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Vibrational Spectroscopy 55 (2011) 69–76

Contents lists available at ScienceDirect

Vibrational Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /v ibspec

imulation of the Raman spectra of zwitterionic glycine + nH2O (n = 1, 2, . . ., 5) byeans of DFT calculations and comparison to the experimentally observed

aman spectra of glycine in aqueous medium

idhi Vyasa, Animesh K. Ojhaa,b,∗, Arnulf Maternyb

Department of Physics, Motilal Nehru National Institute of Technology, Allahabad 211004, IndiaCenter of Functional Materials and Nanomolecular Science, Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany

r t i c l e i n f o

rticle history:eceived 25 June 2010eceived in revised form 6 August 2010ccepted 10 August 2010vailable online 26 August 2010

eywords:lycineFTaman spectra

a b s t r a c t

Raman spectra of an aqueous solution of glycine (Gly) have been recorded in the range of 400–2000 cm−1.In aqueous solution, glycine molecules exist in their zwitterionic form, having two opposite charged poles,COO− and NH3

+. The zwitterionic structure of glycine (ZGly) is stabilized by the hydrogen bond interac-tion of water (W) molecules. In the present report, we have optimized the ground state geometries ofdifferent hydrogen bonded complexes of [ZGly + (W)n=1–5] in aqueous medium using DFT calculations atthe B3LYP/6-311++G(d) level of theory. A comparative discussion on the structural details and bindingenergies (BEs) of each conformer has been also done. The theoretical Raman spectra were calculatedcorresponding to the most stable [ZGly + (W)n=1–5] conformers. The theoretically simulated Raman spec-tra of each stable conformer were compared with experimentally observed Raman spectra to explorethe number of water molecules needed for stabilizing the structure of ZGly. The theoretically simulated

Raman spectra corresponding to the most stable conformer of [ZGly + (W)5] having a BE of −22.8 kcal/mol,are matching nicely with the experimentally observed Raman spectra. Thus, on the basis of the aboveobservations, we conclude that the conformer, [ZGly + (W)5] is the most probable conformer in the aque-ous medium. We also believe that in the conformer, [ZGly + (W)5] the five water molecules are arrangedaround the ZGly in such a way that the effect of steric hindrance is less compared to the other conformers.

tion pn=1–5]

The dipole–dipole interacbond for each [ZGly + (W)

. Introduction

The phenomenon of hydrogen bonding is omnipresent in nature.t plays a critical role in stabilizing the structures of biological

olecules, e.g. proteins or nucleic acid [1–5]. It is essentially a weakond between two molecules, one containing an electronegativetom and the other having a H atom [2–5]. Hydrogen bonds playn important role in the area of crystal engineering, where theyre used to generate a variety of supramolecular assemblies [4].lthough the hydrogen bond interactions are weak, the fact that

hey are numerous in number results in a considerable stabiliza-ion of the protein molecule as well as the ribonucleic acid (RNA)

nd deoxyribonucleic acid (DNA) molecules by forming hydrogenonds between the pyrimidine and purine bases [5]. The aminocids are the building blocks of proteins. They contain a basic aminoroup (–NH2) and an acidic carboxylic group (–COOH), which

∗ Corresponding author at: Department of Physics, Motilal Nehru National Insti-ute of Technology, Allahabad, UP 211004, India. Tel.: +91 5322771289.

E-mail address: animesh@mnnit.ac.in (A.K. Ojha).

924-2031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.vibspec.2010.08.007

otential (DDP) is also calculated corresponding to the strongest hydrogenconformer.

© 2010 Elsevier B.V. All rights reserved.

are the main functional groups in biological important molecules[6,7].

A diverse application of hydrogen bonding has been a matter ofgreat interest in both chemical and biochemical sciences [8]. In viewof the role of water in the structural and functional properties ofmacromolecules and their interactions, considerable attention hasbeen paid to the exploration of the properties of biomolecules inaqueous medium. The amino acids exist as heavily solvated zwitte-rions in aqueous solutions. It is also well established that the aminoacids exist predominantly in the neutral form in gas phase, whilein zwitterionic form in crystal or solution phase.

Glycine is the simplest amino acid with a small hydrocar-bon backbone and is a very good biochemical model compoundfor a theoretical study and, therefore, many such studies canbe found in the literature. Several theoretical investigations onthe environmental effects on the molecular structure of glycine

have been performed [9–14]. They have shown that the aque-ous media stabilize the zwitterionic form of glycine. The bindingof few water molecules at the appropriate sites of the glycinemolecule through hydrogen bonds would represent the micro-scopic solvent effect [15–16], which affects the stable energy of

7 l Spect

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0 N. Vyas et al. / Vibrationa

omplexes in a bulk environment. The 1:1 glycine–water complexas studied by Ding and Karsten [17] employing both, second orderøller–Plesset (MP2) and Hartree–Fock (HF) theoretical methodsith the basis sets 6-31++G(d,p). They observed that a single waterolecule bridges the carboxylic acid and the amino groups. The

lycine–water complex was 26.75 kJ/mol lower in energy than theeutral glycine molecules at the HF/6-31++G (d,p) level of theory.

