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J . Am . Chem. SOC.1980, 102 6207-6210 6207

B,(HC = N ) = 118.8’, and cis &( HC N ) = 124.6°.24 Thusit seems clear that the singlet electronic state is the species re-sponsible for the widely recognized extreme re a~ ti vi ty l- ~f me-thylnitrene. In his sense it appears that singlet alkylnitrenes areeven more susceptible to 1,2 hydrogen shifts that th e comparablealkylcarbenes, which have small but finite barriers to isomeri-zation.

Mo re complete levels of theory were also applied to ‘E CH 3Nand ground state CH,=N H, and the results are summarized inTable 11. Th e singlet predictions, at th e unlinked cluster correctedD Z + P C I level of theory, are included with the triplet resultsin Figure 2, an electronic state energy level diag ram . Figure 2illustrates particularly clearly the crossing of the triplet and singletpotential surfaces during the CH 3 N somerization. Should the3A” and ‘A’ statesbe connected sufficiently strongly by spin-orb itcoupling or some other interaction, a crossing from the triplet tosinglet potential energy surface might occur.

Among recent experimental s t u d i e ~ ~ ~ ~ ~esigned to test whether

24) This constrained planar theoretical structure agrees well with ex-periment: R. Pearson andF. J . Lovas, J . Chem. Phys. 66, 4149 1977).However, it should be noted that Pearson and Lovas have shown experimen-tally and B otschwina2’ has shown theoretically th at singlet CH 2N H has a CH,tilt angle of about3O.

25) R. M. Moriarty and R.C. Reardon, Tetrahedron 26, 1379 1970).26) R. A. Abramovitch andE. P. Kyba, J . Am. Chem.SOC. 3, 1537

1971).

27) R. A. Abramovitch and E. P. Kyba,J . Am. Chem.SOC. 6, 4801974).28) A. Pancrazi and Q. Khuong-Huu, Tetrahedron 31 2041, 2049

1975).

alkylnitrenes exist as discrete intermediates, one aspect of thewidely cited2v3w ork of M oria rty an d R e a r d ~ n ~ ~tands in con-tradiction to the present theoretical predictions. Examining th eproducts of photolysis of ten alkyl azides including t he n-butylcompound, Moriarty and Reardon concluded that migration ofthe a substituent occurs synchronously with the departure ofmolecular nitrogen. Th e seemingly inexplicable result of theirexperiments is that the triplet photosensitized process, designedto generatea triplet nitrene intermediate, yielded the sa me productdistribution as the direct photolytic decomposition. In spite ofthe above experimental data, it seems extremely unlikely to usthat the room -temperature triplet 1,2 hydrogen shift of n-butylnitrene could occur with the rate constant suggested,25namely,106 s-1.

This apparen t conflict with the experiments of Moriarty andR e a r d ~ n ~ ~ight be explained if the triplet decomposition of thealkyl azide is concertedor if the “triplet” sensitization actuallyinvolves energy transfer from a singlet, which is not an impossibleoccurrence with polycyclic arom atic sensitizers. Mo riarty andReard on (page 1384, ref 25) note tha t the triplet photosensitizerchrysene may actually yield the singlet state of the azide.

Acknowledgment. This research was supported by theU SNational Science Foundation, Grant C HE 76-22621, and by theRobert A. Welch Foundation.

29) F. C. Montgomery and W . H. Saunder,J . Org. Chem. 41, 236830) E. P. Kyba and R. A. Abramovitch,J . Am. Chem.SOC. 02, 735

1976).

1980).

