 H2N NH NH2 ---NNaa--- H2N NH 2 501 H2N NH NH2 Na+ H2N NH CCOOOONNaa NH2 2 HN CCOOOONNaa 2 --- +--- H2N OO N COONa NH2 501 H2N OO N COONa NH2 --- Na---+ H N O N NH2 501 H N O N COONa NH2 ---Na---+ H2 N CCOONaOONa NH2 501 501 H2 N O COONa NH 2 ---Na---+ H2 N O COONa NH2 501 H2 N O COONa NH2 ---Na--- H2N O 2 H2N O COONa 2 Na H2N COONa H2N O O COONa O COONa O COONa O COONa O COONa OO O COONa OO CCHH3O3 O COH O CH33 O CCHH333 CH O 3 O CH3 HH O CH3 HH O CH 3 H O CH3 H CH3 HNN O CH3 HNN O CH 3 NH O CH3 NH O CH3 NH O NNHH222 718718 CH3 NH O NNHH------+++------HH22NN CH3 HN O NH2 718 CH O NH222--- NNaaa+--- H22N CH3 HN O NH 22HH222NN CH3 HN O 2 Na+ CH HN NH2 718 2 H2N CH3 HN NH2--- +--- H2N 3 H NH2 718 H N CH3 HN NH2--- Na+--- H2N NH COOH 2 718 2 H2 N NH NH2--- Na --- H2N O NH CCOOOOHH NH2 718 2 H2N NH CCOOOONNaa NH --- Na+--- H2N OO N NH2 718 2 H2N OO HN COONa NH 2--- Na+--- H N N COOH NH2 718 2 2 O N COONa NH2--- Na+--- H2 N O COOH NH2 718 2 H2N O N NH2--- Na+--- H2 N O COOH NH2 718 718 2 H2N COONa NH2--- Na+--- H2N O COOH NH2 718 2 H2N O COONa NH2--- Na+--- H2N O 2 H2N O COONa 2 Na COOH 2 2 O COONa O COOH O COONa O COOH O COONa OO O O CCHH3 O O CH333 O CH3 O CH3 HH O CH3 H O CH3 HNN O CH3 NH O 740740 CH3 NH O NNHH222 740 3 HH22NN CH3 HN 2 740 33H22N CH3 HN NH2 740 H2N H NH2 740 3 H2N NH NH2 740 3 H2N NH CCOOOONNaa NH2 740 3 H2N OO N COONa NH2 740740 3 H2N O N COONa NH2 740 3 H2N O COONa NH2 740 740 3H2N O COONa NH2 3 H2N O COONa OO COONa O COONa O COONa