In a more recent study, the structural properties, energies,nd vibrational frequencies of the optimized stable structuresf glycine–(H2O)3 complexes were obtained from MP2 and den-ity functional theory (DFT) methods, such as B3LYP, PW91,PW1PW91, and PBE using the 6-311++G(d,p) basis set [18]. In thisork, the functional groups NH3

+ and COO− were used to inves-igate the effect of microsolvation on the zwitterionic glycine. Innother study, the DFT method (PBE1PBE) was used to analyze theicrosolvation of neutral and zwitterionic glycine [19]. There, itas found that neutral glycine was favored when it associated with

ero to six water molecules. However, with seven water molecules,he two structures were isoenergetic [19].

Deng et al. [20] have calculated vibrational Raman opti-al activity (VROA) spectra of N-acetyl-N′-methyl-l-alaninamideNANMLA) by means of ab initio calculations. The theoreticallyalculated VROA spectra were compared with the experimen-ally recorded spectra in order to determine the conformation ofANMLA molecule. They found the presence of different stable con-

ormers of NANMLA in aqueous and chloroform solution. Jalkanennd Suhai [21] presented a theoretical study on N-acetyl-l-alanine′-methylamide (l-AANMA) molecule. They calculated the opti-ized geometries, vibrational frequencies, vibrational absorption

VA) intensities and vibrational circular dichroism (VCD) for theight low energy conformers of (l-AANMA) in gas phase at 6-1G(d)/B3LYP level of theory. In this study they concluded thatour water molecules are needed to stabilize one conformerf (l-AANMA) molecule. The agreement between the calculatedotational strengths of the various amide modes for which thexperimental measurements have been made is very good in casef right-handed a-helical conformation of a (l-amino acid). Theylso predicted correctly the negative couplet for the amide A band,he positive couplet for the amide I band and the negative monosig-ate signal for the amide II band. Han et al. [22] studied the effectf hydration on the N-acetyl-l-alanine N-methylamide (AAMA)eometries, relative energies, and vibrational properties by apply-ng the solvent continuum model by adding four water molecules toach of the conformers, and finally they combined two approacheso draw the conclusion. The explicit water molecules play anmportant role in stabilizing different conformations of AAMA that

ay not be stable in the isolated state. They also concluded thathe interaction between the water molecules and the AAMA willtrongly influence the observed Raman, VCD, and ROA spectra anduggested that the PII structure of AAMA is the dominant one inhe aqueous solution. Tajkhorshid et al. [23] performed a theo-etical investigation on zwitterionic form l-alanine (ALAZW). Theyave calculated optimized geometries, vibrational frequencies andbsorption intensity of ALAZW stabilized by four neighboring waterolecules at B3LYP/6-31G(d) level of theory. The calculated val-

es of vibrational absorption, VCD and polarized Raman scatteringntensities for the ALAZW were found to be very sensitive to theelative arrangement of the neighboring water molecules. Poon etl. [24] reported about the nine dipolar couplings in the protonuclear NMR spectrum of alanyl dipeptide dissolved in water-ased liquid crystal can be quantitatively understood. They have

lso investigated the effect of solvent on equilibrium geometryf the alanyl dipeptide in terms of its dominant conformer. Theyxplained the NMR spectrum quantitatively in terms of one dom-nant conformer of alanyl dipeptide. Frimand et al. [25] calculatedA and VCD spectra of l-alanine at B3LYP/6-31G*(d) level of theory