Ab Initio Self-Consistent Field Calculations on Som e SmallAmino Acids

Lance R. Wright and Raymond F. Borkman*

Contribution fr om the School of Che mistry Georgia Institute of TechnologyAtlanta Georgia 30332. Received A pril 21 1980

Abstract: Ab initio self-consistent ield calculations have been performedon five small amino acids: glycine, alanine, serine,cysteine, and threonine.SCF energies are reported for the nonionic and zwitterionic forms of the molecules usinga split-valence6-31G Gaussian basis set. The ionization potential, proton affinity, and dipole moment were computed for each amino acid.The gaseous zwitterion of each amino acid was calculated tobe 34-43 kcal/m ol less stable than the neutral form. The differencein energy between the ne utral and zwitte rionic forms of the am ino acids was used to de termin e the ene rgy difference betweenthe gaseo us zwitterion and its solid and solvated states. The calculated properties of the neutral amino acids varied slightlyamong the five studied: the average ionization potential was 8.3 0.6 eV and the average proton affinity was222 f kcal/mol.The individual differences are consistent with the inductive effects of th e am ino acid side chains. Comparison between othercomputational and experimental resultsis also given.

Introduction

Ab initio self-consistent field(SCF) calculations have frequentlybeen performedon atom s and small molecules, but only in recentyears have these calculations been extended to larger polyatomicmolecules. This is due in large part to advance s made in the speedand capacity of com puters and increased efficiency in software.A class of molecules now feasible to study a t the a b initio theo-retical level are those of biological interest; amino acids,NH ,CH RC OOH , for example, are ideally suited for this purpose.

Glycine (R= H), the smallest amino acid, has been the subjectof several ab initio calculations. The confo rmation, relative en-ergetics, and S C F energies of glycine and its zwitterion have beencomputed using a 4-31G Gaussian basis set.’> Ryan an d Whitten

(1) Vishveshwara, S.; Pople, J. A. J . A m . C h e m . SOC. 1977, 9 92422-2426.

have studied the na ture of the bonding in the glycine zwitterionand in th e simplest dipeptide, glycylglycine, using both S C F andlimited CI technique^.^Semiempirical molecular orbital theoryhas also been used to study glycine:Oegerle and Sabin haveperformed C N D O calculations to determine the conformation andmolecular propertiesof this amin o acid,4 while Chun g et a l. em-ployed the INDO method to study the protonation of g l y ~ i n eExamples of other theoretical calculationson glycine includeextended Hiickel molecular orbital theory6 and the ab initio

2) Tse, Y. C.; Newton, M.D.; Vishveshwara,S.; Pople, J. A J . A m .

3) Ryan, J . A,; Whitten,J . L. J . Am. Chem.SOC. 972, 94 2396-2400.4) Oegerle, W . R.; Sabin,J . R. J . Mol. Struct . 1973, 15 131-136.5) Chung, K.; Hedges, R. M.; Macfarlane, R . D.J . Am. Chem.Soc. 1976,

6) See for example, Kier, L.B.; George, J.M. Theor. Chim. Acfo1969,

Chem. SOC.1978, 100 4329-4331.

98 7523-7525.

14 258-260.

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6208

Table 1. Calcula ted SC F Energ ies and Dipole Momentsof t h eFive Neut ra l Amin o Acids an d Thei r Zwi t te r ionsin the Gas Phase

neut ra l zwi t te r ion

J . Am. Chem. SOC., ol. 102, No. 20, 1980

dipole d ipo lemo- mo- AE,

amino ment , ment , kca l /ac id energy, au D energy, au D m o l

Wright, Borkman

Bond lengths and angles were taken from the sta nda rd values givenby Pople and Gordon,l' where available. For sulfur, the Following bondlengths were used: S-H, 1.34A and S-C, 1.82A; 8the S2 angle wastaken to be 1 0 9 . 4 7 O . Use of the standard geometric para meter s resultedin the heavy atoms of glycine lying in a plane. By varying the dihe dralangles, Vishveshwara and Pople, using a 4-31 G basis set, calculated themost stable orientations of the neutra l glycine molecule and its zwitter-ion,',* as depicted in structuresI and 11. Calculations carried out using

R H STRUCTURElycine -282.688 47 1 .17 -282 .61 9 95 13.85 42.99a lan ine -321 .710 41 1 .83 -321 .644 56 13 .45 41.32ser ine -396 .517 5 3 2 .22 -396 .462 75 12 .91 34.37cysteine -718.731 37 1.96 -718 .617 59 12 .47 33 .74t h r e o ni n e - 4 3 5 . 5 4 1 1 9 2 . 0 4 -435 .485 47 12 .78 34 .96

