Vibrational spectra of acid and alkaline glycine salts

20 Pages·1998·0.44 MB·English

VibrationalSpectroscopy16(cid:14)1998.35–54
Vibrational spectra of acid and alkaline glycine salts
Ma´rio T. Rosado a, Maria Leonor T.S. Duarte a, Rui Fausto b,)
a DepartamentodeQu´ımicaeBioqu´ımica,FaculdadedeCieˆnciasdaUni˝ersidadedeLisboa,Ed.C1-58,CampoGrande,1700Lisbon,
Portugal
bDepartamentodeQu´ımica,Uni˝ersidadedeCoimbra,3049Coimbra,Portugal
Received5September1997;accepted5December1997
Abstract
The infrared and Raman spectra of crystalline acid (cid:14)qNH CH COOHPCly. and alkaline (cid:14)NaqPNH CH COOy.
3 2 2 2
glycine salts were recorded and interpreted. The assignments were confirmed by comparison with the infrared and Raman
spectraofcrystallineglycine(cid:14)zwitterion,qNH CH COOy.andtheinfraredspectrumofthismoleculeisolatedinanargon
3 2
matrixat lowtemperature(cid:14)10 K. (cid:14)neutralform, NH CH COOH.. Furtherinsighton the featuresof the vibrationalspectra
2 2
of these substances was achieved by ab initio (cid:14)H)Fr6-31G . frequency and intensity calculations and potential energy
distributioncalculationsresultingfromnormalcoordinateanalysis. q1998ElsevierScienceB.V.
Keywords: Glycine (cid:14)neutral and zwitterionic forms.; Acid and alkaline glycine salts; Vibrational (cid:14)IR and Raman. spectra; Ab initio
predictionofvibrationalspectra
w x
1. Introduction 10,11, have previously been studied, although the
main focus has been on the zwitterion, due to its
Glycine is the simplest protein structural unit and
greater biological importance.
thesmallestaminoacidthatcanactasneuroinhibitor
w x The assignment of bands in the infrared spectrum
in mammalian central nervous systems 1,2. Since
of crystalline glycine (cid:14)zwitterion. presents difficul-
vibrational spectroscopy can provide the key to elu-
ties, due to the presence of some regions with very
cidation of both structural and dynamical properties
broad and structured bands (cid:14)e.g. nOH, nNH and
andcanbeusedtoestablishfundamentalcorrelations
nCHstretchingregions..Thestudyofthevibrational
between these and the chemical or biochemical be-
spectra of glycine alkaline and acid salts appeared to
haviour of these substances, considerable effort has
be a promising way of understanding complex fea-
been dedicated to such studies on glycine under
various experimental conditions w2–12x. Interesting tures of the zwitterion spectra, since -NH2, -COOy
and -COOH, -NHq groups are present in alkaline
vibrationalfeaturesoftheinfraredspectraofglycine, 3
wx and acid salts, respectively. However, glycine salts
both in the zwitterionic 2–9 and neutral forms
have not previously received much attention.
Williams et al. tried to correlate vibrational spec-
w x
)Correspondingauthor. Tel.: q351-39-22826;fax: q351-39- tra of the salts with ab initio calculations 12. How-
27703;e-mail:[email protected]. ever, these focused only on spectra in aqueous solu-
0924-2031r98r$19.00q1998ElsevierScienceB.V.Allrightsreserved.
PII S0924-2031(cid:14)97.00050-7
36 M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54
tion and the calculations were performed with a modification of this compound w14x (cid:14)glycine crystal-
relatively modest basis set (cid:14)4-31G.. lizesinthreedifferentmodifications,the a, bandg
In this work, FT-IR and Raman spectra of crys- forms w2x, the a-form corresponding to the stable
talline glycine salts were assigned with the help of modification at room temperature.. For zwitterionic
polarized wavefunction double-z HFr6-31G) ab glycine, the infrared and Raman spectra of the crys-
initio vibrational frequency calculations and normal tal (cid:14)a-modification. were obtained, while the neutral
coordinate analysis. All calculations were applied to form was studied by matrix isolation infrared spec-
the most stable conformer of each molecule studied troscopy (cid:14)MI-IR., in Argon, at 10 K.