Polymers 2017, 9, 711 8 of 16

Polymers 2017, 9, 711 8 of 16 PolymersAccording 2017, 9, 711to the calculation of the ion peaks, the peak of 218 Da came from the AG molecules,8 of 16 According to the calculation of the ion peaks, the peak of 218 Da came from the AG molecules, which meant the existence of the free AG. Although the intensity from this peak was not high, it could which meant the existence of the free AG. Although the intensity from this peak was not high, it be detectedAccording in all to of the the calculation samples. In of this the work,ion peaks, the acid the conditionspeak of 218 were Da came adjusted from by the toluene-p-sulfonic AG molecules, could be detected in all of the samples. In this work, the acid conditions were adjusted by whichacid with meant molecule the existence weight 172of the Da. free The AG. ions Although with 293 andthe intensity 533 Da from from the this samples peak was prepared not high, under it toluene-p-sulfonic acid with molecule weight 172 Da. The ions with 293 and 533 Da from the couldacid conditions be detected were in then all beof assigned the samples. as the In combination this work, of the the acid toluene-p-sulfonic conditions were acid adjusted and FA by or samples prepared under acid conditions were then be assigned as the combination of the toluenetheir self-condensates.-p-sulfonic acid with molecule weight 172 Da. The ions with 293 and 533 Da from the toluene-p-sulfonic acid and FA or their self-condensates. samplesDuring prepared the self-condensation under acid conditions of furfuryl werealcohol then molecules, be assigned formaldehyde as the might combination be given of off the for During the self-condensation of furfuryl alcohol molecules, formaldehyde might be given off toluenethe relatively-p-sulfonic unstable acid methylene and FA or ethertheir link,self-condensates. seen as below: for theDuring relatively the self unstable-condensation methylene of etherfurfuryl link, alcohol seen as molecules, below: formaldehyde might be given off for the relativelyO unstable methyleneO ether link,O seen as below:O O O OH HO O O + O O O O O + HCHO OH HO O + + HCHO In Table 1, the peak of 298 Da was assigned as the co-condensation product of furfuryl alcohol In Table1, the peak of 298 Da was assigned+ as the co-condensation product of furfuryl alcohol and AGIn Table and that1, the of peak 617 Daof 298came Da from was assignedthe Na complex as the co with-condensation two molecules product of theof furfuryl products alcohol. Seen and AG and that of 617 Da came from the Na+ complex with two molecules of the products. Seen from andfrom AG Figure and 1,that the of ions 617 with Da came298 Da from could the be Na detected+ complex clearly with fortwo the molecules samples ofprepared the products under. pHSeen 1 Figure1, the ions with 298 Da could be detected clearly for the samples prepared under pH 1 and 3. fromand 3. Figure Then, 1, the the intensity ions with of 298the Daion could peak bedecreased detected for clearly the sample for the prepared samples preparedunder pH under 5. Even pH no 1 Then, the intensity of the ion peak decreased for the sample prepared under pH 5. Even no peak at andpeak 3. at Then, 298 Da the could intensity be detected of the ionfor peakthe sample decreased prepared for the under sample pH prepared7. Although under the peakpH 5. at Even around no 298 Da could be detected for the sample prepared under pH 7. Although the peak at around 298 Da peak298 Da at reappeared298 Da could for be the detected samples for prepared the sample under prepared pH 9, under11, and pH 13, 7. judged Although by thethe intensitypeak at around of this reappeared for the samples prepared under pH 9, 11, and 13, judged by the intensity of this peak, the 298peak, Da the reappeared reaction between for the samplesfurfuryl preparedalcohol and under AG underpH 9, 11,alkaline and 13, cond judgeditions bywas the not intensity as easy ofas thisthat reaction between furfuryl alcohol and AG under alkaline conditions was not as easy as that under acid peak,under the acid reaction conditions. between The furfurylpeaks of alcohol 252 and and 310 AG Da under could alkaline be assigned cond itionsas carbonium was not ionsas easy that as were that conditions. The peaks of 252 and 310 Da could be assigned as carbonium ions that were gotten from undergotten acidfrom conditions. the co-condensation The peaks ofproduct 252 and of 310furfuryl Da could alcohol be andassigned AG. Theas carbonium possible reaction ions that for were the the co-condensation product of furfuryl alcohol and AG. The possible reaction for the formation of the gottenformation from of thethe cocarbonium-condensation ions ofproduct 310 Da of was furfuryl as below: alcohol and AG. The possible reaction for the carbonium ions of 310 Da was as below: O formation of the carboniumO ions of 310 Da was as below: CH3 CH3 + H O H O +H N O CH2 N O CH3 CH H N NH 3 NH OH 2 H O H2N H + O -+HH20 N COOH CH2 N COOH O NH O NH OH H2N H2N -H 0 2 O COOH Table 2 wasO theCO OassignmentsH on the main ion peaks of the samples with furfuryl alcohol and AG preparedTable 2 was under the assignments alkaline conditions. on the main It was ion regretful peaks of thatthe samples there was with no furfuryl co-condensate alcohol withand AGfurfuryl preparedTable al2cohol was under theand assignments AG alkaline was detected. conditions. on the mainThe It peaks ionwas peaks regretfulof 218, of the240, that samples and there 256 with wasDa came furfuryl no co from-condensate alcohol one molecule and with AG furfurylpreparedof AG. The al undercohol peaks and alkaline of AG435, conditions.was457, detected.and 479 It Da wasThe came regretfulpeaks from of 218,thatthe complex240, there and was of256 no two Da co-condensate molecules came from of one withAG molecule with furfuryl H+. + ofalcoholThe AG. peaks The and ofpeaks AG 501, was of718, 435, detected. and 457, 740 and TheDa 479 were peaks Da fromcame of 218, the from 240,complex the and complex 256of two Da of camemolecules two frommolecules of one AG molecule of with AG Nawith of. AG.TheH+. + Thepeaks peaks of 299, ofof 435,315,501, 457, 318,718, and 397,and 479 and 740 Da 413Da came Dawere were from from from the the complex furfuryl complex of alcohol twoof two molecules and molecules its derivatives. of AG of withAG with H . TheNa+ peaks. The + peaksof 501,In of 718, all, 299, and the 315, analysis740 318, Da 397, were on and the from 413 ESI theDa-MS complexwere spectra from of furfuryl indicated two molecules alcohol that the ofand AG co its- withcondensation derivatives. Na . The occured peaks of only 299, 315,under 318,In acidic all, 397, the and conditions. analysis 413 Da onwere However, the from ESI furfuryl- MS since spectra one alcohol AG indicated and molecule its derivatives.that owns the cothree-condensation different amino occured groups, only underspecifically,In acidic all, the primary conditions. analysis amido, on However, the secondary ESI-MS since spectra amido, one indicated and AG aliphatic molecule that theamino owns co-condensation groups, three differentand all occured of amino them only have groups, under the specifically,acidicpossibility conditions. to primary react However, with amido, furfuryl secondary since alcohol one AGamido, in molecule theory, and aliphatic ownsthe analysis three amino different of groups, the mechanism amino and all groups, of ofthem the speciﬁcally, have reaction the possibilityprimarybetween amido,AG to and react secondary furfuryl with furfuryl al amido,cohol alcoholis and still aliphatic impossible in theory, amino with the groups, analysisthe limited and of all information the of themmechanism have that the ofis possibilitygiven the reaction by the to betweenreactESI-MS with results AG furfuryl and with furfuryl alcohol so many al incohol isomers. theory, is still the impossible analysis of with the mechanism the limited ofinformation the reaction that between is given AG by andthe ESIfurfuryl-MS results alcohol with is still so impossible many isomers. with the limited information that is given by the ESI-MS results with so3.2. many The 13 isomers.C NMR Analysis 3.2. The 13C NMR Analysis 3.2. TheThe13 13CC NMR NMR Analysis result of AG is given in Figure 2. The clarity of the 13C NMR results of the AG sample reflected13 the purity of the raw material used in this study. All the13 carbons in an AG molecule The 13CC NMR NMR result result of of AG AG is isgiven given in in Figure Figure 2.2 .The The clarity clarity of ofthe the C 13NMRC NMR results results of the of AG the and their assignments were labeled in Figure 2. Seen from the chemical structure of AG, both amino sampleAG sample reflected reﬂected the pur theity purity of the ofraw the material raw material used in used this study. in this All study. the carbons All the in carbons an AG inmolecule an AG and carboxyl groups of AG molecules are possible to react with furfuryl alcohol molecules. When andmolecule their andassignments their assignments were labeled were in labeled Figure in 2. FigureSeen from2. Seen the from chemical the chemical structure structure of AG, ofboth AG, amino both considering the unpaired electron of the amino group, the amino groups will show better andamino carboxyl and carboxyl groups groups of AG moleculesof AG molecules are possible are possible to react to with react furfuryl with furfuryl alcoholalcohol molecules. molecules. When nucleophilicity than carboxyl groups and be more liable to react with furfuryl alcohol molecules. consideringWhen considering the un thepaired unpaired electron electron of the of aminothe amino group, group, the the amino amino groups groups will will show show better nucleophilicity than than carboxyl carboxyl groups groups and and be be more more liable liable to to react react with with furfuryl furfuryl alcohol alcohol molecules molecules..