The gas-phase d i ffe rence be tween the neu t ra l and zw i t te r ion icamino ac ids,E PA - ) - E A ) , n kca l /mol .

molecular fragment approach.' Glycine was also used as a testmolecule in calculations employing charge-conserving integralapproximations.* Mo re recently, Palla et al. compared the resultsof several theoretical approaches usedin the study of internalrotation in

Calculations on other amino acids are more limited. Ab initioS C Fenergies have been reported for 21 neutral amino acids1 andfor the glycine and serine zwitterions using a contracted Gaussianbasis set with a number of time-saving, approximate integralroutines. Th e results of these calculations were subsequently used

to study the interactio n of water with these amino acids.l .l' Th enonempiricalPRDDO method was employed in the calculationof the electronic struc ture and bonding of several amino acids.I2

Herein are presented results of accu rate, completely ab initioSCFcalculations on five amin o acids: glycine(R = H) , alanine(R = CH,), serine (R = C H 2 0 H ) ,cysteine(R = C H 2 S H ) ,andthreonine(R = C H , C H O H ) . SCF energies and dipole momentsar e given for both th e nonionic and zwitterionic formsof themolecules; ionization potentials and proton affinities are comp utedfor the neutral amino acids. Estimates of the energy required forthe transitionof the solid amino acid to the gaseous zwitterionand the energy released when the gaseous zwitterionis solvatedare also given.

There are currently few experimental results in the literaturefor amin o acids larger than glycine. Glycine has, however, beenwidely studied. Th us, to assess the accuracy of the calculated

properties of the larger amino acids studied here, glycine wasinvestigated in order to compare with the available experimentalresults.

Method of CalculationAll calculations were performed on a Control Data Cyber 70, Model

74, computer. Th e calculations employed theHONDO (Version 5.0)progra m w ritten by Dupuis et aI.l3 using the restricted S C F techniquesdeveloped by R00th aan .l~A split-valence 6-31G Gaussian basis set wasused for all atom s with the exception of sulfur and hydrogen, wh ere the4-31G and 31G basis sets were used, re~pective1y.l~o determine thequality of the 6-31G basis set, test calculations were performed onN,,CO , and H,CO; these calculations gave results within0.13 of theHartre e-Foc k energies of these molecules.I6 It is believed tha t theenergies reported for the five amino acids in the present work a re ofcomparable quali ty.

(7) Shipman, L. L.; Christoffersen,R. E. Theor. Chim. Acta 1973, 31,

8 ) Wilhite, D.L.; Euwema, R.N . Chem. Phys. Lett. 1973, 20, 610-614.(9) Palla, P.; Petrongolo,C.; Tomasi, J . J Phys. Chem. 1980,84,435-442.(10) Clementi,E.; Cavallone, F.; Scordamaglia, R.J . A m . C h e m .SOC

(1 1) Carozzo, L.; Corong iu, C.; Petrongolo, C .; Clementi,E. J . Chem.

(12) Dixon,D. A,; Lipscomb,W. N. J . Bioi. Chem. 1976,251, 5992-6000.(13) See, for example, Dupuis,M.; Rys, J.; King, H. F. J . Chem . Phys.

(14) Roothaan, C. C.J . Rec. Mod . Phys. 1951, 23, 69-185.(15) Hehre,W . J.; Ditchfield, R.; Pople,J . A . J . Chem. Phys. 1972, 56,

2257-2261. Ditchfield, R.; Heh re,W. J.;Pople,J. A . Ibid. 1971, 54, 724-728.(1 6) For N2 and CO , Hartree-Fock energies were taken from Christiansen,

P. A.; McCullough, E. A J . Chem. Phys. 1977 ,67 ,187771882, For H 2 C 0 ,the Hartree-Fock energy was taken from Garrison,B. J.; Schaefer, H. F.;Lester, W. A J . Chem . Phys. 1974, 61 3039-3042.