(cid:14)Fig. 1. and, since the present studies used the
crystalline compounds, the cyclic dimer of the acid
salt (cid:14)see Fig. 1. was also considered. 2. Experimental and computational methods
The assignment of vibrational spectra was helped
byextensivecomparisonwiththespectraofzwitteri- a-Glycine p.a. q98% was obtained from Merck.
onic and neutral glycine. The zwitterionic form of The alkaline salt, the glycinate (cid:14)NaqP
glycineconstitutesauniquesituation,becauseitdoes NH CH COOy., was prepared by dissolving com-
2 2
not correspond to a minimum in the potential energy mercialglycinep.a. ina saturatedsolutionof NaOH.
surface of the isolated molecule w13x. The calcula- The solution (cid:14)pH)12. was dried in vacuum until
tions were carried out using the basic molecular complete solvent evaporation, and the crystals
geometry previously determined by neutron diffrac- washed with methanol. The acid glycine salt
tion for an individual molecule in the a-crystalline (cid:14)qNH CH COOHPCly. was prepared by dissolu-
3 2
tion of the amino acid in concentrated HCl solution
(cid:14)33%. and the resultingsolvent(cid:14)pH-3. was evapo-
rated as for the alkaline salt.
Fourier transform infrared (cid:14)FT-IR. spectra of
crystalline materials as potassium bromide pellets
were recorded with a Perkin Elmer PE 1760 FT-IR
spectrometer, in the 4000–400 cmy1 region, with a
resolution of 2 cmy1.
Raman spectra were obtained using a SPEX 1403
double monochromator spectrometer (cid:14)focal distance
0.85 m, aperture fr7.8., equipped with holographic
gratings with 1800 grooves mmy1 (cid:14)Ref. 1800-
1SHD.. The 514.5 nm argon laser (cid:14)Spectra-Physics,
model 164-05. line, adjusted to provide 220 mW
power at the sample, was used as excitation radia-
tion. Detection was effected using a thermoelectri-
cally cooled Hamamatsu R928 photomultiplier.
Spectra were recorded using increments of 1 cmy1
and integration times of 1 s. Under these conditions,
the estimated errors in wavenumbers are "1 cmy1.
The matrix isolation spectrum (cid:14)MI-IR. of the
neutral form of glycine was recorded with a disper-
sive Specord IR75 (cid:14)updated. infrared spectrometer
and a resolution of 0.2 cmy1. The complete descrip-
tion of the set-up used for these experiments is
w x
described elsewhere 10.
Fig. 1. Molecular structures of the various systems studied: (cid:14)a.
Ab initio calculations were performed with a PC
zwitterionicglycine;(cid:14)b.neutralglycine;(cid:14)c.glycinealkalinesalt;
(cid:14)d.glycineacidsalt(cid:14)monomer.and(cid:14)e.glycineacidsalt(cid:14)dimer.. equipped with a Am486DX4r100 MHz processor,
M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54 37
using Gaussian 92 for WindowsrRevision G-3 w15x. intensities, their potential energy distribution and
These calculations were executed at the Hartree– proposed assignments are presented in Tables 2–5.
Fock level of theory with the 6-31G) basis set w16x. Ingeneral,theassignmentsmadeforglycine(cid:14)both
Molecular geometries were fully optimized by the neutral and zwitterionic form. agree with those pre-
force gradient method using Berny’s algorithm w17x. viously proposed w2–9x and, together with the results
The largest residual coordinate forces were always of the calculations, were used as a basis for assign-
less than 3=10y4 hartree bohry1 (cid:14)1 hartrees ment of the spectra of the two salts. The detailed
2625.5001 kJ moly1; 1 bohrs5.29177=10y11 m. analysis and discussion of the results will be divided
or hartree rady1, for bond stretches and angle bends, into sections, corresponding approximately to differ-
respectively. The stopping criterion for the SCF iter- ent spectral regions.
ative process required a density matrix convergence
of less than 10y8.
