Stabilization of Zwitterionic Versus Canonical Proline by Water Molecules Gang Yang1*, Lijun Zhou1 and Yang Chen2

Yang et al. SpringerPlus (2016) 5:19 DOI 10.1186/s40064-015-1661-8

RESEARCH Open Access Stabilization of zwitterionic versus canonical proline by water molecules Gang Yang1*, Lijun Zhou1 and Yang Chen2

Abstract At physiological conditions, a majority of biomolecules (e.g., amino acids, peptides and proteins) exist predominantly in the zwitterionic form that usually decides the biological functions. However, zwitterionic amino acids are not geometrically stable in gas phase and this seriously hampers the understanding of their structures, properties and biological functions. To this end, one of the recent research focuses is to demonstrate the stabilization effects of zwitterionic amino acids. Relative stabilities of canonical conformers are dependent on water contents, while zwitterionic stability improves monotonously and pronouncedly with increase of water contents. We find that one water molecule can render zwitterionic proline geometrically stable, and stabilities of different zwitterionic amino acids increase as glycine

Background On the other hand, amino acids in gas phase con- In aqueous solutions, a wide range of biomolecules sist entirely of canonical conformers (Császár 1992; Hu (e.g., amino acids, peptides and proteins) exist predomi- et al. 1995; Yu et al. 1995), which is totally different from nantly in the zwitterionic form, and water molecules the condition in aqueous solutions. This is obviously an are essential to maintain their native conformations and obstacle for us to comprehend the electronic proper- physiological functions (Timasheff1970 ; Rand 2004). ties and biological functions of zwitterionic structures. Interaction of biomolecules and water has attracted gen- Zwitterions can generate strong electric fields around eral interest (Teeter 1991; Levy and Onuchic 2004; Cor- that usually decide the biological functions of biomol- radini et al. 2013), and amino acids, the structural unit ecules. Owing to the particular importance, one of the of proteins, are generally the protypes for ab initio and recent research focuses is to demonstrate the stabiliza- density functional investigation of peptides and proteins tion effects of zwitterionic amino acids (Yu et al. 1995; (Császár 1992; Hu et al. 1995; Yu et al. 1995; Zhang and Zhang and Chung-Phillips 1998; Remko and Rode 2001; Chung-Phillips 1998; Remko and Rode 2001; Hoyau and Hoyau and Ohanessian 1998; Ai et al. 2003; Constan- Ohanessian 1998; Ai et al. 2003; Constantino et al. 2005; tino et al. 2005; Remko and Rode 2006; Corral et al. Remko and Rode 2006). 2006; Yang et al. 2009; Jensen and Gordon 1995; Snoek et al. 2002; Yamabe et al. 2003; Balta and Aviyente 2004; Im et al. 2008; Gutowski et al. 2000; Rimola et al. 2008; *Correspondence: [email protected] Wu and McMahon 2007; Rimola et al. 2013; Kass 2005; 1 College of Resource and Environment and Chongqing Key Yang et al. 2008; Tian et al. 2009; Hwang et al. 2011; Li Laboratory of Soil Multi‑scale Interfacial Process, Southwest University, et al. 2011; Kim et al. 2014; Yang and Zhou 2014), and 400715 Chongqing, People’s Republic of China Full list of author information is available at the end of the article a variety of attempts have been made in this aspect,

© 2016 Yang et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Yang et al. SpringerPlus (2016) 5:19 Page 2 of 7