Polymers 2017, 9, 711 9 of 16 Polymers 2017, 9, 711 9 of 16

Polymers 2017, 9, 711 9 of 16

Figure 2. 13C NMR spectra of N-(2)-L-alanyl-L-glutamine. Figure 2. 13C NMR spectra of N-(2)-L-alanyl-L-glutamine.

Figure 3 was the 13C NMR results of the material furfuryl alcohol (FA) and the samples FA1 and 13 13 FA2.Figure FA1,3 andwas FA2 the wereC NMR Figurethe products results 2. C NMR of gotten the spectra material with of N furfuryl-(2) furfuryl-L-alanyl alcohol- alcoholL-glut amine.under (FA) strong and the acid samples and alkaline FA1 and FA2.conditions, FA1, and respectively. FA2 were theThe products 13C NMR gotten result withof FA2 furfuryl showed alcohol no obvious under difference strong acid as that and of alkaline FA, Figure 3 was the 13C NMR results of the material furfuryl alcohol (FA) and the samples FA1 and conditions,which indicated respectively. that thereThe 13 wasC NMR no selfresult-condensation of FA2 showed between no obvious furfuryl difference alcohol as molecules that of FA, under which FA2. FA1, and FA2 were the products gotten with furfuryl alcohol under strong acid and alkaline indicatedstrong alkaline that there conditions. was no self-condensation On the contrary, betweenthe 13C NMR furfuryl result alcohol of FA1 molecules showed undergreat difference strong alkaline as conditions, respectively. The 13C NMR result of FA2 showed no obvious difference as that of FA, 13 13 13 conditions.thatwhich of FA. Onindicated In thethe contrary, C that NMR there theof FA1, wasC NMR28.88 no self and result-condensation 31.52 of FA1ppm showed could between be great assigned furfuryl difference as alcohol the asmethylene molecules that of FA. link under In from the C NMRthestrong ofself FA1,-condensation alkaline 28.88 andconditions. of 31.52 furfuryl ppm On the alcohol could contrary, be molecules. assigned the 13C The asNMR the shift result methylene at 64.69of FA1 ppm link showed fromcould great the be self-condensationassigneddifference as as the ofmethylene furfurylthat of alcoholFA. ether In the link, molecules. 13C which NMR ofcame The FA1, shiftfrom 28.88 atthe and 64.69 self 31.52-condensation ppm ppm could could bebe of assignedfurfuryl asalcohol, as the the methylene methylene too. Since link the ether from peak link, whichareathe cameof self methylene-condensation from the link self-condensation ofwas furfuryl obviously alcohol bigger of molecules. furfuryl than thatThe alcohol, shiftof ether at too. 64.69 link, Since ppm methylene the could peak be link assigned area might of methyleneas be the the linkmain wasmethylene form obviously for ether the selflink, bigger-condensation which than came that from of of furfuryl etherthe self link, -alcoholcondensation methylene under of pH linkfurfuryl 1. In might all, alcohol, the be 13 thetCoo. NMR mainSince proved the form peak forthat the self-condensationthearea self -ofcondensation methylene of furfuryl link reaction was alcohol obviously of furfur under biggeryl pH alcohol 1. than In all,under that the of strong13 etherC NMR link, acid proved methylene conditions. that link the The self-condensationmight shift beat the 82.83 13 reactionppmmain was of form furfurylassigned for the alcohol as self the-condensation formaldehyde under strong of furfurylhydrate, acid conditions. alcohol namely under methylene The pH shift 1. In atglycol, all, 82.83 the which ppmC NMR wasindicated proved assigned that that asthe the the self-condensation reaction of furfuryl alcohol under strong acid conditions. The shift at 82.83 formaldehydehydroxymethyl hydrate, groups namely might methylene be released glycol, from which the indicated furfuryl that alcohol the hydroxymethyl molecules or groups its selfppm-condensation was assigned products as the formaldehyde under strong hydrate, acid conditions. namely methylene That also glycol, caused which the indicated appearance that ofthe the might be released from the furfuryl alcohol molecules or its self-condensation products under strong shifthydroxymethyl at 46.59, 71.48, groupsand 92.66 might ppm, which be released all came from for formaldehyde the furfuryl and alcohol its derivatives. molecules or its acid conditions.self-condensation That alsoproducts caused under the strong appearance acid conditions. of the shift That at 46.59, also caused 71.48, theand appearance 92.66 ppm, of which the all cameshift for formaldehydeat 46.59, 71.48, and and 92.66 its derivatives. ppm, which all came for formaldehyde and its derivatives.

Figure 3. 13C NMR spectra of furfuryl alcohol and the samples FA1 and FA2. Figure 3. 13C NMR spectra of furfuryl alcohol and the samples FA1 and FA2. Figure 3. 13C NMR spectra of furfuryl alcohol and the samples FA1 and FA2. The 13C NMR results of the samples prepared with furfuryl alcohol and AG under alkaline 13 conditionsThe wereC NM givenR results as Figure of the 4. samples prepared with furfuryl alcohol and AG under alkaline Theconditions13C NMR were given results as ofFigure the 4. samples prepared with furfuryl alcohol and AG under alkaline conditions were given as Figure4.

