polymers Article Reactions of Soy Flour and Soy Protein by Non-Volatile Aldehydes Generation by Specific Oxidation Charles R. Frihart 1, Antonio Pizzi 2,3,* , Xuedong Xi 2 and Linda F. Lorenz 1 1 Forest Products Laboratory, USDA, 1 Gifford Pinchot Ave., Madison, WI 53726, USA 2 LERMAB-ENSTIB, University of Lorraine, 27 rue Philippe Seguin, 88000 Epinal, France 3 Department of Physics, King Abdulaziz University, Al Ehtifalat St, Jeddah 21589, Saudi Arabia * Correspondence: [email protected]; Tel.: +33-623126940 Received: 30 August 2019; Accepted: 7 September 2019; Published: 10 September 2019 Abstract: Soy protein isolate (SPI) and insoluble soy flour polymeric carbohydrates have been reacted with sodium periodate for the specific oxidation of vicinal –OH groups to investigate the reactions involved in this approach to soy flour adhesives. The reactions have been shown to generate carbohydrate oligomer fractions presenting one, two or multiple aldehyde groups. With the exception of the small molecular weight heptanedial, the smaller molecular weight aldehydes generated from mono- and disaccharides by the same reaction do not appear to form from the insoluble soy flour carbohydrates, or have already reacted. The reaction of periodate with soy protein isolate has been shown to generate some aldehydes too. When the mix of SPI and soy insoluble carbohydrates is treated with periodate, the majority of the observed aldehyde carrying species appear to be higher molecular weight carbohydrate oligomer fractions. Keywords: soyflour; soy protein isolate; insoluble carbohydrates; periodateoxidation; aldehydesgeneration; condensation reactions; soy adhesives 1. Introduction Recently, to meet new environmental standards, a number of different approaches to the use of natural materials have been used to prepare wood adhesives without the use of formaldehyde. The use of carbohydrate oxidants is a long well-known practice for a number of different applications. Thus, a number of different oxidation systems have been used to generate aldehydes from polymeric carbohydrate, the literature on this being rather abundant. These include sulfoxide-carbodiimide and related methods [1], nitroxyl radical mediated aqueous oxidation [2], special salts oxidation [3] and more recently nitro-oxidation methods [4,5], this latter being well suited for nanocellulose generation. Among these, the periodate specific oxidation of oligomeric carbohydrates is a well-known system. Thus, such a system is reported from the older periodate oxidation literature on aldehyde generation [6–8] to the more recent literature on nanocellulose generation by such a system followed by ozonization [9]. Among the more recent applications, the use of specific oxidants of monomeric or oligomeric carbohydrates in soy flour have shown particularly interesting results, allowing not only the protein fraction but also the carbohydrate fraction to participate positively in the preparation of wood panel adhesives [10–12]. In this approach, soy flour adhesives have been successfully prepared by treating soy flour with either potassium permanganate or sodium periodate [12]. Sodium periodate is a specific oxidant for carbohydrates reacting with adjacent vicinal hydroxyl groups to form dialdehydes [6,12] according to the reaction: Polymers 2019, 11, 1478; doi:10.3390/polym11091478 www.mdpi.com/journal/polymers Polymers 2019, 11, 1478 2 of 18 Polymers 20192019,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 22 ofof 1919 Polymers 2019, 11, x FOR PEER REVIEW 2 of 19 O O _ O O M _ H22O OOM O O M H2O OOM I II I II OO OO MM HO OH HO OH O O HO II OHOH HOHO OHOH MIO OO O O MIOMIO44 O OO H H H H H H H HH H H RR1 RR2 RR11 R R22 M = H, Na 1 1 22 R 1 RR 2 1 2 M = H, Na 1 22 WhileWhile this thisthis reaction reaction reaction andand and outcome outcome areare better arebetter better known known known for forfor carbohydrate carbohydratecarbohydrate for carbohydrate monomers monomersmonomers monomers [6,13] [6,13][6,13] and andand [dimers6,13 dimersdimers] and [13]dimers[13][13] suchsuch such [13 as]asas such glucoseglucoseglucose as glucose andandand sucrose,sucrose, and sucrose, equivalentequivalentequivalent equivalent re rereactionsactions reactions are are known are known known also also alsofor for forhigher higher higher carbohydrate carbohydrate carbohydrate oligomersoligomers up up to to cellulosecellulosecellulose itself [12,14,15]. [[12,14,15].12,14,15]. In In the the case case of of cellulose cellulosecellulose and andand long longlong carbohydrate carbohydratecarbohydrate oligomers oligomersoligomers alone,alone, the the reactions reactions that that havehave been shown shown to to occur occur are are the the condensation condensation condensation of of the the aldehydes aldehydes