such as protonation and deprotonation (Yu et al. 1995; level, which has been sufficiently validated before (Li Zhang and Chung-Phillips 1998), metalation (Hoyau and et al. 2011; Kim et al. 2014; Yang et al. 2009; Rankin et al. Ohanessian 1998; Ai et al. 2003; Constantino et al. 2005; 2002; Yang et al. 2010; Ajitha and Suresh 2011) and in this Remko and Rode 2006; Corral et al. 2006; Yang et al. work. 2009), hydration (Jensen and Gordon 1995; Snoek et al. 2002; Yamabe et al. 2003; Balta and Aviyente 2004; Im Results and discussion et al. 2008; Hwang et al. 2011; Li et al. 2011; Kim et al. As shown in Scheme 1, three proline conformers have 2014) and anion attachment (Kass 2005; Yang et al. 2008; been considered in this work: two canonical (PA and PB) Tian et al. 2009). A minimum of two water molecules while the third zwitterionic (PC), which are in line with is required to stabilize glycine in the zwitterionic form previous studies (Rankin et al. 2002; Yang et al. 2010; (Jensen and Gordon 1995), while one is enough to cause Ajitha and Suresh 2011; Yang et al. 2015). PA and PB are zwitterionic arginine as the lowest-energy conformer (Im canonical conformers that predominant in gas phase et al. 2008). Interaction of proline and water has been (Eszter et al. 2003; Alln et al. 2004; Kapitán et al. 2006) investigated by different groups (Li et al.2011 ; Kim et al. and become the choice for catalytic studies (Rankin 2014), and two water molecules were considered neces- et al. 2002; Yang et al. 2010; Ajitha and Suresh 2011). PA sary for rendering zwitterionic proline to be geometri- is slightly less stable than PB with the relative energy of cally stable. 1.6 kcal/mol (Table 1 and Additional file 1: Table S1). In this work, density functional calculations were PC (zwitterionic) does not represent a local minimum employed to comprehend the gas-phase interaction of dif- on the potential energy surface (PES) (Eszter et al. 2003; ferent proline conformers and water with a wide range Alln et al. 2004), and so its relative stability is evaluated of contents (n = 0–5). Five water molecules were found by fixing the N-H1 distance at 1.030 Å, with production enough to fill up the first shell of proline functional sites of a higher relative energy than PB (13.1 kcal/mol). Rela- (carboxylic and amido), as in the case of glycine (Kokpol tive energies of these proline conformers are also calcu- et al. 1988). We determined that one water molecule can lated at other theoretical levels, which are in line with stabilize proline in zwitterionic form; meanwhile, stabilities the default B3LYP/bs2//B3LYP/bs1 method (Additional of zwitterionic glycine, proline and arginine were compared file 1: Table S1). with each other. Then, the numbers of water molecules For each proline confirmer, interactions with water necessary to render zwitterionic proline energetically pref- can result in the various structures that are discerned erential and conformationally predominant were respec- by suffixingnW N, wherein n (n = 1, 2, …) repre- tively determined, and the relationship of zwitterionic sents the number of water molecules and N (=I, II, …) stability vs. water content was demonstrated. Finally, transi- ranks the stability of different structures. For instance, tion states for the transformation from canonical to zwitte- PA2WIII/PC5WI stands for the third/first stable con- rionic proline at all water contents were located, testifying former for PA/PC interactions with two/five water that zwitterionic formation in gas phase is impeded mainly molecules. by the thermodynamic rather than kinetic stability. Necessary for zwitterionic stabilization Computational methods Figure 1 shows the interacted structures of proline con- B3LYP density functional, in combination with 6-31+G formers (PA, PB and PC) and one water molecule. PA (d,p) basis set, was used for structural optimizations and PB each result in four stable interacted structures, and frequency calculations (Frisch et al. 2013). Energy and their relative stabilities increase in the order of minima were confirmed to have all positive frequencies PA1WIV (5.6)