Polymers 2017, 9, 711 10 of 16 Polymers 2017, 9, 711 10 of 16

Figure 4. 13C NMR spectra of the samples with furfuryl alcohol and N-(2)-L-alanyl-L-glutamine Figure 4. 13C NMR spectra of the samples with furfuryl alcohol and N-(2)-L-alanyl-L-glutamine prepared under alkaline conditions. prepared under alkaline conditions. Seen from Figure 4, the 13C NMR results of the samples that were prepared with furfuryl alcohol 13 and AGSeen under from Figure pH 74 , and the 9 showedC NMR results no much of the difference samples that from were that prepared of furfuryl with alcohol furfuryl and alcohol AG themselves,and AG under meaning pH 7 and no 9reaction showed between no much furfuryl difference alcohol from thatand ofAG furfuryl under pH alcohol 7 and and 9. AGFor themselves,the FA/AG samplesmeaning that no reaction were prepared between underfurfuryl pH alcohol 11 and and 13, AG new under peaks pH 7around and 9. 35.6 For the ppm FA/AG appeared. samples To determinethat were prepared the assignment under pH of this 11 and new 13, peak, new the peaks software around of 35.6 Mestrenove ppm appeared. was used To determineto predict the chemicalassignment shifts of thisof all new the peak,possible the products software gotten of Mestrenove with furfur wasyl alcohol used to and predict AG. theThe chemical chemical shifts shift wasof all actually the possible determined products greatly gotten by withthe inductivity furfuryl alcohol of the andaliphatic AG. Theamino, chemical primary shift and was secondary actually amidodetermined groups greatly from by AG the molecules. inductivity The of the calculation aliphatic of amino, the software primary Mestrenove and secondary on amido the 13C groups NMR 13 chemicalfrom AG shifts molecules. of the The possible calculation reactions of the between software AG Mestrenove and furfuryl on alcohol the C NMRwas then chemical the results shifts of respectingthe possible this reactions law. between According AG toand the furfuryl calculation alcohol wasresults, then the the results possible of respecting chemical shift this law. of FuranAccording–CH2 to–NH(CO) the calculation–R from results, the reaction the possible between chemical the primary shift of amido Furan– groupCH2–NH(CO)–R of AG and from furfuryl the alcoholreaction was between 36–37 the ppm. primary The possible amido groupchemical of AGshift and of R furfuryl–NH–C alcoholH2–Furan was from 36–37 the ppm. reaction The between possible thechemical aliphatic shift amine of R–NH– groupC Hof2 AG–Furan andfrom furfuryl the reactionalcohol was between 43–44 the ppm. aliphatic The possible amine groupchemical of AGshift and of Furanfurfuryl–CH alcohol2–NR’(CO) was– 43–44R from ppm. the The reaction possible between chemical secondary shift of amido Furan– groupCH2–NR’(CO)–R of AG and from furfuryl the alcoholreaction was between 40–41 secondary ppm. Therefore, amido the group new of peak AG andat 35.58 furfuryl ppm alcohol for the wassample 40–41 prepared ppm. Therefore, at pH 11 and the 35.56new peak ppm at for 35.58 the ppm sample for the prepared sample prepared at pH 13 at could pH 11 beand assigned 35.56 ppm as for the the Furan sample–CH prepared2–NH(CO) at pH–R structure13 could befrom assigned the reaction as the between Furan–C Hthe2–NH(CO)–R primary amido structure group from of AG the and reaction furfuryl between alcohol. the The primary peak atamido 57.20 group ppm could of AG be and assigned furfuryl as alcohol. –CH2 bond The peakof furfuryl at 57.20 alcohol, ppm could which be meant assigned the asexistance –CH2 bond of the of freefurfuryl furfuryl alcohol, alcohol which in meant the system. the existance It is normal of the free and furfuryl reasonable alcohol for in the the existence system. It of is the normal reaction and monomer.reasonable for the existence of the reaction monomer. There was was a a peak peak at at 182.76 182.76 ppm ppm (pH (pH = = 11 11 and and 13). 13). Seen Seen from from the thestructure structure of AG, of AG, the selfthe- self-condensationcondensation of intramolecular of intramolecular cyclisation cyclisation of ofAG AG between between carboxylic carboxylic acid acid and and primary primary amide amide could be be occurred. occurred. However, However, the peak the peakat 182.76 at ppm 182.76 should ppm not should be from not carbonyl be from of the carbonyl self-condensate of the 13 sofelf AG.-condensate When compared of AG. When with compared the Figure with2 of the the FigureC NMR 2 of spectra the 13C ofNMR AG, spectra 182.76 ppmof AG, appeared 182.76 ppm at a appearedrelatively lowerat a relatively ﬁeld. In lower theory, field. if the In self-condensation theory, if the self of-condensation intramolecular of cyclisationintramolecular of AG cyclisation occurred, ofthe AG carbonyl occurred, of the the self-condensate carbonyl of the would self-condensate appeared at would a relatively appeared higher at a ﬁeld relatively because higher of the field p–π becauseconjugated of the effect p– inπ theconjugated new formed effect –CO–NH– in the new groups. formed Similary, –CO–NH the– peakgroups. at 182.76 Similary, ppm the should peak not at 182.76come from ppm theshould C1 reacted not come with from methylol the C1 groupsreacted of with FA methylol for the p– groupsπ conjugated of FA for effect. the p–π conjugated effect.There was a great possibility for the peak at 182.76 ppm from the shift of C1 of AG that was causedThere by reactionwas a great between possibility C8 of AG for and the FA. peak Their at 182.76 reaction ppm mechanism from the was shift similar of C1 with of AG that that between was causedurea and by formaldehyde reaction between under C8 alkaline of AG conditions. and FA. Their reaction mechanism was similar with that between urea and formaldehyde under alkaline conditions.

