Structure and tautomeric properties of thymine derivatives generated by hydroxyl radical in anaerobic conditions

Abstract

The tautomeric properties of three hydroxyl radical thymine derivatives were characterised by gradient geometry optimisation in the 6311G** basis set and MP2 (6311G**) energy calculations. The solvent effects were also estimated based on SCIPCM (6311G**) model. Calculations were applied to following main products of thymine degradation of hydroxyl radical in anaerobic conditions: 5-hydroxy-6-hydrothymine ( 1), 6-hydroxy-5-hydrothymine ( 2) and 5,6-dihydrothymine ( 3). The most stable tautomer of 5-hydroxy-6-hydrothymine ( 1) is in diketo form with 5eq conformation of the chiral C 5 carbon atom. Among all studied tautomers of 6-hydroxy-5-hydrothymine ( 2) the 15eq6ax tautomer (diketo isomer with C 5 equatorial and C 6 axial orientations) is the preferred one. The most probable 5,6-dihydrothymine ( 3) tautomer is in diketo form with equatorial orientation of the C 5 chiral centre. No significant impact of the solvent polarity on the order of tautomers was observed. Since in all cases the most stable tautomeric form corresponds to diketo isomer, the coding abilities are the same and may be described as (donors on O 2 and O 4 oxygen atoms and acceptor on H 3 hydrogen atom).