roscopy 55 (2011) 69–76

employing three different approaches; the zwitterion surroundedby explicit water molecules only, the zwitterions embedded in aself-consistent reaction field and zwitterion plus the explicit watermolecules embedded in a self-consistent reaction field. The cal-culated VA and VCD spectra of l-alanine plus water complexesare in better agreement with experimentally VA and VCD spectrareported previously. Jalkanen et al. [26] have investigated theo-retically the chiral sensitive Raman and Raman optical activity(ROA) spectra as a probe for secondary structural features of thebiomolecules. In this report the combination of Raman and ROAspectroscopy has been shown to very sensitive to the secondarystructure changes. The combination of vibrational absorption andVCD offers a complementary source of information for investigat-ing the secondary structure and chiral feature of the biomolecules.Weise and Weisshaar [27] had performed a conformational analy-sis of alanine dipeptide from dipolar coupling in water-based liquidcrystals. They investigated the stability of peptide–water interac-tion by performing the theoretical calculations. Degtyarenko et al.[28] had studied l-alanine in a droplet of water by performing den-sity functional molecular dynamics. They reported the results ofa Born–Oppenheimer molecular dynamics study on an l-alanineamino acid in neutral aqueous solution and observed that thehydrophobic side chain defines the trajectory path of the molecule.They also reported that the number of water molecules in thefirst hydration shell of an alanine molecule is constantly chang-ing and the average number was found to be seven. Deplazes et al.[29] performed a combined theoretical and experimental study ofthe vibrational absorption (VA)/IR, VCD, Raman and Raman opticalactivity (ROA) spectra of l-histidine in aqueous solution solvatedwith five different solvation models. By analyzing the results of thetheoretical calculations they reported that the structural parame-ters are substantially affected by solvating the molecule. In anotherreport Jalkanen et al. [30] had investigated the role of hydrationin determining the structure and vibrational spectra of l-alanineand N-acetyl-l-alanine N′-methylamide in aqueous solution. Theyobserved that the first principle locations of the water molecules inthe first solvation shell are responsible for stabilizing the zwitteri-onic structure of l-alanine. Joti et al. [31] investigated the hydrationeffects on protein dynamics by comparing the frequency depen-dence of the calculated neutron scattering spectra between full andminimal hydration states at temperatures between 100 and 300 K.They found that the protein boson peak is observed in the frequencyrange 1–4 meV at 100 K in both the minimal and full hydrationstate (FHS). Mukhopadhyay et al. [32] analyzed the Raman opti-cal activity (ROA) spectra of N-acetylalanine-N-methylamide (Aladipeptide) in H2O and D2O using DFT on Monte Carlo (MC) sam-pled geometries to examine the propensity of Ala dipeptide toadopt compact right-handed (˛R) and left-handed (˛L) helical con-formations and suggested that alanine dipeptide populates the ˛Rand PPII conformations but no substantial population of ˛L or ˇstructures, despite sampling ˛L- and ˇ-structures in our MC sim-ulations. Mourik et al. [33] studied the canonical (keto) and rare(enol) tautomers of 5-bromouracil in clusters consisting of 50 and100 water at BLYP/6-31G(d,p) and B3LYP/6-31G(d,p) level of the-ories. They found that at both levels of theory, the keto formof uracil is favored over the enol tautomer, whereas this pref-erence is reversed for 5-bromouracil. Recently Zaccai Serdyuck[34] published a nice book on methods in molecular biophysicsin which they reported about multi-method and techniques forapproaching different problems in molecular biophysics. Althoughthe neutral or zwitterionic forms of many amino acids with the

water molecules have been investigated thoroughly by performingmany theoretically calculations, the experimental observation isstill lacking to verify all those theoretical results. The spectroscopictechniques combined with theoretical calculations are capableof exploring how many water molecules are needed to stabilize

N. Vyas et al. / Vibrational Spectroscopy 55 (2011) 69–76 71

F rogen[

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ig. 1. Optimized ground state geometries of ZGly (A) and its different stable hydZGly + (W)3], (E) [ZGly + (W)4], and (F) [ZGly + (W)5] in aqueous medium.

he zwitterionic form of the glycine, which is the smallest aminocids.

The objective of the present work was to first study the differ-nt hydrogen bonded complexes of zwitterionic glycine and wateromplexes, [ZGly + (W)n], using DFT. The theoretically calculatedaman spectra of each [ZGly + (W)n] complex were then comparedo the experimentally recorded Raman spectra of glycine in aque-us medium, where most of the glycine molecules existed in thewitterionic form, to determine how many water molecules areeeded to stabilize the zwitterionic form of the glycine. The othertructural details, such as bond angles, bond lengths, binding ener-ies, and electronic charges of the [ZGly + (W)n] complexes havelso been investigated and the dipole–dipole interaction potentialas calculated.

. Experimental details

Glycine (99.5%) was procured from Aldrich Co. and was usedithout further purification. Raman spectra were recorded using a

abRam micro-Raman set-up (HORIBA Jobin Yvon). The laser linet 514 nm from an Ar ion laser was used as the Raman excitationource for recording the spectra. The laser power at the sample was15 mW. Raman spectra of 2 M aqueous solution of glycine wereeasured in back-scattering geometry. We also measured the pH of

he aqueous solution of glycine and found it to be ∼6.0. At this par-icular pH the glycine molecules exist in their zwitterionic form. Thepectrometer was equipped with a 1200 grooves/mm holographicrating, a holographic super-notch filter, and a Peltier-cooled CCDetector. The data acquisition time for each spectrum was 20 s. The

xpected spectral resolution is around 1.0 cm−1 in the present set-p. Raman scattered intensities were recorded by making a twofoldverlap of the single window frame. The Raman spectra of thequeous solution of glycine were recorded in the spectral regionetween 400 and 2000 cm−1.

bonded complexes with water molecules, (B) [ZGly + (W)1], (C) [ZGly + (W)2], (D)