The force constants (cid:14)symmetry internal coordi- 3.1. Region abo˝e 2000 cmy1
nates. used in the normal coordinate analysis were
obtained from the ab initio Cartesian harmonic force Only neutral glycine and its acid salt contain the
constants using the program TRANSFORMER (cid:14)ver- OH group. In the MI-IRspectrumof neutralglycine,
sion2.0.w18x.Thisprogramwasalsousedtoprepare as consequence of the isolated status in the Ar
input data for the normal coordinate analysis pro- matrix, the OH stretching mode appears as a sharp
grams: BUILD-G and VIBRAT (cid:14)Refs. w19x 1 and band at 3560 cmy1. In the infrared spectrum of the
w20x., which enabled the calculation of the potential acid salt, this appears as a broad band centered at
energy distribution (cid:14)PED. of each of the normal 3439 cmy1. In this case, the band shape clearly
modes of vibration. shows the effect of association and hydrogen bond-
The ab initio calculated force fields were scaled ing. It is interesting to note that the calculated value
down using a single scale factor (cid:14)0.89., which is for the frequency of this mode for a monomer in the
acceptedto be thebestfor the Hartree–Focklevelof acid salt calculations (cid:14)3558 cmy1; see Table 5. is
w x
calculations 21. This very simple scaling preserves considerablyhigherthanthe experimentalone, while
the potential energy distributions as they emerge thecalculatedvalueforthedimer(cid:14)3437cmy1;Table
from the ab initio calculation and has an important 6. is in good agreement.
advantage over more elaborate force field scaling Assignment of the NH and CH stretching modes
procedures that use more than one scale factor and in the neutral glycine infrared spectrum is straight-
usually gives rise to marked PED distortions from forward, since these give rise to bands in a clear
the ab initio calculated values. spectral region. In the alkaline salt, n CH and
as 2
nCH are observed at 2945 and 2927 cmy1, in the
s 2
infrared spectrum and give rise to strong Raman
3. Results and discussion bands at 2941 and 2911 cmy1. Calculations on this
molecule suggest the nNH bands should appear at
2
The optimized molecular geometries for the low- lower frequencies than in isolated neutral glycine
est energy conformation of the various molecules (cid:14)seeTables3and4.. Thus,thesemodesareassigned
studiedareshowninTable1.Thevibrationalspectra to the two broad medium intensity bands at 3325
of neutral and zwitterionic glycine (cid:14)infrared and cmy1 (cid:14)n NH . and 3212 cmy1 (cid:14)nNH . in the
as 2 s 2
Raman. are shown in Figs. 2–4 and those of the Raman spectrum of this compound. In the corre-
alkaline and acid salts are presented in Figs. 5–8. sponding infrared spectrum, the nNH vibrations
2
Experimental and calculated vibrational frequencies, give rise to the very broad feature from ca. 3000 to
3400 cmy1. The two infrared bands at 3439 and
3391 cmy1 are at too high frequencies for these
modes and are attributed to the first overtone of the
1These programs incorporate several routines from programs nasCOOy stretchingfundamental,whichappearsasa
GMATandFPERTw19x. doublet at 1597 and 1561 cmy1.
38 M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54
The infrared spectra of both crystalline glycine broadening and complex structure of the nNH
3
and its acid salt show a very broad and structured stretching region in the infrared spectrum of crys-
band spanning from 3400 to 2400 cmy1. This is talline amino acids has been extensively discussed
w x w x
typicalofcrystallineaminoacids 5,22 andhasbeen 5,23–26.Althoughstillcontroversial,itisgenerally
ascribed to NH stretching modes, which sometimes considered that the observed band contour results
3
obscure the CH stretching bands. In the Raman essentiallyfrom Fermi resonancebetweenthe nNH
2 3
spectra, the nCH modes increase and the nNH stretching modes and combination bands involving
2 3
modes decrease in intensity, which allows a clear mainly the dNH bending vibrations w5x. This inter-
3
observation of the first. The origin of the extensive pretation is reinforced by the present results. Al-
Table1
Calculated(cid:14)abinitio6-31G).optimizedmoleculargeometriesa
Coordinate Glycine(cid:14)neutralform. Glycineb(cid:14)zwitterion. Alkalinesalt Acidsalt(cid:14)monomer. Acidsalt(cid:14)dimer.