Table 1 Relative stabilities for interacted structures of dif- is from PA instead of PB that represents the most sta- ferent proline conformers (ERS) and water with a wide ble conformer in the isolated state (Eszter et al. 2003; range of contents (n 0–5) = Alln et al. 2004). In PA1WI, water forms two H-bonds n 0a n 1 n 2 n 3 n 4 n 5 with the carboxylic site of proline (O H •••O and ======1 1 3 O H •••O ). Both P 1W and P 1W have only one Gas phase P nW 1.6 1.2 3.3 0.1 0.4 0.5 3 3 2 A II A IV A I − − − − intermolecular H-bond, while H-bonding interactions of P nW 13.1 8.7 2.8 1.1 2.6 6.0 C I − − the former is apparently stronger as reflected from their Energy units in kcal/mol distances (O3H3•••N: 1.868 Å vs. O3H3•••O1: 2.057 Å). For a given water content, PBnWI is used as energy benchmark In , water interacts mainly with one of the two a PB1WI In zwitterionic proline (PC), the N-H1 distance is fixed at 1.030 Å during structural optimizations carboxylic-O atoms, while structures involving both carboxylic-O atoms cannot be located as energy minima. The less efficient intermolecular H-bonding interactions in P 1W cause it to have a higher relative energy than (2.6)

O1 1.775 O1 O3 O1 2.303 2.281 H3 O2 1.993 N O2 O2 2.205 N 1.926 N 2.227 1.868 H2 H2 H2 H3 O3 H3 2.181 O3

a b c H3 H3 O3 O3 O3 2.057 H3 O1 1.986 O1 1.889 H4 2.290 O2 3.144 O2 O2 N H1 N 2.297 N O1 H1 1.841 H2 1.838 d e f O2 O2 O2 O1 1.925 1.602 O1 H1 H3 N N O1 2.515 2.322 H1 N 1.082 H2 2.855 1.824 O3 H3 2.087 2.255 2.470 O3 O3 H2 g h i

Fig. 1 Interacted structures of proline conformers (PA, PB and PC) with one water molecule. Relative energies (kcal/mol) are given in parentheses, using PB1WI as benchmark. H-bonds (Å) are marked with dashed lines Yang et al. SpringerPlus (2016) 5:19 Page 4 of 7

that resemble the condition in PB1WIII; however, both 1.647 1.761 H-bonds have been significantly reinforced, with the dis- O3 O1 O1 H3 1.752 O3 tances being optimized respectively at 2.255 and 1.925 Å H3 1.800 O4 N O2 2.026 vs. 2.470 and 2.322 Å in PB1WIII. Despite that, PC1WI H5 2.180 N O2 1.861 still has a much higher energy than PB1WI (8.7 kcal/ 2.214 H2 O4 H5 mol, see Table 1 and Additional file 1: Table S2). For H2 glycine, arginine and proline, relative stabilities of their ab zwitterionic conformers should differ from each other. O1 Both zwitterionic proline and arigine require one water 1.757 O3 O3 H3 molecule in order to remain geometrically stable, while O2 H3 1.757 O1 2.044 2.044 2.529 zwitterionic glycine necessitates two water molecules N 1.906 H5 O4 (Jensen and Gordon 1995; Im et al. 2008). In addition, H2 H5 N O2 2.529 2.240 1.906 one water molecule is already sufficient to cause zwitte- O4 H2 rionic arginine as the global energy minimum (Im et al. cd 2008). Accordingly, relative stabilities of these zwitterionic conformers should increase in the order as O4 O1 O4 1.839 2.208 glycine

O2 O2 1.877 O2 O2 O1 H3 H3 1.988 O1 O3 O1 H1 1.758 O3 N O1 1.859 N H3 O3 N N 1.847 H5 H5 H3 O3 O4 H2 H2 1.841 H2 O4 H5 2.287 O4 2.111 H2 O4 H5 aba b O3 O3 1.793 O1 1.881 H3 H3 O3 H5 1.865 O2 1.951 1.950 H3 1.935 H3 H5 1.824 O4 O2 O2 H5 1.679 H5 O4 O3 H1 2.510 O4 O2 H6 N 1.954 N 2.953 O1 O4 H6 N 2.229 N 2.117 H2 O1 1.817 3.679 H6 H2 O1 1.809 1.533 c d H1 c d 1.872 O2 O1 O1 2.480 1.886 H5 H3 O2 O1 H1 O2 H1 2.918 O2 H3 1.775 O3 H3 2.272 H3 1.562 1.793 1.782 O3 H2 2.414 O1 H1 O3 2.110 O3 1.916 N 2.063 N H2 N 1.688 1.965 N 2.526 H2 O4 H2 O4 2.197 H5 e f O4 e f