3. Computational details

Density functional theory (DFT) has been accepted by the quan-tum chemistry community as a cost effective and meaningfulapproach for the computation of molecular structures, vibrationalfrequencies, and energies of chemical reactions. It has been alsoshown that the molecular structures calculated by the DFT methodsare more reliable than the ab initio methods in terms of computa-tional costs and accuracy [35–38]. DFT at the B3LYP level has beenalso used to study several hydrogen bonded complexes [39–42]and was shown to be effective at accurately predicting structuresand energies. In the present work, the structures of [ZGly + (W)n]hydrogen bonded complexes have been optimized using DFT atthe B3LYP level of theory employing the 6-311++G(d) basis set. Allcalculations have been performed in aqueous medium for includ-ing the bulk solvent effect using the polarizable continuum model(PCM). In the DFT method, the B3LYP hybrid functional [43–46]consists of the Hartree–Fock and non-local exchange and corre-lation parts, which are used by implementing the 6-311++G(d)basis set. For the hydrogen bonded system it is expected thatboth diffuse and polarization functions may be necessary in thebasis set. It has been proven by Wang et al. [43] that the resultswith the 6-311++G(d) basis set are more accurate with the B3LYPmethod for [Gly + (W)n=1–5] complexes. To make the calculationsmore realistic, the gas optimized geometries of different complexes[ZGly + (W)n] have been solvated in aqueous medium using thepolarizable continuum model (PCM) [47]. Non-electrostatic termswere included for the calculation of the solvation energies. ThePCM creates a realistic molecule-shaped cavity within the solventcontinuum from interlocking spheres centered on atoms or func-

tional groups of the solute molecules [48]. The reaction field dueto the distribution of solute charges is represented as an appar-ent charge on the surface of the cavity. The electrostatic potentialhas been used to calculate the surface charge distribution. Thecontinuum model usually provides a reliable evaluation of the sol-

72 N. Vyas et al. / Vibrational Spectroscopy 55 (2011) 69–76

F entalls f glyc(

vadtgbtth(f(f–cm

ig. 2. (a) (i) Simulated Raman spectra of [ZGly + (W)1] along with (ii) the experimpectra of [ZGly + (W)3] along with (ii) the experimentally recorded Raman spectra oii) the experimentally recorded Raman spectra of glycine in aqueous medium.

ation free energy, provided appropriate atomic radii parametersre used. Moreover, explicit hydrogen bonds may represent a fun-amental contribution to stabilize zwitterions in water [49]. Allhe calculations have been carried out using the GAUSSIAN 03 pro-ram [50]. The initial geometries of the [ZGly + (W)n=1–5] hydrogenonded complexes were constructed assuming three schemes onhe basis of favorable sites for the hydrogen bond formation: (i)he water molecules are kept in such a way that they form theydrogen bonds only with the –NH3

+ functional group of ZGly,ii) the water molecules were placed in such a way that theyorm the hydrogen bonds with the –COO− functional group, and

iii) the water molecules were placed in such a way that theyorm the hydrogen bond with both functional groups, –NH3

+ andCOO−. Many minimum energy conformers of the [ZGly + (W)n=1–5]omplexes have been found at the potential energy surface. Theinimum energy conformers were confirmed by performing the

y recorded Raman spectra of glycine in aqueous medium; (b) (i) simulated Ramanine in aqueous medium; (c) (i) simulated Raman spectra of [ZGly + (W)5] along with

calculation of the vibrational frequencies where no any imagi-nary frequency was observed. The vibrational frequencies of moststable conformers corresponding to [ZGly + (W)n] complexes withn = 1–5, are for the first time shown and discussed in the presentstudy. The calculated Raman frequencies of these conformers arecompared with the experimentally recorded Raman spectra ofglycine in aqueous medium, where most of the glycine moleculesexist in their zwitterionic form. The optimized geometries of themost stable [ZGly + (W)n=1–5] complexes calculated according tothe above discussed schemes are presented in Fig. 1. The panels(A)–(F) represent the optimized ground state geometries of ZGly

and the [ZGly + (W)1], [ZGly + (W)2], [ZGly + (W)3], [ZGly + (W)4],and [ZGly + (W)5] complexes, respectively. Several conformers ofthe [ZGly + (W)n=1–5] complexes exist for n = 1–5 with differentarrangements of the hydrogen bond interaction. Only those geome-tries of [ZGly + (W)n=1–5] complexes are reported in the presented

Spect

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N. Vyas et al. / Vibrational

tudy, which have a maximum binding energy (BE) among all pos-ible conformers and have a final structure, which nicely matchesith our assumptions (i)–(iii). The length of the hydrogen bonds

s analyzed to determine the strength of the hydrogen bonds inach conformer. The BEs of the [ZGly + (W)n=1–5] hydrogen bondedomplexes are calculated using the following relationship:

E = EAB − (EA + EB),

here AB stands for the complex and A and B stand for each of thewo monomer molecules.