75-82.

1977, 99, 5531-5545.

Phys. 1978, 68, 787-793.

1976, 1-1 16.

c

STRUCTUREI

I II

Ithe 4-21GI9 as well as the present 6-31G basis sets concurred with theseresults. These conformations, therefore, were adop ted for the otheramin o acids studied. Use of the stand ard bond lengths and angles re-

sulted in the heavy atoms of the R-g rou p side chainsalso being planar.The side chains were chosen to be oriented in a plane parallel to tha t ofthe glycine fragment in an effort to minimize steric interactions. Thisapproach w as adopted as it was felt that a complete study of the rotationbarriers of each of the am ino acids would be prohibitively expensive. Th ecalculations consumed from 4200CPU seconds per S C F energy forglycine to over 300 00 CP U seconds per S C F energyfor threonine.

Convergence of the S C F iterative cycles has been a considerableproblem in previous calculationson polyatomic molecules. Som e recentrefinements made in computer programs are directly related to thisproblem.' By properly choosing the initial guess input vectors, however,one can effectively avoid most difficulties in convergence. Thi s has beenthe case in eachof the amino acids studied here.Using a lan ine as anexample, appropriate vectors from calculations on glycine and meth anewere combined an d used as the initial guess coefficients. Th e glycinevectors were, i n turn. generated from calculations which used methyl-amin e and formic acid vectors as starting coefficients. On ce obtained,

these vectors were usedfor the calculation of the zwitterion as wellasfor the singly positive and protonated species.In this way, the numberof iterations required to achieve convergence was typically reduced by

ResultsThe SCF energies for the neutral and zwitterionic forms of the

amino acids are presented in T ableI along with the com puteddipole mom ents. For each of th e five amino acids, the gaseouszwitterion was calculated to be34-43 kcal/mol less stable thanthe neutral molecule. A comparison of the energies of glycine

30-50%.

~ ~~

(17) Pople,J. A , ; Gordon,M . J . Am . Chem.SOC 967, 89, 4253-4261.(18) Huheey,J. E. Inorganic Chemistry ; Harper and Row: New York,

(19) Sellers,H. L.; Schafer,L. J . Am . Chem.SOC. 978, 100, 7728-1729.1972; pp 691-702.

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SCF Calculations on Small Amino Acids J . A m . Chem. SOC., ol 102, No. 20, 1980 6209

Table V. Calculated SCF Proton Affinities for the Protonationof the Nitrogen Atom o f the Neutral Amino Acids

Table 11. Comparison of Previous Calculatio ns on Glycine andthe Glycine Zwitterion with Present Results

energy,aubasis set orglycine calcn meth od used neutral zwitterion

PRDDO -281.721L”CCS/CCPCb.C

WEd

sseRW (SCF)fRW (CI)fV P / T N V Ppresent workHartree-Fock

(estd)’

contracted -281.779 -281.684Gaussian

integral approx-imations

charge conserving -282.119 23

4-21G -282.158 053 41P -282.120 5

-282.168 94-31G -282.400 I 1 -282.354 246-31G -282.688 41 -282.619 95

-283.05 -282.98

Reference 12. Reference 10. Reference11. Reference8. Reference 19. Reference 3. Reference1. Reference2. This estimate is basedon the results of calculations on mole-cules whose Hartree-Fock energies are known;see text.

Table111. Calculated SCF Rotatio n B arriersof the GlycineMolecule, Relative to the Most Stable Orientation . Comparisonbetween t he Results of the 4 -31G and 6-31 G Basis Set Calculations

4-31G,b 6-31GConformationa angle, deg

0 kcal/mol kcal/mol

0 0 0 0.0 0.0180 0 0 8.1 8.2

0 180 0 2.6 2.1180 180 0 1.5 1.9180 180 18 0 2.2 2.5

The geometric parameters are defined in structureI. Refe-rence 1. Present work.

