Vibrational spectra of acid and alkaline glycine salts

TL;DR: In this article, the infrared and Raman spectra of crystalline acid NH CH COOHP Cl and alkaline Na P NHCH COO 32 2 2 glycine salts were recorded and interpreted.

About: This article is published in Vibrational Spectroscopy.The article was published on 1998-03-01 and is currently open access. It has received 142 citations till now. The article focuses on the topics: Zwitterion & Raman spectroscopy.

Summary

1. Introduction

• Glycine is the simplest protein structural unit and the smallest amino acid that can act as neuroinhibitor w xin mammalian central nervous systems 1,2 .
• Interesting vibrational features of the infrared spectra of glycine, w xboth in the zwitterionic 2–9 and neutral forms ) Corresponding author.
• Glycine salts have not previously received much attention.
• The calculations were carried out using the basic molecular geometry previously determined by neutron diffraction for an individual molecule in the a-crystalline Ž .

2. Experimental and computational methods

• A-Glycine p.a. q98% was obtained from Merck.
• Ž .The solution pH)12 was dried in vacuum until complete solvent evaporation, and the crystals washed with methanol.
• Molecular geometries were fully optimized by the w xforce gradient method using Berny’s algorithm 17 .
• ŽThe force constants symmetry internal coordi.nates used in the normal coordinate analysis were obtained from the ab initio Cartesian harmonic force Žconstants using the program TRANSFORMER ver. w xsion 2.0 18 .

3. Results and discussion

• The optimized molecular geometries for the lowest energy conformation of the various molecules studied are shown in Table 1.
• ŽIn general, the assignments made for glycine both .neutral and zwitterionic form agree with those prew xviously proposed 2–9 and, together with the results of the calculations, were used as a basis for assignment of the spectra of the two salts.
• When only one internal mode in the PED have a contribution )10%, its value is ommited.
• As with the n C5O mode, the calculations also overestimate the frequency of the antisymmetrical COOy stretching mode of the zwitterionic form and of the alkaline salt, while they underestimate the frequencies of the symmetrical COOy stretching Ž .vibration see Tables 2 and 4 .

4. General conclusions

• In addition, although generally not as accurate as the frequencies, the theoretically predicted intensities Ž .both in infrared and in Raman have proved useful in assignment of the spectra.
• Finally, a dimeric structure of the acid salt of glycine in the crystalline state was found to give best results in the prediction of the vibrational spectra of this compound.

Figures

Table 5 Ž .aCalculated spectra and potential energy distribution for the glycine acid salt monomer ‘model’

Table 4 Spectra assignment and potential energy distribution for the glycine alkaline salta

Table 3 Spectra assignment and potential energy distribution for the neutral form of glycinea

Table 1 Ž ) . aCalculated ab initio 6-31G optimized molecular geometries

Table 2 Spectra assignment and potential energy distribution for the zwitterionic form of glycinea

Table 6 Spectra assignment and potential energy distribution for the glycine acid salt dimera

Frequently asked questions

Q1. What are the contributions in "Vibrational spectra of acid and alkaline glycine salts" ?

In this paper, the vibrational spectra of crystalline glycine zwitterion, NH CH COO and the infrared spectrum of this molecule isolated in an argon 3 2, 4, 5 matrix at low temperature 10 K neutral form, was recorded and interpreted. 

Q2. What is the key to elucidation of structural and dynamic properties of gly?

Since vibrational spectroscopy can provide the key to elucidation of both structural and dynamical properties and can be used to establish fundamental correlations between these and the chemical or biochemical behaviour of these substances, considerable effort has been dedicated to such studies on glycine under w xvarious experimental conditions 2–12 . 

Q3. What is the frequency of the OH stretching mode in the IR spectrum?

In the MI-IR spectrum of neutral glycine, as consequence of the isolated status in the Ar matrix, the OH stretching mode appears as a sharp band at 3560 cmy1. 

Q4. What was the w xsion of the program?

ŽThe force constants symmetry internal coordi.nates used in the normal coordinate analysis wereobtained from the ab initio Cartesian harmonic force Žconstants using the program TRANSFORMER ver. w xsion 2.0 18 . 

Q5. What is the spectral region of the glycine infrared spectrum?

Assignment of the NH and CH stretching modes in the neutral glycine infrared spectrum is straightforward, since these give rise to bands in a clear spectral region. 

Q6. How many grooves were used to obtain the Raman spectra of zwitterio?

Raman spectra were obtained using a SPEX 1403 Ždouble monochromator spectrometer focal distance .0.85 m, aperture fr7.8 , equipped with holographic y1 Žgratings with 1800 grooves mm Ref. 1800- . 

Q7. What was the method for calculating the force fields?

The ab initio calculated force fields were scaled Ž .down using a single scale factor 0.89 , which isaccepted to be the best for the Hartree–Fock level of w xcalculations 21 . 

Q8. Why does the C–O vibration in glycine occur at a much higher frequency?

Because carboxylic acid group intra-dimer inter-actions in the acid salt of glycine considerably inŽ .crease the double bond character of the C–O Hbond when compared with a pure carboxylic acid moiety, the n C–O vibration in this molecule occurs at a much higher frequency than in the isolated neutral form of glycine. 

Q9. What is the frequency of the n NH stretching modes in glycine?

sThe carbonyl stretching band is present in both neutral glycine and acid salt spectra and can be assigned to the intense infrared bands at 1780 cmy1 Ž . y1 Ž .glycine and 1745 cm acid salt; B mode and tou the doublet of bands at 1737 and 1717 cmy1 in the( )M .T .R osado et al.r V ibrationalSpectroscopy 16 1998 35 – 54 50( )M .T .R osado et al.r V ibrationalSpectroscopy 16 1998 35 – 54 51Ž . 

Q10. What is the frequency of the t C–N mode calculated?

The t C–N mode calculated value: 206 y1 .cm was previously assigned to the infrared band y1 Ž y1 .at ca. 520 cm Raman: 509 cm , on the basis of neutron diffraction studies. 

Q11. What is the frequency of the Raman band in glycine?

In the zwitterionic glycine, this vibration is predicted to occur at a higher freŽquency than in the acid salt calculated value: 320 y1 .cm and is here assigned to the Raman band at 364 cmy1, whose infrared counterpart occurs at 358 y1 w x Žcm 9 . 

Q12. What is the frequency of the n NH stretching2 modes in glycine?

The frequencies of the n NH stretching2 modes in the alkaline salt of glycine and, in particular, the n NH vibrations in both crystalline glycine3 and its acid salt are considerably overestimated by the calculations, since these groups participate in intermolecular hydrogen bonding that is not properly considered in the calculations. 

Q13. What is the spectra of zwitterionic glycine?

For zwitterionicglycine, the infrared and Raman spectra of the crysŽ .tal a-modification were obtained, while the neutralform was studied by matrix isolation infrared specŽ .troscopy MI-IR , in Argon, at 10 K.a-Glycine p.a. q98% was obtained from Merck. 

Q14. What is the frequency of the Raman band in the alkaline salt?

In the alkaline salt, this mode gives rise to the infrared band at 1431 cmy1 and to the Raman doublet at 1439 and 1410 cmy1, while in the acid salt it gives the band at 1421 cmy1 Ž .both in the infrared and Raman spectra .