. Results and discussion

According to the three different schemes for the formation ofydrogen bonds between ZGly and the water molecules as dis-ussed in the above section, the water molecules are placed (i)lose to the NH3

+ functional group, (ii) close to the COO− group,nd (iii) close to both the groups, –NH3

+ and –COO− for searchinghe minimum energy configuration of [ZGly + (W)1]. The calcula-ion has been performed at the B3LYP/6-311++G(d) level of theory.he vibrational frequencies of the lowest energy conformer amongll possible [ZGly + (W)1] complexes were also calculated for com-arison with the experimentally recorded Raman spectra of theGly molecule. For scheme (i), the water molecule was kept nearo the –NH3

+ group of the ZGly molecule in such a way that theater molecule acts as proton acceptor and the –NH3

+ group actss proton donor for the formation of the hydrogen bond. Thus,n this case the O atom of the water molecule bridges with onef the three H atoms of the –NH3

+ group of the ZGly molecule.he binding energy of this conformer was calculated using Eq. (1)nd it turns out to be −5.4 kcal/mol. The length of the hydrogenond was found to be 1.734 A. In scheme (ii), the calculation ofhe geometry optimization of the [ZGly + (W)1] complex was per-ormed by keeping the water molecule closer to the –COO− groupf the ZGly molecule. In this conformer the water molecule acts asroton donor and the –COO− group acts as proton acceptor. Theatom of the water molecule attaches to the hydrogen bond to

he O atom of the –COO− group. The BE of this conformer wasalculated to be −2.9 kcal/mol and the hydrogen bond length wasetermined to be 1.790 A. Further, in scheme (iii), the geometryas optimized by placing the water molecule close to both theNH3

+ and –COO− groups. In this conformer, the water molecules orientated in such a way that it forms two hydrogen bonds withhe ZGly molecule. For one hydrogen bond the water moleculects a proton donor and for the other it acts as proton accep-or. In one hydrogen bond, the H atom of the water moleculettaches to the O atom of the –COO− group with a bond lengthf 2.002 A while for the other hydrogen bond, the O atom of theater molecule attaches to the H atom of –NH3

+ group with aydrogen bond length of 1.749 A. Thus, in case where the waterolecule acts as proton donor as well as proton acceptor, the

ydrogen bond strength is found to be stronger than that foundor its acceptor function. The BE of this conformer was calculatedo be −4.8 kcal/mol. Thus, among the three [ZGly + (W)1] hydrogenonded conformers studied under the different schemes, the con-ormer where the water molecule attaches to the H atom of theNH3

+ group possesses the maximum BE compared to the otherwo conformers. The maximum BE conformer is shown in Fig. 1(B).he maximum BE conformer, (B) was further used to calculate theaman frequencies. This simulated Raman spectrum was compared

ith the experimentally recorded Raman spectrum of the zwitte-

ionic glycine.For the [(ZGly + (W)2)] hydrogen bonded complexes, six dif-

erent conformers have been optimized by keeping the waterolecules near to ZGly in accordance to the three different

roscopy 55 (2011) 69–76 73

schemes. Under scheme (i), the water molecules were kept closerto the –NH3

+ functional group of ZGly; here, two conformers havebeen optimized. In these two conformers, the water molecules actas proton acceptor and the –NH3

+ group behaves like a protondonor. The two H atoms of the –NH3

+ group are linked with thehydrogen bonds to the O atom of each water molecule. The BEenergies of these two complexes were calculated to be 11.4 and10.3 kcal/mol. The lengths of the hydrogen bonds were found tobe 1.765 and 1.766 A for the conformer having BE 11.4 kcal/moland 1.799 and 1.754 A for the conformer having BE 10.3 kcal/mol,respectively. The difference in the BEs of these two conformers maybe due to the different orientation of the water molecules result-ing in different lengths of the hydrogen bonds and consequentlyin different hydrogen bond strengths. Further, three conform-ers of [ZGly + (W)2] have been optimized by keeping two watermolecules near to the –COO− group of the ZGly molecule as dis-cussed in scheme (ii). In all these three cases, the water moleculesact as proton donors and ZGly acts as proton acceptor. In thevery first structure, each water molecule bridges with its hydro-gen bonds with the O atoms of the –COO− group resulting in aBE of 5.4 kcal/mol. Thus, in this case, the water molecules maketwo hydrogen bonds of lengths 1.812 and 1.813 A. The hydrogenbond is stronger (1.812) when the H atom of the water moleculeis attached to the O atom of the COO− group, which is linkedwith the carbon atom through a single bond. However, in the sec-ond [ZGly + (W)2] conformer only one water molecule was directlyattached to the O atom of the –COO− group of ZGly and the otherwater molecule is associated through the hydrogen bond with thefirst water molecule. In this particular conformer, the second watermolecule acts as proton donor and the first water molecule acts asproton acceptor. The hydrogen bond length, with which the firstwater molecule was linked to the –COO− group, was 1.749 A. Thelength of the hydrogen bond, by which the two water moleculeswere linked to each other, was 1.773 A. Thus, the hydrogen bondbetween the first water molecule and ZGly is stronger compared tothe strength of the hydrogen bond formed between the two watermolecules. The BE of this conformer is calculated to be 7.3 kcal/mol.Finally, in the third [ZGly + (W)2] conformer, both water moleculesare attached to the same O atom of the –COO− group under theformation of two hydrogen bonds of lengths 1.803 and 1.813 A.The difference between the hydrogen bond lengths may be due tothe different orientation of the water molecules and the differentelectronegativity of the O atoms bonded through single and doublebonds with the carbon atom. The BE of this conformer is calculatedto be 5.8 kcal/mol. According to scheme (iii), the optimization of the[ZGly + (W)2] conformers was done by placing one water moleculenear to the –NH3

