Recognition modes of hypoxanthine,xanthine and their derivatives by amino acid carboxylic group: UV spectroscopic and quantum chemical data

UV absorption spectra of Hyp, Xan, their nucleosides and methyl derivatives were studied in anhydrous dimethylsuloxide and the changes in these spectra on the interactions with neutral and deprotonated carboxylic groups of amino acids were traced. By the semiempirical quantum-chemical method MNDO/H it was shown, that interaction with carboxylate-ion fixes Hyp as the rare N7H enolic tautomer and converts Xan into its N9H diketo tautomeric form with a probable admixture of the N7H O6-enolic form. Significant changes in the UV spectra of Xan, m3Xan, m9Xan and X under interaction with carboxylate-ion are determined by essential contribution to complex formation of proton transfer from bases to ligands, m9Xan and X proving to be slightly protonated even by the solvent. The methylation of the N7 position in m7I and m7X was established to result in the practical absence of their interactions with carboxylate-ion and initiation of a new ability of forming complex with the neutral carboxylic group. Substitution of the C8H group by N in 8-azaXan does not change the interaction specificity of the base with two forms of carboxylic group.