Amino Acids and Aminophenols

Aliphatic amino acids are categorized according to the position of the -NH2 group relative to the carboxylic group. The a-amino acids comprise the subgroup 

An enormous number of experimental and theoretical papers claim unanimously that the a-amino acids form predominantly zwitterions at their isoelectric pH, near 7 in aqueous solution. For size limitations, only a few relevant papers are referred here [78-80]. The importance of the paper by Tunon et al. [79] is emphasized, where the mechanism of the neutral to zwitterion tautomeric transformation was followed through MD simulations. 

Aspartic acid can be considered both as a- and as P-amino acid. Accordingly, two zwitterionic forms exist there, where the structure depends on the actual deprotonation of one ofthe two-COOH groups. Nagy and Noszal [81] calculated the relative protonation constant for the H Asp -1- H Asp (zwitterion) equilib- 

In the y-amino acid series, the tautomeric equilibrium for the y-aminobutyric acid has been studied theoretically. The (CH2)3 spacer prevents the formation of an intramolecular hydrogen bond for the most stable neutral gas-phase structure [37]. The extended coirformers were found predominating also in aqueous solution [34, 35], and the solute overwhelmingly adopts the zwitterionic form at the model pH of 7. 

In the aromatic series, Nagy and Takacs-Novak [49] pointed out both theoretically and experimentally that the 3- and 4-COOH pyridine carboxylic acids (nicotinic and isonicotinic acids) form the zwitterionic species in aqueous solution. The distances of the -COOH groups from the pyridine nitrogens prevent the formation of a favorable dimeric form (except for the 3-COOH isomer with and anti carboxylic conformation) where a double-proton-relay 

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