+ group and the other one close to the –COO−

group of the ZGly molecule. For this particular structure, one watermolecule acts as proton donor and the other acts as proton accep-tor. The length of the hydrogen bond, where the water moleculeis associated to the –COO− group of ZGly by donating a proton, is1.786 A. The length of the hydrogen bond where the water moleculeis linked through a hydrogen bond to the –NH3

+ group by acceptinga proton is 1.726 A. Thus, the hydrogen bond is stronger for the casewhere the water molecule is acting as proton acceptor than for thecase where it plays the role of a proton donor. The BE of this con-former was calculated to be 9.1 kcal/mol. For all six [ZGly + (W)2]conformers optimized according to schemes (i), (ii), and (iii), theconformer following scheme (i), where both water molecules areattached to the –NH3

+ group by accepting a proton has the maxi-mum BE, 11.4 kcal/mol. Therefore, this conformer is the most stable

conformer among all possible [ZGly + (W)2] complexes; its struc-ture is displayed in panel (C) of Fig. 1. This conformer was usedto calculate the Raman frequencies. Again, the simulated Ramanspectrum was compared with the experimentally recorded Ramanspectrum of an aqueous solution of glycine.

74 N. Vyas et al. / Vibrational Spectroscopy 55 (2011) 69–76

Table 1The experimentally observed Raman line positions of glycine in aqueous medium along with the calculated values for different hydrogen bonded conformers of ZGly andwater.

Observed Raman line position (cm−1) Calculated Raman line positions (cm−1)

[ZGly + (W)] [ZGly + (W)2] [ZGly + (W)3] [ZGly + (W)4] [ZGly + (W)5]

506 490 528 510 510 505584 580 584 590 588 585670 670 670 650 670 670899 870 878 870 878 900

1033 1010 1027 1000 1012 10351123 1120 1131 1100 1124 11701329 1350 1354 1360 1357 1369

bwhdmtsaiwHbhgAidcttwmbimhHpg1bsf1ftirFwfg–ofif1mg

1413 1400 14101433 1480 14811618 1630 1620

Next, three [ZGly + (W)3] hydrogen bonded conformers haveeen optimized to explore the interaction of three water moleculesith the ZGly molecule. Again, the three [ZGly + (W)3] conformersave been investigated taking into consideration the three schemesiscussed above. According to scheme (i), each of the three waterolecules was put near to one of the H atoms of the –NH3

+ group ashe initial guess for the optimization. The finally stable [ZGly + (W)3]tructure, studied under scheme (i), was found to be the one wherell three water molecules were forming hydrogen bonds by accept-ng protons from the –NH3

+ group, it means in this case the threeater molecules were attached through the hydrogen bonds to theatom of the –NH3

+ group. After a closer examination of hydrogenond length it is very interesting to note that the strength of theseydrogen bonds was different for each water molecule. The hydro-en bond lengths were determined to be 1.781, 1.784, and 1.838 A.gain, here the different orientations of the water molecules dur-

ng the hydrogen bond formation may be used as explanation forifferent lengths of hydrogen bonds. The BE of this conformer isalculated to be 15.3 kcal/mol. In scheme (ii), for the optimiza-ion of [ZGly + (W)3] the three water molecules were kept near tohe –COO− functional group of the ZGly molecule, such that twoater molecules are directly involved in the hydrogen bond for-ation with the –COO− group while the third water molecule is

ridging with two other water molecules via hydrogen bonds lead-ng to a closed loop hydrogen bonded structure. The two water

olecules, which are directly involved in the formation of theydrogen bond with the ZGly molecule, are acting as proton donors.owever, the third water molecule is acting as proton donor androton acceptor at the same time. In this conformer, four hydro-en bonds are present with bond lengths 1.777, 1.827, 1.827, and.774 A. It has been observed that the lengths of the two hydrogenonds formed between two water molecules are those with theame value, 1.827 A. However, the lengths of the hydrogen bondsormed between the ZGly and the water molecules are 1.777 and.774 A. Thus, the strengths of the hydrogen bonds are not the sameor both water molecules attached to the ZGly molecule. Next tohe different orientations of the water molecules, the differencesn electron density at the O atom of the ZGly molecule might beesponsible for this. The BE of this conformer is 13.10 kcal/mol.inally, [ZGly + (W)3] was also optimized assuming scheme (iii),here the water molecules were placed in such a way that they

ormed hydrogen bonds with both –NH3+ and –COO− functional

roups. In this conformer, one water molecule is attached to theNH3

+ group and another one is associated with the –COO− groupf the ZGly molecule. The third water molecule is bridging the

rst water molecule with the –COO− functional group. This con-

ormer has four hydrogen bonds of lengths 1.733, 1.766, 1.879, and.806 A. The hydrogen bond is stronger in the case where the waterolecule is attached to the H atom of the –NH3