C5O 1.188 1.233 1.233 1.184 1.197
C–C 1.515 1.553 1.553 1.517 1.515
C–O 1.330 1.237 1.237 1.301 1.289
C–N 1.439 1.496 1.463 1.496 1.495
O–H 0.952 0.957 0.964
C–H 1.085 1.080 1.086 1.080 1.081
1.085 1.080 1.089 1.080 1.081
N–H 1.002 1.012 1.005 1.012 1.013
op
1.002 1.012 1.005 1.012 1.013
N–H 1.011 1.011 1.013
ip
H...O5 1.948
C–C–O 111.8 117.5 115.6 111.4 111.6
C–C5O 125.4 117.1 116.7 120.8 121.1
O5C–O 122.8 125.4 129.7 127.8 127.2
N–C–C 115.0 111.9 115.7 106.8 108.6
C–O–H 108.4 111.3 112.4
C–C–H 107.7 108.8 109.0 111.4 110.7
107.7 110.5 107.4 111.4 110.7
C–N–H 110.4 110.4 104.6 110.9 111.6
op
110.4 111.7 106.9 110.9 111.6
C–N–H 112.1 112.2 111.5
ip
O–H...O5 170.3
C5O...H 130.1
C–C(cid:14)5O.–O 180.0 180.0 178.2 180.0 180.0
N–C–C5O 0.0 18.9 11.8 0.0 0.0
O5C–O–H 0.0 0.0 0.0
H–C–C5O y123.0 y101.0 y107.9 y118.8 y119.5
123.0 140.6 133.7 118.8 119.5
H –N–C–C y58.6 y58.1 y37.7 y58.5 y59.3
op
H –N–C–C y177.3 180.0 180.0
ip
aBondlengthsinA˚,anglesindegrees;geometriesrefertothemoststableconformerintheRHFr6-31G) PESofeachmoleculestudied.
bZwitterionicformofglycinedoesnotcorrespondtoaminimuminthePESofthismoleculew13x.Thegeometryherepresentedandusedto
undertakethe vibrationalcalculationswas built as follows:(cid:14)i. valenceangles and dihedralswere taken from the neutrondiffractiondata
relativetocrystallineglycine(cid:14)a-form.w14x;(cid:14)ii.bondlengthsweretransferredfromthe6-31G) optimizedstructuresherecalculatedforthe
alkalinesalt(cid:14)COOy group.andacidsalt(cid:14)NHq group.. Thecalculatedbondlengthsobtainedforthetwosaltswereusedinsteadofthe
3
experimentaldataobtainedfromneutrondiffractionsinceotherwiseitwouldnotbepossibletoadoptthesingleforcefieldscalingfactor
usedinthisstudytofitthecalculatedfrequenciestotheexperimentalspectraalsoforthezwitterionicformofglycine.Indeed,thescale
factorrequires6-31G) consistentdata,inparticularinthecaseofbondlengths,whoseparticularvaluesstronglyinfluencethecalculated
frequencies.
M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54 39
Fig.2.Infraredspectraofneutralformofglycine:(cid:14)a.experimental(cid:14)MI-IRspectrumofglycineisolatedinanArmatrixat10K.and(cid:14)b.
6-31G) predictedinfraredspectrum(cid:14)scaled..
40 M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54
Fig. 3. Infrared spectra of zwitterionic glycine: (cid:14)a. experimental (cid:14)FT-IR spectrum of crystalline glycine at room temperature. and (cid:14)b.
6-31G) predictedinfraredspectrum(cid:14)scaled..
M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54 41
Fig. 4. Raman spectra of zwitterionic glycine: (cid:14)a. experimental (cid:14)Raman spectrum of crystalline glycine at room temperature. and (cid:14)b.
6-31G) predictedRamanspectrum(cid:14)scaled..
42 M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54
Fig.5.Infraredspectraofglycinealkalinesalt:(cid:14)a.experimental(cid:14)FT-IRspectrumofcrystallineNaqPNH CH COOyatroomtemperature.
2 2
and(cid:14)b.6-31G) predictedinfraredspectrum(cid:14)scaled..
M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54 43
Fig.6.Ramanspectraofglycinealkalinesalt:(cid:14)a.experimental(cid:14)RamanspectrumofcrystallineNaqPNH CH COOyatroomtemperature.
2 2
and(cid:14)b.6-31G) predictedRamanspectrum(cid:14)scaled..
44 M.T.Rosadoetal.rVibrationalSpectroscopy16(1998)35–54
Fig.7.Infraredspectraofglycineacidsalt:(cid:14)a.experimental(cid:14)FT-IRspectrumofcrystallineqNH CH COOHPCly atroomtemperature.
3 2
and(cid:14)b.6-31G) predictedinfraredspectrum(cid:14)dimer‘model’;scaled..