Table IV Calculated SCF Ionization Potentials forthe Amino Acids

neutral zwitterion

direct,a Koopmans’bpc Koo pma nsfbamino acid eV theorem, eV theorem, eV

glycine 8.61 10.15 (10.0) 8.77alanine 8.46 10.09 (9.8) 8.11serine 8.76 10.33 9.51cy st eine 8.00 9.41 8.88threonine d 10.23 8.69

a Calculated ionization potential from E(A)- E(A+). Ioniza-tion potential com puted using Koopmans’ theorem and the “8%rule”. Experim ental values in parenthese s from ref 25. 0mitted becauseof excessive computation time required.

an d its zwitterion calculated using a 6 -31G basis set with theresults of previous calculationsis summarized in Table11. T h epresent S C F energy for glycine was 8-25 eV lower than previoustheoretical calculations and was estimated to lie about10 eV abovethe Hartree-Fock limit (see Table 11).

A number of glycine conformations were studied using the6-31G basis set to determin e the most stable orientation of theneutral molecule. A summ ary of the calculated values of therotatio n barriers is presented in Tabl e 111. The dihedral angles0, 4, and I are defined in structureI; the most stable orientationis the one in which0 = 4 = = 0’ The results of these cal-culations were within 0.5 kcal/mol to those of Vishveshwara Pople,who used th e smaller 4-31G basis set.’

Th e first vertical ionization potential for each amino ac id ispresented in Tabl e IV. Th e ionization potential was determinedin two ways: first as the difference betweenSCF calculations onthe n eutra l and th e singly positive amin o acid, and second by usingKoopmans’ theorem*O an d th e empirically de termin ed “8% rule”.*’

2 0 ) Koopmans,T. Physica 1934, I 104 1 13.

proton affinity,a internuclearamino acid kcal/mo l distance,A

glycine 222.30 (208 .2) 1.012alanine 225.78 (212.2) 0.987serine 219.99 1.017cysteine 219.76 1.011threonine 221.04 1.014

a Experim ental values in parentheses from ref 2 4. The dis-tance between the proton (tetrahedral approach) and the nitrogenatom of th e neutral amino acid.

Table VI. Calculated SCF Energy D ifferences between th eGaseous Zwitterions and Their Solid and Solvated States”

AE, cal/mol

amino acid [ y s ) [+A-(,)zwitterion A-WI +A-a q Iglycine 66.0 4 2 . 2alanine 66.3 -64.5serine 54.5 -49.2cysteine 56.7 -51.2threonine 58.0 b

Determined by combining the calculated neutral and zwitterionenergies in Tab leI with the experimentaldata ofre f 22.b AH ^^^,.,)for threonine was not given.

Th e two methods predicted the sam e trend in ionization potentials,but differed in each case by about 1.55 eV, with the directlycalculated ionization potential being the smaller of the two. Th eionization potentials of the zwitterions were computed using onlythe Koopmans’ theorem/‘%% rule” approach . These ionizationpotentials, included in Table IV, are seen to be lower than thevalues for the corresponding neutra l molecules by an av erageof1.1 eV.

Reported in Table V is the minimumSCF energy and theequilibrium in ternuclear distance for the tetrah edral approach ofa proton to the nitrogen atom of each neutral amin o acid. Thes einteraction energies were calculated at 3-4 internuclear distances.Th e proton affinities were then determined from the m inima in

curves fit smoothly throu gh these computed points. Th e protonaffinities were remarkably constant for the five amino acidsstudied, with an average value of 222 4 kcal/mol. Th e energybarrier to rotation of the protonated ami ne group about the C-Naxis of glycine was calculated to be less than 0.2 kcal /mol. Thisbarrier was not calculated for the other four am ino acids.

Th e energy associated with the transition of th e solid aminoacid to the gaseous zwitterion,[ fA-(s) A- ] and the heatevolved when this gaseous species is solvated,[ A (gj A-(aqj],have been estimated by combining the present calculated energiesof the neutral and zwitterionic amin o acids with the experimentaldat a of Gaffney et aLzz These estimated energies are given inTable VI. The values for glycine and alanine are noted to bemuch larger (abou t 20%) than thosefor the other am ino acids.