+ group. The hydro-en bond between two water molecules is again stronger compared

1410 1421 14151470 1485 14701630 1622 1620

to that between the water molecule and the –COO− group. Here,two water molecules are attached to the O atom of the –COO−

group with bond lengths, 1.879 and 1.806 A. The BE of this con-former is 13.6 kcal/mol. Thus, among all three optimized structuresof [ZGly + (W)3] using schemes (i)–(iii), the conformer where thethree water molecules are associated with the –NH3

+ group bythree hydrogen bonds of lengths, 1.781, 1.784 and 1.838 A is theenergetically most stable one having a BE of 15.3 kcal/mol; its struc-ture is presented in Fig. 1(D). Therefore, this conformer was usedto calculate the Raman spectrum, which again could be comparedto the experimental Raman spectrum.

Now, we have tried to optimize the [ZGly + (W)4] hydro-gen bonded complexes. Also here, the most stable configurationwas found to be according to scheme (iii) where the fourwater molecules are attached to both functional groups, –NH3

+

and –COO− of the ZGly molecule. Among the different possi-ble [ZGly + (W)4] conformers, the configuration where two watermolecules are bridged through hydrogen bonds with the –COO−

group and the other two water molecules are connected to the–NH3

+ group is found to have the largest BE (18.7 kcal/mol). There-fore, this conformer is the energetically most favored structureamong the [ZGly + (W)4] hydrogen bonded complexes. Two watermolecules act as proton donor, and the other two act as protonacceptor. The strengths of the hydrogen bonds are stronger for thewater molecules attaching to the –NH3

+ group compared to thoseconnected to the –COO− group. The most stable [ZGly + (W)4] con-former is presented in panel (E) of Fig. 1. This conformer was usedto calculate a Raman spectrum.

Finally, also for the [ZGly + (W)5] complexes scheme (iii) yieldedthe energetically best results. Here, many possible conformersof [ZGly + (W)5] were optimized by placing the water moleculesnear to both the –NH3

+ and –COO− functional groups of the ZGlymolecule. Among the different possible [ZGly + (W)5] conform-ers, the structure where two water molecules are attached to the–COO− group and the other three water molecules are attached tothe –NH3

+ group was found to have a maximum BE of 22.8 kcal/mol.Also here, the hydrogen bonds of the water molecules attached tothe –NH3

+ group are stronger compared to those bridging to the–COO− group. In this particular conformer, more water moleculesact as proton donors than as proton acceptors. The most stable[ZGly + (W)5] conformer is presented in Fig. 1(F) and was used tocalculate the Raman spectrum.

5. Calculated Raman spectra of [ZGly + (W)n=1–5] complexes

The ground state geometries of the energetically most favoredstructures of the [ZGly + (W)n=1–5] complexes calculated using DFTat the B3LYP/6-311+G(d) level of theory have been presented inFig. 1(B–F). For these five conformers the Raman spectra have beencalculated using DFT with the B3LYP functional employing the 6-

N. Vyas et al. / Vibrational Spectroscopy 55 (2011) 69–76 75

Table 2Potential energy of the dipole–dipole interaction for the two polar molecules glycine and water calculated using the optimized bond lengths and angles for the respectiveconformers.

[ZGly + (W)n=1–5] Hydrogen bond length (Å) Bond angle (�) Dipole–dipole interactionpotential V/100 (kcal/mol)

[ZGly + (W)] (B) H1· · ·O1 (1.7330) 179.26 −2.16

3eaiTmTo[tciwps

hanlito[teeaTsrOtbe[Rwutf[btuwm

6

l

[ZGly + (W)2] (C) H2· · ·O2 (1.7657)[ZGly + (W)3] (D) H3· · ·O3 (1.7746)[ZGly + (W)4] (E) H4· · ·O4 (1.7250)[ZGly + (W)5] (F) H5· · ·O5 (1.7747)

11++G(d) basis set. These spectra were then compared with thexperimentally obtained Raman spectra of the ZGly molecule inqueous medium for estimating the number of water moleculesnvolved in the hydrogen bond interaction with one ZGly molecule.he experimentally observed wavenumbers of different vibrationalodes of the ZGly molecule in aqueous medium are presented in

able 1 along with the theoretically calculated Raman line positionsf the five most stable conformers, [ZGly + (W)1], [ZGly + (W)2],ZGly + (W)3], [ZGly + (W)4], and [ZGly + (W)5]. It has to be notedhat the DFT calculations yield Raman scattering amplitudes, whichannot be taken directly to be the Raman intensities. The Ramanntensities are proportional to the scattering cross-section, d�/dω,

hich may be calculated from the Raman scattering amplitude andredicted wavenumbers for each normal modes using the relation-hip [51,52]:

d�j

dω= 24�4

45

((�0 − �j)