aldehydes formed formed with with otherother carbohydrate carbohydrate chains chainschains to to yield yield crosslinking,crosslinking, crosslinking, leading leading leading to to tosolid solid solid panels panels panels in inthe in the the case case case of celluloseof of cellulose cellulose [15]. [15]. [ 15]. ByBy increasing increasing the the levellevel ofof oxidationoxidation withwith further further periodate periodate in in the the presence presencepresence of ofofan ananaldehyde-reactive aldehyde-reactivealdehyde-reactive speciesspeciesspecies such such such as asas a aa soy soysoy protein proteinprotein [12], [[12],12], aa a flavonoidflavonoid flavonoidflavonoid tannin tannin tannin [13] [13] [[13]13 or] or or other other other reactive reactive reactive species, species, species, including including lignin, lignin,lignin, cross-linking can also occur, leading to feasible wood adhesives [12,13].27 cross-linkingcross-linking can can alsoalso occur,occur, leadingleadingleading tototo feasiblefeasiblefeasible woodwoodwood adhesivesadhesivesadhesives [ [12,13].27[12,13].2712,13]. Further scission FurtherFurtherFurther aldehydes scissionscission FurtherFurther aldehydesaldehydes NaIO4 OH NaIO4 OH OH 4 OHO OHOHO O O O OH OH O O O O OO O O O OH HO OH HO OH OH O OO O O O NaIO O HOHO OHOH HO O Carbohydrates O 4 O O O O O O crosslinkingCarbohydrates O O NaIO44 O O Carbohydrates O O crosslinkingcrosslinking HO OH O O O O O O HO OH O O O O O with or without HO OH O with orNaIO without4 with or without Aldehydes reaction HO OH NaIO NaIO44 with other species Crosslinking Aldehydes reaction with other species Crosslinking The oxidation with the periodate ion, resulting in a 1,2–glycol scission, is one of the most widely usedThe reactions oxidation in carbohydrate with the periodate chemistry. ion, Theresulting mild reaction in a 1,2–glycol and the scission, aqueous issolvent one of conditions the most most widely widelyfor usedperiodate reactions oxidation in carbohydrate are particularly chemistry. apt for The use wimild th water-soluble reaction reaction and and carbohydrates.the the aqueous aqueous solvent solvent The development conditions conditions for for periodateand wide oxidation application are of particularlyparticularlythe reaction aptareapt due forfor useuseto its withwi highthth water-solublewater-soluble degree of selectivity carbohydrates.carbohydrates. [6,14,15]. The The development development and wide Aldehydes application are well of theknown reaction for reacting areare duedue with toto its itsthe highhigh amino degreedegree group ofof on selectivityselectivity proteins [10–12]. [[6,14,15].6,14,15 The]. aldehydes generated from the carbohydrate fraction of soy flour appear to react with the active sites of the soy Aldehydes are well known for reacting withwith the amino group on proteins [10–12]. [10–12]. The The aldehydes aldehydes protein, leading to cross-linking and yielding adhesives bonding plywood panels that are very generated from from the the carbohydrate carbohydrate fraction fraction of of soy soy flour flour appear appear to toreact react with with the theactive active sites sites of the of soy the encouraging for shear bond strengths, especially under wet testing conditions [12]. Equally, some of protein,soy protein, leading leading to cross-linking to cross-linking and and yielding yielding adhesives adhesives bonding bonding plywood plywood panels panels that that are are very the aldehydes generated by the periodate treatment of glucose and sucrose also react with a encouraging forfor shearshear bond bond strengths, strengths, especially especially under under wet wet testing testing conditions conditions [12 [12].]. Equally, Equally, some some of the of polyflavonoid tannin, yielding equally encouraging plywood bond strengths [13]. thethe aldehydesaldehydes generatedgenerated byby thethe periodateperiodate treatmenttreatment ofof glucoseglucose andand sucrosesucrose alsoalso reactreact withwith aa aldehydesIn the generated case of Na by theperiodate periodate acting treatment on glucos of glucosee at 120 and °C sucrosefor 1 hour, also reacta number with aof polyflavonoid different polyflavonoid tannin, yielding equally encouragingcouraging plywoodplywood bondbond strengthsstrengths [13].[13]. tannin,aldehydes yielding have equallybeen shown encouraging to be generated, plywood for bond example:28 strengths [13]. InIn thethethe casecasecase of ofof Na NaNa periodate periodateperiodate acting actingacting on onon glucose glucosglucos ate 120 at 120◦C for °C 1 for h, a1 number hour, a of number different of aldehydes different aldehydes have been shown to be generated, for example:28 aldehydeshave been shownhave been to be shown generated, to be generated, forCHO example: for example:28OH OH CHO