Polymers 2017, 9, 711 11 of 16 Polymers 2017, 9, 711 11 of 16

However, for the 13C NMR, the chemical shift of carbonyl groups would be much easier to be However, for the 13C NMR, the chemical shift of carbonyl groups would be much easier to be affectedHowever, by its environments, for the 13C NMR, such the as chemicalthe resolvent, shift ofpH, carbonyl and others, groups than would that of be methylene much easier groups. to be affected by its environments, such as the resolvent, pH, and others, than that of methylene groups. Theaffected judgements by its environments, on the reaction such based as the on resolvent, the analysis pH, and of the others, peaks than of thatmethylene of methylene groups groups. would The be The judgements on the reaction based on the analysis of the peaks of methylene groups would be mojudgementsre reliable on than the that reaction of carbonyls based on groups. the analysis of the peaks of methylene groups would be more more reliable than that of carbonyls groups. reliableThere than was that no of peak carbonyls from methylene groups. and ether link caused by the self-condensation of furfuryl There was no peak from methylene and ether link caused by the self-condensation of furfuryl alcoholThere molecules was no in peak Figure from 4. methylene and ether link caused by the self-condensation of furfuryl alcohol molecules in Figure 4. alcoholThe molecules analysis inon Figure Figure4. 4 indicated that the reaction between furfuryl alcohol and AG was The analysis on Figure 4 indicated that the reaction between furfuryl alcohol and AG was difficultThe to analysis happen on under Figure alkaline4 indicated conditions that the and reaction only betweenwhen the furfuryl alkaline alcohol condition and AGwas wasas strong difﬁcult as difficult to happen under alkaline conditions and only when the alkaline condition was as strong as pHto happen = 11 or underhigher, alkaline were a conditionsfew co-condensation and only whenproducts the alkalinethat were condition detected wasin a asmixing strong system as pH with = 11 pH = 11 or higher, were a few co-condensation products that were detected in a mixing system with furfurylor higher, alcohol were a fewand co-condensation AG. The stability products of furfuryl thatwere alcohol detected under in alkalinea mixing conditions system with might furfuryl be furfuryl alcohol and AG. The stability of furfuryl alcohol under alkaline conditions might be responsiblealcohol and for AG. that The [17,24]. stability Under of furfuryl relatively alcohol weak under alkaline alkaline conditions, conditions it seemed might beto responsiblebe difficult for responsible for that [17,24]. Under relatively weak alkaline conditions, it seemed to be difficult for AGthat molecule [17,24]. Under to form relatively a relatively weak stable alkaline primary conditions, amide itanions seemed reaction to be difﬁcultintermediate for AG or molecule even there to AG molecule to form a relatively stable primary amide anions reaction intermediate or even there wereform a some relatively amide stable anio primaryns in the amide system, anions their reaction concentration intermediate was not or even as high there as were to promote some amide the were some amide anions in the system, their concentration was not as high as to promote the condensationanions in the system,reaction their with concentration furfuryl alcohol. was notUnder as high strong as toalkaline promote conditions, the condensation to form reactionmore amide with condensation reaction with furfuryl alcohol. Under strong alkaline conditions, to form more amide anionsfurfuryl would alcohol. get Under easier, strong which alkaline would conditions,be helpful for to formtheir morecombination amide anions with furfuryl would getalcohol. easier, For which the anions would get easier, which would be helpful for their combination with furfuryl alcohol. For the wouldaliphatic be amino helpful groups for their of combinationAG, it was hard with to furfuryl form reaction alcohol. intermediate. For the aliphatic For the amino secondary groups amido of AG, aliphatic amino groups of AG, it was hard to form reaction intermediate. For the secondary amido groups,it was hard the tosteric form hindrance reaction intermediate. might be the For main the reason secondary for amidothe stop groups, of condensation the steric hindrance between mightthem groups, the steric hindrance might be the main reason for the stop of condensation between them andbe the furfuryl main reasonalcohol for molecules. the stop of condensation between them and furfuryl alcohol molecules. and furfuryl alcohol molecules. Based on the analysisanalysis above, the reaction mechanism between furfuryl alcohol and AG was Based on the analysis above, the reaction mechanism between furfuryl alcohol and AG was presumed, as below. presumed, as below.

In all, under alkaline conditions, the condensation reaction between FA and AG was slow. In all, under alkaline conditions, the condensation reaction between FAFA andand AGAG waswas slow.slow. The 13C NMR results of the samples prepared with furfuryl alcohol and AG under pH 1, 3, and 5 The 13CC NMR NMR results results of of the the samples samples prepared prepared with with furfuryl furfuryl alcohol and AG under pH 1, 3, and 5 were given as Figure 5. were given as Figure5 5..