2

1 − e−hc�jkT

)h

8�2c�jSj (1)

ere, �0 is the exciting frequency, �j is the vibrational frequencynd Sj the corresponding Raman scattering amplitude of the jthormal mode. h, c, and k are the Planck constant, the speed of

ight, and the Boltzmann constant, respectively. The relative Ramanntensities obtained using this relationship match the experimen-ally observed intensities nicely. The simulated Raman spectraf the lowest energy conformers [ZGly + (W)1], [ZGly + (W)3] andZGly + (W)5] along with the experimentally observed Raman spec-ra are shown in panels a–c of Fig. 2. The corresponding lowestnergy structure of the conformer is also shown in an inset. Bothxperimental and simulated Raman spectra, shown in Fig. 2(a–c),re normalized with respect to the most intense peak at ∼899 cm−1.he simulated Raman spectrum of conformer [ZGly + (W)5] pre-ented in Fig. 1(c) is in good accordance with the experimentalesult except for the wavenumber shift for few vibrational bands.ut of 10 experimentally observed Raman bands, eight bands of

he ZGly molecule are matching nicely with the simulated Ramanands corresponding to the optimal [ZGly + (W)5] complex. How-ver, in case of all other four complexes, [ZGly + (W)], [ZGly + (W)2],ZGly + (W)3], and [ZGly + (W)4], the positions and intensities of theaman bands are quite different from the experimentally observedavenumber positions. For a better comparison between the sim-lated and observed wavenumber positions of the Raman bands,he simulated wavenumber positions corresponding to each con-ormer, [ZGly + (W)], [ZGly + (W)2], [ZGly + (W)3], [ZGly + (W)4], andZGly + (W)5], along with the experimentally observed wavenume-er positions have been listed in Table 1. It becomes obvious thathe stability, coordination number of water molecules, and the sim-lated Raman bands strongly point to the fact that the conformer,hich is most likely present in the aqueous solution of the glycineolecule is [ZGly + (W)5].

. Calculations of dipole–dipole interaction potential

It is believed that the dipole–dipole interaction favors the co-inear structures. Considering the different angles and hydrogen

178.19 −2.05175.18 −1.99174.17 −2.17179.26 −2.02

bond distances from the optimized results, the potential energy ofinteraction V between the two polar molecules glycine and wateris calculated using [51–53]

V =(�W ∗ �gly

4�ε0

)∗(

f

r3

)(2)

here, f = 1 − 3cos2�, and �w and �gly are the dipole moment ofwater and ZGly, respectively. Due to the statistical distributionof the freely rotating polar molecules, attractions and repulsionscompensate each other on average. This is expressed by the factthat averaging over angles � results in f = 0. Here we use, for thecalculation of the dipole–dipole interaction potential, the angle cor-responding to the strongest hydrogen bond interactions for the[ZGly + (W)n] conformers, B–F. The calculated values are listed inTable 2. Looking to these results, one finds that the dipole–dipoleinteraction potential energy is higher for the conformer E havinga hydrogen bond length of 1.7250 A and a bond angle of 174.17◦

than for conformer F, which has bond length and angle of 1.7747 Aand 179.26◦, respectively. In general, we find that the values of thedipole–dipole interaction potential more strongly depend on thehydrogen bond length than on bond angle.

7. Conclusions

For the present study, the Raman spectrum of neat glycinemolecules in aqueous medium, where the glycine molecules existin their zwitterionic form, has been recorded. In aqueous medium,the two charged poles, COO− and NH3

+ of zwitterionic glycineare known to form hydrogen bonds with water molecules, whichstabilize the zwitterionic structure. In this context, the groundstate geometries of different hydrogen bonded complexes of[ZGly + (W)n=1–5] have been optimized by DFT calculations at theB3LYP/6-311++G(d) level of theory. Comparing the binding ener-gies to the water molecules, the most stable conformers could beidentified for each complex (n = 1–5). The Raman spectra of thesefive most favorable conformers have been then calculated the-oretically and compared to the experimentally observed Ramanspectrum of glycine in aqueous medium. We found that the theo-retically calculated Raman spectra corresponding to the optimizedconformer of the [ZGly + (W)5] complex is matching best with theexperimentally observed Raman spectrum. Thus, we conclude thatthis conformer is the most probable hydrogen bonded complex ofglycine with water molecules. Thus, our experimental and theoret-ical results show that five water molecules are needed to stabilizethe zwitterionic form of the glycine in aqueous medium.

Acknowledgements

AKO is thankful to the Alexander von Humboldt foundation forthe award of a research fellowship. NV is thankful to MNNIT, Alla-

habad for granting a research fellowship. AKO is also thankful to Dr.Ranjan K. Singh for providing the access of the Gaussian 03 facility.The authors of the manuscript are also thankful to reviewer for pro-viding a list of some important published research articles based onthe subject matter reported in the present manuscript.

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