O2 1.981 H3 H5 O3 O2 N O3 O4 1.797 H3 g 1.830 2.066 H2 H4 H6 2.081 O4 O1 3.019 g O1 2.409 N 1.519 Fig. 3 Interacted structures of PB and two water molecules. Relative H1 energies (kcal/mol) are given in parentheses, using PB2WI as benchmark. H-bonds (Å) are marked with dashed lines Fig. 4 Interacted structures of PC and two water molecules. Relative energies (kcal/mol) are given in parentheses, using PB2WI as benchmark. H-bonds (Å) are marked with dashed lines

file 1: Figures S9–S13, and their activation barriers (EA) and reaction heats (ER) are listed in Table 2. In transi- (H11O7H12) (Yamabe et al. 2003). The changing trends of tion state structures, O-H1 bonds have already ruptured EA at MP2/bs2//B3LYP/bs1 level are identical to that of while N-H1 bonds are being constructed. At a given B3LYP/bs2//B3LYP/bs1; in addition, the exact EA values water content, there may exist several zwitterionic struc- of these two methods are rather close each other, thus tures, and the activation barriers (EA) that are dependent validating the default methodologies. At any water con- on reaction paths may differ significantly; e.g., at n = 3, tent, the activation barriers (EA) for transformation from the EA values vary in the range of 2.6–8.6 kcal/mol, and canonical to zwitterionic structures are rather low, and the maximum (8.6 kcal/mol) is even larger than those of accordingly the transformation to zwitterionic struc- n = 1 and 2. Generally, with increase of water contents, tures in gas phase will not be kinetically hindered. It is transformation to zwitterionic structures becomes more thus assumed that the zwitterionic distribution at a given thermodynamically favourable, as indicated by the calcu- water content (n ≥ 0) are determined principally by their lated reaction heats (ER), see Table 2. thermodynamic stabilities. Figure 5 gives the activation barriers (EA) for transformation to the most stable zwitterionic structures Conclusions (PCnWI) (n = 1–5), which are equal to 6.7, 4.2, 3.8, 1.9 Conformational analyses are performed for interacted and 4.2 kcal/mol for n = 1, 2, 3, 4 and 5, respectively structures of proline conformers (PA, PB and PC) and (Table 2). The larger EA value for PC5WI than for PC4WI water with a wide range of contents (n = 1–5). Five is due to the alteration of direct transformation path water molecules are enough to fill up the first shell of from O1 to N and the involvement of one water molecule proline functional sites (carboxylic and amido). The Yang et al. SpringerPlus (2016) 5:19 Page 6 of 7