13 Figure 5. 13C NMR spectra of the samples with furfuryl alcohol and N-(2)-L-alanyl-L-glutamine Figure 5. 13C NMR spectra of the samples with furfuryl alcohol and N-(2)-L-alanyl-L-glutamine preparedFigure 5. underC NMR acid conditions spectra of the samples with furfuryl alcohol and N-(2)-L-alanyl-L-glutamine prepared under acid conditions prepared under acid conditions In the mixing system with furfuryl alcohol and AG, the reaction intermediate might come from In the mixing system with furfuryl alcohol and AG, the reaction intermediate might come from the hydroxymethyl furfural carbonium ions. They could react with its carbon 5, its hydroxymethyl the hydroxymethyl furfural carbonium ions. They could react with its carbon 5, its hydroxymethyl groups or the amino groups of AG molecules. As seen from Figure 5, in the samples that were groups or the amino groups of AG molecules. As seen from Figure 5, in the samples that were

Polymers 2017, 9, 711 12 of 16

In the mixing system with furfuryl alcohol and AG, the reaction intermediate might come from the hydroxymethyl furfural carbonium ions. They could react with its carbon 5, its hydroxymethyl Polymersgroups 2017 or the, 9, amino711 groups of AG molecules. As seen from Figure5, in the samples that were prepared12 of 16 with furfuryl alcohol and AG under pH 3, the peaks at 35.60 and 43.15 ppm were obviously observed. prepared with furfuryl alcohol and AG under pH 3, the peaks at 35.60 and 43.15 ppm were According to the analysis on the assignments of the chemical shift above, they could be assigned obviously observed. According to the analysis on the assignments of the chemical shift above, they as Furan–CH2–NH(CO)–R and R–NH–CH2–Furan, respectively. Being similar with the results of could be assigned as Furan–CH2–NH(CO)–R and R–NH–CH2–Furan, respectively. Being similar the samples that were prepared under alkaline conditions, as seen from the bigger peak area of the with the results of the samples that were prepared under alkaline conditions, as seen from the bigger former structure than that of the latter, the reaction between the primary amido groups of AG and peak area of the former structure than that of the latter, the reaction between the primary amido furfuryl alcohol would be easier to proceed than that between the aliphatic amino groups of AG and groups of AG and furfuryl alcohol would be easier to proceed than that between the aliphatic amino furfuryl alcohol. groups of AG and furfuryl alcohol. In theory, the amido groups in AG moleculs show weaker nucleophilicity than aliphatic amide In theory, the amido groups in AG moleculs show weaker nucleophilicity than aliphatic amide groups because of the p–π conjugated effect of the amido groups. However, under acid conditions, groups because of the p–π conjugated effect of the amido groups. However, under acid conditions, aliphatic amide has the priority to attract hydrogen ions, and then be inactivated to other groups [23]. aliphatic amide has the priority to attract hydrogen ions, and then be inactivated to other groups Therefore, under acid conditions, when compared with the aliphatic amide groups, the non-protonized [23]. Therefore, under acid conditions, when compared with the aliphatic amide groups, the amido groups would show bigger possibility to react with the furfural carbonium ions to form the non-protonized amido groups would show bigger possibility to react with the furfural carbonium Furan–CH2–NH(CO)–R structure. Being as the same as the results of the samples tha were prepared ions to form the Furan–CH2–NH(CO)–R structure. Being as the same as the results of the samples tha under alkaline conditions, no peak at around 41 ppm from the structure of Furan–CH2–NR’(CO)–R were prepared under alkaline conditions, no peak at around 41 ppm from the structure of was detected. Furan–CH2–NR’(CO)–R was detected. The possible reactions with furfuryl alcohol and AG under acid conditions were as below. The possible reactions with furfuryl alcohol and AG under acid conditions were as below.

Seen from Figure 5, there were both self- and co-condensation products in the mixing system with Seenfurfuryl from alcohol Figure and5, there AG under were bothacid self-conditions, and co-condensation and the sample products prepared in under the mixing pH 3 showed system withthe biggest furfuryl amount alcohol and of the AG underco-condensation acid conditions, reaction and theproducts. sample Obviously, prepared under the pH co-condensation 3 showed the reactionbiggest amount between of furfuryl the co-condensation alcohol and AG reaction had products.to compete Obviously, with the selfthe- co-condensationcondensation of reactionfurfuryl alcohol.between It furfuryl was reported alcohol andthat AGviolent had self to compete-condensation with the reaction self-condensation between furfuryl of furfuryl alcohol alcohol. molecules It was wouldreported happen that violent under self-condensationstrong acid conditions, reaction especially between when furfuryl the value alcohol of pH molecules was lower would than happen 2, and largeunder quantities strong acid of conditions, heat would especially be given when off [25,26]. the value The of strong pH was acid lower conditions than 2, andwould large catalyze quantities the reactionof heat would between be giventhe hydroxymethyl off [25,26]. The furfur strongal acidcarbonium conditions ions wouldand the catalyze furfuryl the alcohol’s reaction carbon between 5. Thethe hydroxymethyl release of the large furfural quantities carbonium of heat ions meant and the that furfuryl the self alcohol’s-condensation carbon products 5. The release of furfuryl of the alcohollarge quantities molecules of heatowned meant relatively that the low self-condensation energy in the opinion products of ofthermodynamics. furfuryl alcohol molecules owned relativelyBoth lowthe relative energy inhighe ther opinion energy of the thermodynamics. co-condensation products of furfuryl alcohol and AG, and the possibleBoth the protonation relative higher would energy lead ofto thethe co-condensationprevailing of the products self-condensation of furfuryl reaction alcohol in and the AG, mixing and systemthe possible with protonationfurfuryl alcohol would and lead AG tounder the prevailing acid conditions. of the self-condensationThe more acid the reactionsystem was, in the the mixing more violentsystem withthe self furfuryl-condensation alcohol and would AG underbe. However, acid conditions. at the same The time, more with acid themore system acid was,in the the mixing more violentsystem, thethere self-condensation would be more hydroxymethyl would be. However, furfural at carbonium the same time, ions, with which more would acid be in favorable the mixing to thesystem, co-condensation there would reaction be more between hydroxymethyl furfuryl furfural alcohol carboniumand AG. Therefore, ions, which there would was an be favorableequilibrium to pH for the competition between the self-condensation and co-condensation in the mixing system. In this study, for more co-condensation products, it was better for furfuryl alcohol and AG to be mixed under pH 3.