Discussion

The use of the 6-31G basis set has resulted in a significantimprovement in the SC F energies calculated for the amino acidscompared to the other split-valence basis sets previously used,without increasing the total number of basis functions. Th eSCFenergy reported for glycine using a 4-21G basis set, -282.15805au,19 s about 6.6 eV higher than the result computed with a 4-31Gbasis set, -282.40077 au.’ Th e 6-31G basis set used in the presentwork lowered this energy by an add itional7 . 8 eV (see Tabl e11).This energy improvement did not come a t the costof a substantialincrease in computatio n time. Calculations on glycine comp aringthe 4-31G and 6-31G basis sets showed only a6% increase in

( 2 1 ) Brundle, C. R.; Robin, M . B . ; Busch, H.J . Chem. Phys. 1970, 53,

(22) Gaffney, J. S.; Pierce, R . C.; Friedman, L. J . A m Chem. SOC. 1977,

p j

21 96-221 3.

99, 4293-4298.

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ZWlTTERlON 4)

Wright Borkman

IV). The work of Carozzo et al. has shown tha t the largestinteraction between a water molecule and a n amino acid zwitterionoccurs at the carboxylate group, with hydrogen bonds formingbetween the carboxylate oxygen atoms and the hydrogens ofwater. With the total negative charge of the carboxylate groupreduced owing to the presence of the electron-withdrawing sidechains, the attraction between the amino acid and the watermolecule is not as great, resulting in lower solvation energies.Conversely, the methyl gro up of alanine donates its electrons tothe am ino acid backbone, increasing the attraction of w ater toth e zwitterion relative to glycine. These conclusions are consistentwith the estimated energies of solvation reported in Table IV.

The experimental proton affinities determined from massspectrometry for the neutra l glycine and alanine molecules in thegas phase , 208 .2 and 2 12.2 k c a l / m ~ l , ~ ~espectively, are consistentwith the calculated values of 222.3 and 225.8 kcal/mol, respec-tively (see Tab le V). Again , the inductive effect of the side chainis expected to affect the proton affinity. Th e electron-donatingmethyl group of alanine makes the nitrogen atoms more attractiv eto the proton, resulting in a more stable protonated species. Th eelectron-withdrawing groupsof the side chain s of serine, cysteine,and threonine have the opposite effect by decreasing the basicityof the amin o acid relative to that of glycine, as indicated by theresults in Tab le V. Unfortun ately, no experime ntal values ar eavailable for comparison. The equilibrium distance between theapproaching proton and the nitrogen ato ms for each of the amino

acids studied (see Table V) is nearly equal to the stand ard N-Hbond length of 1.01A . 1 7This suggests that there exists an equalcharge density distribution over the threeN-H bonds of theprotonated amin e group.

Debies and R abalais, using photoelectron spectroscopy, havedetermine d the first vertical ionization potentials of glycine andalanine to be 10 .0 and 9.8 eV, respectively; he directly calculatedionization potentials for these amino acids, 8.61 and 8.46 eV,respectively, are therefore in error by about14 . The ionizationpotentials calculated using Koo pmans' theorem and th e8% rule'',10.15 and 10.09 eV, respectively, however, agree w ithin 3% ofthe experimental values. Thu s, the ionization potentials for theother amino acids are expected to be most reliably predicted byusing Koopmans' theorem, Le., 10.33, 9.47, and 10.23 eV, forserine, cysteine, and threonine, respectively.No experimentalvalues for these amino acids are curren tly available for comparison.Mulliken population analyses of th e singly positive amin o acidsshowed that th e ionized electron originated from the amin e groupof all of the amino acids with the exception of cysteine, whereit originated from th e sulfhydral group. These results are con-sistent with the conclusions of Debies and Rabalais who foundtha t the m ost loosely bound electrons of glycine and alanine a rethose of the nonbonding nitrogen orbitaLZ5

