SIDE CHAIN CONFORMATIONS IN SOME ACETYLATED AMINO ACIDS: A NORMAL COORDINATE ANALYSIS
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Date
1980
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Ohio State University
Abstract
Infrared spectra of N-acetyl -1-glutamic acid (I), N-acetyl-1-glutamine (II), N-acetyl-1-methionnine (III) and N-acetyl-1-cysteine have been investigated in solid state over the 1800-400 cm-1 region. The group frequencies (amide, $CH_{3} CH_{2}$ and COOH groups) have been identified and the infrared bands characteristic of the infrared bands has been; i) frequency shifts on deuteration of compounds I-IV, ii) comparison of the spectra with the similar compounds (N-acetylglycine, 1-glutamic acid, 1-glutamine, 1-methionne, 1-cycsteine, acetamides, etc.), iii) normal coordinate analysis of N-acety1glycine which has same skeletal structure as compounds I-IV and has no side chain. Whereas the infrared bands sensitive to the side chain conformation show no significant change when 1-glutamine, 1-methionine and 1-cysteine are acetylated, they are very much changed in going from 1-glutamic acid to N-acetyl-1-glutamic acid. This seems to indicate that the conformational changes (side chain) are taking place in 1-glutamic acid on acetylation, Results of the theoretical calculations on the normal vibrations of 1-glutamic acids and N-acetyl-1-glutamic acid (crystal structure unknown) with different side chain conformations show that whereas the side chain modes observed in 1-glutamic acid agreed with GGT conformation, those in the acetylated derivative were best fitted when the side chain was considered TTT (angle of rotation about the bonds $C_{\alpha}-C_{\beta}, C_{\beta} -C\gamma, C_{\gamma}, C_{\delta}$ are gauche, gauche, trans (GGT) and trans, trans trans (TTT). This shows unbuckling of the side chain due to close proximity and consequent repulsion of the oxygen atoms in carboxyl group (side chain) and amide group.
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$^{1}$. R. B. Srivastava et. al., Indian J. Biochem. J. Biophys. 12, 238 (1975).
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