Polymers 2017, 9, 711 13 of 16 the co-condensation reaction between furfuryl alcohol and AG. Therefore, there was an equilibrium pH for the competition between the self-condensation and co-condensation in the mixing system. In this study, for more co-condensation products, it was better for furfuryl alcohol and AG to be mixed under pH 3. Polymers 2017, 9, 711 13 of 16 3.3. FT-IRPolymers Analysis 2017, 9, 711 13 of 16

Figure3.3. FT-6IR gives Analysis the FT-IR results of AG. In Figure6, the absorption at 3334.3 and 3411.5 cm −1 3.3. FT-IR Analysis could be assignedFigure 6 gives as the the symmetric FT-IR results and of asymmetricAG. In Figure vibration 6, the absorption of N–H at bond 3334.3 of and –NH 3411.52 groups. cm−1 The −1 −1 absorptioncouldFigure be at 3227.6assigned 6 gives cm as the the cameFT symmetric-IR fromresults the and of vibrationAG. asymmetric In Figure of N–Hvibration 6, the bond absorption of fromN–H –NH–bond at 3334.3 of groups. –NH and2 groups.3411.5 The absorption cmThe −1 −1 at 1652.7couldabsorption cm be assigned wasat 3227.6 from as cm the the−1 camesymmetric C=O from from theand amido vib asymmetricration groups. of N vibration– TheH bond absorption offrom N– H–NH bond at– 1606.4groups. of –NH cm The2 groups. absorptionwas fromThe the −1 − bendingabsorptionat 1652.7 vibration cm at−1 3227.6 ofwas N–H from cm bond thecame C=O from from from aliphatic the amido vibration amino groups. of groups.N –TheH bond absorption The from absorption – NHat 1606.4– groups. at cm 2939.0 −1The was andabsorption from 1321 the cm 1 −1 −1 were,atbending respectively, 1652.7 vibrationcm fromwas of from O–HN–H the bondbond C=O from and from C–Oaliphatic amido bond aminogroups. from groups. carboxyl The absorption The groups absorption at [27 1606.4,28 at]. 2939.0 Figurecm andwas7 was 1321from the cm the−1 FT-IR −1 resultsbendingwere, of furfuryl respec vibrationtively, alcohol. of from N–H TheO bond–H absorption bond from and aliphatic C at–O 3200 aminobond to from groups. 3300 carboxyl cm The−1 wasabsorption groups from [27,28]. theat 2939.0 vibration Figure and 71321 ofwas hydroxylcm the were,FT-IR r resultsespectively, of furfuryl from O alcohol.–H bond The and absorption C–O bond at from 3200 carboxyl to 3300−1 groups cm−1 was [27,28]. from Figure the vibration 7 was the of from –CH2OH groups. The absorption at 2873.8 and 2929.5 cm could−1 be assigned as the symmetric FT-IR results of furfuryl alcohol. The absorption at 3200 to 3300 cm was−1 from the vibration of and asymmetrichydroxyl from vibration –CH2OH of groups. methylene The absorption bond from at –CH2873.8OH and groups. 2929.5 cm The could absorption be assigned at 1005.9 as the cm −1 2 2 −1 hydroxylsymmetric from and –asymmetricCH OH groups. vibration The absorptionof methylene at 2873.8bond from and 2929.5–CH2OH cm groups. could bThee assigned absorption as the at was from C–O of methylol groups. The absorption at 815.5 cm−1 was from the vibration of C-H of symmetric1005.9 cm−1 and was asymmetric from C–O of vibration methylol of groups methylene. The bondabsorption from at–CH 815.52OH cm groups.−1 was fromThe absorptionthe vibration at furan1005.9of ring. C-H cm of −1furan was ringfrom. C–O of methylol groups. The absorption at 815.5 cm−1 was from the vibration of C-H of furan ring.

Figure 6. FT-IR spectrum of N-(2)-L-alanyl-L-glutamine. Figure 6. FT-IR spectrum of N-(2)-L-alanyl-L-glutamine. Figure 6. FT-IR spectrum of N-(2)-L-alanyl-L-glutamine.

Figure 7. FT-IR spectrum of furfuryl alcohol (FA). FigureFigure 7. 7.FT-IR FT-IR spectrum spectrum of furfuryl alcohol alcohol (FA). (FA). Figure 8 was the FT-IR results of the sample synthesized with furfuryl alcohol and AG under acid Figureconditions 8 was, specifically the FT-IR atresults pH = of 3 .the When sample compared synthesized with Figure with furfuryl 6, the absorpt alcoholion and at 1652.7AG under and acid1606.4 conditions cm−1 from, specifically amido and ataliphatic pH = 3 .amino When groups compared shifted, with respectively, Figure 6, the to absorpt 1722.0ion and at 1678.2 1652.7 cm and−1, 1606.4which cmindicated−1 from thatamido the and combination aliphatic amino of the groupsamido andshifted, aliphatic respectively, amino groupsto 1722.0 with and some 1678.2 groups cm−1, which indicated that the combination of the amido and aliphatic amino groups with some groups

