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Trends in Organic Chemistry   Volumes    Volume 7 
Conformational analysis of amino acid tautomers
Allan D. Headley, Stephen D. Starnes
Pages: 75 - 84
Number of pages: 10
Trends in Organic Chemistry
Volume 7 

Copyright © 1998 Research Trends. All rights reserved

Owing to the low solubility of most α-amino acids and their inability to form zwitterions in the gas phase, the factors that influence their tautomerization and tautomer stability in solution and the gas phase cannot be investigated directly. Compared to most α-amino acids, N-alkylated α-amino acids are very soluble in a wide range of solvents and some form fairly stable gas phase zwitterions; as a result, these type molecules are ideal to analyze the effects that substituents and solvents have on the tautomerization of α-amino acids. An understanding of the importance of hydrogen bonding on the relative stabilities of the various conformers of amino acids was achieved from ab initio analysis of the stability of different conformers of glycine (GLY), N-methylglycine (NMG) and dimethylglycine (DMG). Analysis of anomeric effect in amino acids was achieved from the relative stability of different conformers of the theoretical difluoroglycine (DFG) molecule. A theoretical analysis of the tautomerization of N,N-dimethylglycine reveals that a relatively stable ground state zwitterionic conformer exists. To the best of our knowledge, this ground state zwitterionic α-amino acid is among the first detected in the gas phase at the MP2 level of theory. In DMSO, amino acids that have bulky groups bonded to the α-carbon favor the formation of the unionized tautomers. Effective solvation of the tautomers, especially the zwitterionic tautomer, determines the magnitude of the tautomeric equilibrium. Effective solvation requires that solvent molecules access the region between the charges (or dipole) of the tautomers. Thus, zwitterionic tautomers that have bulky alkyl groups are less solvated than those with less bulky groups.
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