Table 2 Activation barriers (EA) and reaction heats (ER) a O4 for transformation from canonical (PB) to zwitterionic (PC) O3 O3 O4 O3 proline with presence of water molecules O1 O1 O1 O2 O4 + H2O + H2O O2 O2 O7 Reaction paths EA ER O6 O6 H2 O5 One water molecule N N O5 N P 1W [TS1W ]≠ P 1W 6.7 6.1 O5 B III → III–I → C I Two water molecules ≠ PB2WII [TS2WII–I] PC2WI 4.2 2.0 O → → b O3 P 2W [TS2W ]≠ P 2W 6.1 4.3 O3 O6 O3 O6 B I → I–II → C II O4 O2 ≠ PB2WVI [TS2WVI–III] PC2WIII 4.9 3.2 + H2O + H2O → → O2 O2 O4 O4 ≠ O1 O1 O1 PB2WIII [TS2WIII–IV] PC2WIV 6.4 6.0 O5 → → H1 O7 P 2W [TS2W ]≠ P 2W 6.2 3.8 N B VII VII–V C V N N O5 → → O5 P 2W [TS2W ] P 2W 5.8 5.2 B V → V–VI → C VI P 2W [TS2W ]≠ P 2W 6.7 6.5 B IV → IV–VII → C VII Three water molecules O3 O4 ≠ c PB3WII [TS3WII–I] PC3WI 3.8 0.7 O3 O4 → → O4 O3 P 3W [TS3W ]≠ P 3W 4.3 1.9 O6 O2 B I I–II C II O1 + H2O O2 + H2O → → O2 O6 P 3W [TS3W ]≠ P 3W 6.3 1.1 O1 B III → III–III → C III − ≠ H1 PB3WIII [TS3WIII–IV] PC3WIV 8.6 0.7 O5 N N O7 → → N O5 P 3W [TS3W ]≠ P 3W 2.6 1.1 O5 B IV → IV–V → C V − Four water molecules ≠ PB4WI [TS4WI–I] PC4WI 1.9 2.6 Scheme 2 Gradual increase of water to interact with dif- → → − P 4W [TS4W ]≠ P 4W 5.8 3.4 ferent proline conformers: a P 3W P 4W P 5W , b B II → II–I → C I − A I → A I → A I ≠ P 3W P 4W P 5W and c P 3W P 4W P 5W . PB4WII [TS4WII–III] PC4WIII 5.3 2.4 B I → B I → B I C I → C I → C I → → − Relative energies (kcal/mol) are given in parentheses, using P nW as P 4W [TS4W ]≠ P 4W 2.7 2.1 B I B III → III–IV → C IV − benchmark (n 3, 4, 5). H-bonds are marked with dashed lines = Five water molecules P 5W [TS5W ]≠ P 5W 4.2 6.0 B I → I–I → C I − P 5W [TS5W ]≠ P 5W 4.8 6.4 B II → II–II → C II − conformational preferences of proline are altered by Energy units in kcal/mol gradual increase of water contents, and intermolecular H-bonds play a crucial role in deciding their relative stabilities. It is interesting to find that one water molecule has 10 already rendered zwitterionic proline to be geometri- B3LYP/bs2//B3LYP/bs1 cally stable with formation of two strong intermolecular 8 MP2/bs2//B3LYP/bs1 H-bonds. Unlike the canonical conformers, the relative , kcal/mol) stabilities of zwitterionic proline improve monotonously a E and pronouncedly with gradual increase of water con- ( 6 tents. Zwitterionic proline is energetically preferential over canonical structures at n 4 and conformationally = 4 predominant at n = 5; in these cases, rather complex H-bonding networks are constructed between water and proline. Zwitterionic stabilities of different amino 2 acids differ substantially and increase in the order of glycine