Polymers 2017, 9, 711 14 of 16

Figure8 was the FT-IR results of the sample synthesized with furfuryl alcohol and AG under acid conditions, speciﬁcally at pH = 3. When compared with Figure6, the absorption at 1652.7 and 1606.4 cm−1 from amido and aliphatic amino groups shifted, respectively, to 1722.0 and 1678.2 cm−1, whichPolymers indicated 2017, 9 that, 711 the combination of the amido and aliphatic amino groups with some groups14 of 16 with stronger inductivity than that of hydrogen atom. The absorption of 1420.4 cm−1 in Figure6 from the vibrationwith stronger of C–N inductivity groups shifted than that to 1404.3 of hydrogen cm−1 atom.in Figure The absorption8, which indicatedof 1420.4 cm that−1 in theFigure possible 6 from new −1 groupthe combined vibration withof C–N the groups amido shifted groups to 1404.3 would cm weaken in Figure the p–8, whichπ conjugated indicated effect that the between possible amino new and group combined with the amido groups would weaken the p–π conjugated effect between amino carbonyl bond. New absorptions appeared at 1205 and 1174.2 cm−1, which could be assigned as the and carbonyl bond. New absorptions appeared at 1205 and 1174.2 cm−1, which could be assigned as symmetric and asymmetric vibration of methylene bond from –NH–CH2–C=C– groups. In all, the the symmetric and asymmetric vibration of methylene bond from –NH–CH2–C=C– groups. In all, FT-IRthe results FT-IR were results another were another proof ofproof the of reaction the reaction between between furfuryl furfuryl alcohol alcohol and and AG. AG.

Figure 8. FT-IR spectrum of FA/AG prepared at pH = 3. Figure 8. FT-IR spectrum of FA/AG prepared at pH = 3. 4. Conclusions 4. Conclusions In this paper, to guide the preparation of soy protein-based adhesive, the reaction between Inprotein this paper,and its to cross guide-linker the furfuryl preparation alcohol of soywere protein-based studied. To simplify adhesive, the study, the reaction a simple between dipeptide protein and itsAG cross-linker was used as furfuryla model compound alcohol were of protein. studied. To simplify the study, a simple dipeptide AG was used as aBased model on compound the analysis of on protein. the results of ESI-MS, 13C NMR, and FT-IR of the samples that were Basedprepared on withthe analysis furfuryl on alcohol the results and AG of underESI-MS, different13C NMR, pHs, and it was FT-IR found of the that samples the medium that were preparedenvironment with furfuryl had great alcohol effects and on AG the under competition different of the pHs, co it-condensation was found that reaction the medium between environmentfurfuryl alcohol and AG and self-condensation reaction of furfuryl alcohol moleculs in the mixing system had great effects on the competition of the co-condensation reaction between furfuryl alcohol and AG with furfuryl alcohol and AG. Under alkaline conditions, both co- and self-condensation were not and self-condensation reaction of furfuryl alcohol moleculs in the mixing system with furfuryl alcohol obviously detected. Only when the value of pH was higher than 11, were a few co-condensation and AG.reaction Under products alkaline gotten. conditions, The reaction both co-occurred and self-condensation mainly between furfuryl were not alcohol obviously and the detected. primary Only whenamido the value groups of pHof AG. was Under higher acid than conditions, 11, were both a few co- co-condensation and self-condensat reactionion were products observed. gotten. When The reactionfurfuryl occurred alcohol mainly and AG between were mixed furfuryl under alcohol pH 1, the and self the-condensation primary amido of furfuryl groups alcohol of AG. molecules Under acid conditions,prevailed both over co- the and co self-condensation-condensation between were furfuryl observed. alcohol When and furfuryl AG. When alcohol furfuryl and alcohol AG were and mixed underAG pH were 1, the mixed self-condensation under pH 5, both of furfurylco- and self alcohol-condensation molecules were prevailed not outstanding. over the In co-condensation this study, betweenunder furfuryl pH 3, the alcohol co- and and self AG.-condensation When furfuryl found alcohol equilibrium. and AG There were was mixed a great under possibility pH 5, both for the co- and self-condensationprimary amido were and not aliphatic outstanding. amino In groups this study, of AG under molecules pH 3, the to co- react and with self-condensation furfuryl alcohol found molecules. No reaction was detected between the secondary amido groups of AG and furfuryl equilibrium. There was a great possibility for the primary amido and aliphatic amino groups of AG alcohol. molecules to react with furfuryl alcohol molecules. No reaction was detected between the secondary amidoAcknowledgments: groups of AG and This furfuryl work was alcohol. supported by National Natural Science Foundation of China (No. 31660176), Forestry Department Foundation of Guizhou Province of China (No. [2017]14), Education Acknowledgments:Department FoundationThis work of Guizhou was supported Province byof China National (No Natural. [2017]114) Science and Introduction Foundation ofof Guizhou China (No.University 31660176), ForestryScientific Department Research Foundation Grants Project of Guizhou of China Province(No. [2016]20). of China (No. [2017]14), Education Department Foundation of GuizhouAuthor Province Contributions: of China Jiankun (No. [2017]114) Liang and andZhigang Introduction Wu contributed of Guizhou the pre Universityparation of Scientiﬁc the testing Research and the Grants Projectanalysis of China of the (No. ESI [2016]20).-MS and 13C-NMR. Xuedong Xi and Taohong Li contributed the test and the analysis of the FT-IR. Hong Lei contributed the design of the experiment and the analysis of the results. Guanben Du contributed the analysis of the results and the edit of the paper.

Polymers 2017, 9, 711 15 of 16

Author Contributions: Jiankun Liang and Zhigang Wu contributed the preparation of the testing and the analysis of the ESI-MS and 13C-NMR. Xuedong Xi and Taohong Li contributed the test and the analysis of the FT-IR. Hong Lei contributed the design of the experiment and the analysis of the results. Guanben Du contributed the analysis of the results and the edit of the paper. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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