The zwitterions of the am ino acids are expected to have a muchlarger dipole moment than the neutral species. This was the casefor each amino acid studied (see Table I). As the experimentaldipole moments of the gaseous amino acids have not been reliablydetermined , a comparison here is difficult. One report suggeststha t the experimental dipole moment of glycine in the gas phaseis actually determined by a mixture of a t least two conformationso f g1 y~ in e.I ~ he reasoning here is based on th e small barriers

to rotation in the neutral species (for some examples, see Table111). In addition, since the dipole moments appear to be highlybasis set dependent, comparison among the other computationalmetho ds is equally difficult. Thus, the gas-ph ase dipole momentsof the amino acids must still be regarded as highly uncertain.

Acknowledgment, The authors gratefully acknowledge theassistance of the Georgia Tech Office of Computing Servicesduring the course of this work.

I1 NEUTRAL( 9 )

/

Figure 1. Energy level diagram for the transitions of the solid aminoacids to the gaseous and solvated states. The experimen tal enthalpies,AHsUb HH nd AHsoIy ere taken from ref 24. The difference inenergy between the gaseous neutral and zwitterionic amino acids,A Ehas been combined with th e experimental enthalpies to estimateaE2 ndA E 3

computation time. As indicated previously, the S C F energiesobtained for glycine and the other amino acids in Table I usinga 6 -31G basis set are believed to be within 0.13% of the H ar-tree-Fock energies.

The difference in energy between glycine and the glycinezwitterion has been the subject of much interest? In the crystallinestate and in solution, glycine normally exists as a zwitterion incontrast to the neutral form preferred in the gas phase. Thecalculated energy differences between the gas-phase neutral a ndzwitterion species vary considerably am ong th e various theoreticalpredictions. Th e semiempirical methods result in differencesof101: 73,23 n d 675 kcal/m ol, with the neutral species being morestable. Ab initio calculations report smaller energy differencesbetween the two species of 57109*1nd 292 kcal/mol, while thepresent work puts this value at43 kcal/mol. Again, the neutralmolecule is predicted to have the lower energy. Th e difference

in energy between the gaseous neutral a nd zwitterionic amino acidshas not been experimentally determined, but th e calculated energydifferences can be used together with the experimental heats ofsublimation and heats of solution to provide estimates of theenergies associated with th e transition of the solid amino acid tothe gaseous zwitterion, ['A-,,, A-Q~], and the solvation energyof the gaseous zwitterion, ['A-(,) A- as)]. These quantitiesare depicted in Figu re 1 and the estimates for each amino acidstudied are reported in Tab le VI. Th e predicted energy differencebetween th e gaseous glycine zwitterion and the solvated ion, -62.2kcal/mol, is about 14 kcal/mol larger than that estimated byGaffney et aLZ2 sing the calculated results of Tse et aL2Noprevious estimates are available for comparison for the other am inoacids studied here.

Th e zwitterions of th e am ino acids with polar side chains (serine,cysteine, and threonine) were ab out8 kcal/mol more stable thanthose with nonpolar R groups (glycine and alanine), relative tothe neutral m olecules (see TableI). Th e hydroxyl groups of serineand threonine and the sulfhy dral group of cysteine apparently serveto distribute the charge of the zwitterion overa greater volume,thu s stabilizing the entire molecule. This effect can also beobserved to a lesser extent in the computed dipole moments; thedipole moments given in Table I of the glycine and alaninezwitterions are larger than those of the other three amino acids,presumably owing to the greater charge separation. This chargestabilization also has an effect on the solva tion energies (see Table

(23) Imamura,A.; Fujita, H.; Nogata, C. Bull. Chem. SOC. pn. 1969,3 118-3 123.

(24) Meot-Ner,M.; Hunter, E. P.; Field, F. H. J . A m . Chem. SOC. 979,

(25) Debies, T P.: Rabalais, J. W . J . Electron Spectrosc. 1974, 3IOl 686-689.

315-322.