Additional file Hwang TK, Eom GY, Choi MS, Jang SW, Kim JY, Lee S (2011) Microsolvation of lysine by water: computational study of stabilized zwitterion. J Phys Chem B 115:10147–10153 Additional file 1. Relative stabilities for different proline conformers Im S, Jang SW, Lee S, Lee Y, Kim B (2008) Arginine zwitterion is more stable with presence of 0, 1, 2, 3, 4 and 5 water molecules (Tables S1–S5) as than the canonical form when solvated by a water molecule. J Phys well as structures of proline conformers with presence of 3, 4 and 5 water Chem A 112:9767–9770 molecules (Figures S1–S8) and transition states for the conformational Jensen JH, Gordon MS (1995) On the number of water molecules necessary to transformation from canonical to zwitterionic proline with presence of 1, stabilize the glycine zwitterion. J Am Chem Soc 117:8159–8170 2, 3, 4, 5 water molecules (Figures S9–S13). Kapitán J, Baumruk V, Kopecký V Jr, Pohl R, Bouř P (2006) Proline zwitterion dynam- ics in solution, glass and crystalline state. J Am Chem Soc 128:13451–13462 Kass SR (2005) Zwitterion-dianion complexes and anion-anion clusters with negative dissociation energies. J Am Chem Soc 127:13098–13099 Author’s contributions Kim JY, Lee Y, Lee S (2014) Effects of microsolvation on the relative stability of GY performed the experiments, analyzed the data and drafted the manuscript. zwitterionic vs canonical proline. Chem Phys Lett 608:177–185 LJZ and YC helped analyze the data and contributed in writing the manu- Kokpol SU, Doungdee PB, Hannongbua SV, Rode BM, Imtrakul LJP (1988) script. All authors read and approved the final manuscript. Ab initio study of the hydration of the glycine zwitterion. J Chem Soc Faraday Trans 2(84):1789–1792 Author details Levy Y, Onuchic JN (2004) Water and proteins: a love-hate relationship. Proc 1 College of Resource and Environment and Chongqing Key Laboratory of Soil Natl Acad Sci USA 101:3325–3326 Multi‑scale Interfacial Process, Southwest University, 400715 Chongqing, Li XJ, Zhong ZJ, Wu HZ (2011) DFT and MP2 investigations of l-proline and its People’s Republic of China. 2 College of Chemistry Chemical Engineering hydrated complexes. J Mol Model 17:2623–2630 and Environmental Engineering, Liaoning Shihua University, 113001 Fushun, Rand RP (2004) Probing the role of water in protein conformation and func- Liaoning, People’s Republic of China. tion. Philos Trans R Soc Lond B Biol Sci 359:1277–1284 Rankin KN, Gauld JW, Boyd RJ (2002) Density functional study of the proline- Acknowledgements catalyzed direct aldol reaction. J Phys Chem A 106:5155–5159 This work was sponsored by the National Natural Science Foundation of Remko M, Rode BM (2001) Catalyzed peptide bond formation in the gas China (21473137) and Fourth Excellent Talents Program of Higher Education phase. Phys Chem Chem Phys 3:4667–4673 2 2 2 in Chongqing (2014-03) and Fundamental Research Funds for the Central Col- Remko M, Rode BM (2006) Effect of metal ions (Li+, Na+, K+, Mg +, Ca +, Ni +, 2 2 leges (SWU113049 and XDJK2014C106). Cu +, and Zn +) and water coordination on the structure of glycine and zwitterionic glycine. J Phys Chem A 110:1960–1967 Competing interests Rimola A, Corno M, Zicovich-Wilson CM, Ugliengo P (2008) Ab initio modeling The authors declare that they have no competing interests. of protein/biomaterial interactions: glycine adsorption at hydroxyapatite surfaces. J Am Chem Soc 130:16181–16183 Received: 2 November 2015 Accepted: 28 December 2015 Rimola A, Costa D, Sodupe M, Lambert JF, Ugliengo P (2013) Silica surface features and their role in the adsorption of biomolecules: computational modeling and experiments. Chem Rev 113:42160–44313 Snoek LC, Kroemery RT, Simons JPA (2002) Spectroscopic and computational exploration of tryptophan-water cluster structures