Biopolymers and cell. 2001; 17 (1): 43 - 60

Influence of methylation and interactions with amino acid carboxylic group on UV spectra of purine bases and nucleosides in dimethylsulfoxide. 3. Hypoxanthine and xanthine

A. V. Stepanyugin, I. M, Kolomiets', A. L. Potyahaylo, S. A. Tryhubenko, T. V. Bohdan, S. P. Samijlenko

UV absorption spectra of hypoxanthine, xanthine, their nucleosides and a number of their methyl derivatives were studied in anhydrous DMSO, and the spectral changes under the interaction with neutral and deprotonated (carboxylate-ion) amino acid carboxylic group were traced. By the semi-empirical quantum-chemical method MNDOlH it was shown, that the interaction with carboxylate-ion fixes Hyp in the rare enolic form and shifts the N7H «* N9H tautomeric equilibrium to the left while in the case of Xan provokes the N7H -»N9H transition, which is blocked up by its methyl substitution at the position N3. Significant changes in the UV spectra of Xan, m Xan, m Xan and X under the interaction with carboxylate-ion are determined by the essential contribution to a complex formation of the proton transfer from a base to the ligand, m Xan and X proving to be partly deprotonated even on the account of the solvent. It wasestablished that methyl substitution at the position N7 in ml and m X resulted in the practical absence of their interaction with carboxylate-ion and the rise of a new ability of forming complexes with the neutral carboxylic group. The substitution of the C8H group for N in S-azaXan does not change the interaction specificity of this base with two forms of carboxylic group.