in the gas phase. Phys Chem Chem Phys 4:2130–2139 References Teeter MM (1991) Water-protein interactions: theory and experiment. Annu 2 Ai HQ, Bu YX, Han KL (2003) Glycine-Zn+/Zn + and their hydrates: on the num- Rev Biophys Biophys Chem 20:577–600 ber of water molecules necessary to stabilize the switterionic glycine- Tian SX, Li HB, Yang JL (2009) Monoanion BH4− can stabilize zwitterionic 2 Zn+/Zn + over the nonzwitterionic ones. J Chem Phys 118:10973–10985 glycine with dihydrogen bonds. Chemphyschem 10:1435–1437 Ajitha MJ, Suresh CH (2011) A higher energy conformer of (S)-proline is the Timasheff SN (1970) Protein-solvent interactions and protein conformation. active catalyst in intermolecular aldol reaction: evidence from DFT calcu- Acc Chem Res 3:62–68 lations. J Mol Catal A: Chem 345:37–43 Wu RH, McMahon TB (2007) Stabilization of the zwitterionic structure of proline Alln WD, Czinki E, Csásázr AG (2004) Molecular structure of proline. Chem Eur by an alkylammonium ion in the gas phase. Angew Chem Int Ed Engl J 10:4512–4517 46:3668–3671 Balta B, Aviyente V (2004) Solvent Effects on Glycine II. Water-assisted Tau- Yamabe S, Ono N, Tsuchida N (2003) Molecular interactions between glycine tomerization. J Comput Chem 15:690–703 and h2o affording the zwitterion. J Phys Chem A 107:7915–7922 Constantino E, Rodriguez-Santiago L, Sodupe M, Tortajada J (2005) Interac- Yang G, Zhou LJ (2014) Zwitterionic versus canonical amino acids over the 2 tion of Co+ and Co + with Glycine. A Theoretical Study. J Phys Chem A various defects in zeolites: a two-layer ONIOM calculation. Sci Rep 4:6594 109:224–230 Yang G, Zu YG, Liu CB, Fu YJ, Zhou LJ (2008) Stabilization of amino acid zwit- Corradini D, Strekalova EG, Stanley HE, Gallo P (2013) Microscopic mechanism of terions with varieties of anionic species: the intrinsic mechanism. J Phys protein cryopreservation in an aqueous solution with trehalose. Sci Rep 3:1218 Chem B 112:7104–7110 Corral L, Mό O, Yáňez M, Moran D, Radom L, Salpin JY, Tortajada J (2006) An Yang G, Zu YG, Fu YJ, Zhou LJ, Zhu RX, Liu CB (2009a) Assembly and stabiliza- 2 experimental and theoretical investigation of gas-phase reactions of Ca + tion of multi-amino acid zwitterions by the zn(ii) ion: a computational with glycine. Chem Eur J 12:6787–6796 exploration. J Phys Chem B 113:4899–4906 Császár AG (1992) Conformers of gaseous glycine. J Am Chem Soc 114:9568–9575 Yang ZW, Wu XM, Zhou LJ, Yang G (2009b) A Proline-based neuraminidase Eszter C, Czinki E, Csásázr AG (2003) Conformers of gaseous proline. Chem Eur inhibitor: DFT studies on the zwitterion conformation, stability and J 9:1008–1019 formation. Int J Mol Sci 10:3918–3930 Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Cheeseman MAJR, Scalmani G, Yang G, Yang ZW, Zhou LJ, Zhu RX, Liu CB (2010) A revisit to proline-catalyzed Barone V, Mennucci B, Petersson GA et al (2013) Gaussian 09, Revision A.9. aldol reaction: interactions with acetone and catalytic mechanisms. J Mol Gaussian, Inc., Wallingford Catal A: Chem 316:112–117 Gutowski M, Skurski P, Simons J (2000) Dipole-bound anions of glycine Yang G, Zhu C, Zhou LJ (2015) Stabilization of zwitterionic proline by DMSO. based on the zwitterion and neutral structures. J Am Chem Soc Int J Quantum Chem 115:1746–1752 122:10159–10162 Yu D, Rauk A, Armstrong DA (1995) Radicals and ions of glycine: an ab initio Hoyau S, Ohanessian G (1998) Interaction of alkali metal cations (Li+-Cs+) with study of the structures and gas-phase thermochemistry. J Am Chem Soc glycine in the gas phase: a theoretical study. Chem Eur J 4:1561–1569 117:1789–1996 Hu CH, Shen M, Schaefer HF III (1995) Glycine conformational analysis. J Am Zhang K, Chung-Phillips A (1998) Conformers of gaseous protonated glycine. J Chem Soc 115:2923–2929 Comput Chem 19:1862–1876