AB-INITIO STUDIES ON THE STRUCTURE AND PROPERTIES OF THE HYDROXYL-RADICAL-MODIFIED ADENINE-DERIVATIVES IN DIFFERENT TAUTOMERIC FORMS

The geometrical and electrical properties of four tautomers of 2-OH-ad enine (2-OH-A) and 8-oxoadenine (8-oxo-A) and three tautomers of fapy- adenine (fapy-A) have been estimated on the basis of a 6-31G ab initio RHF method followed by single-point 6-31G(*) and MP2 calculations. T he resulting relative stability of different tautomeric forms suggests that the most favorable in vacuum are the enol-amino forms of 2-OH-A, the keto-amino tautomer of 8-oxo-A, and the diamino form of fapy-A, A fter taking into account for the polarization functions and electron c orrelation contribution the energetical succession of the studied taut omers was nor changed, The smaller contribution of the electron correl ation was observed for the most stable tautomers and the highest for e nol-imino isomers. The dipole moment strongly depends on the tautomeri c form and has the lowest value for all enol-amino forms of adenine de rivatives. The nonplanarity of all studied tautomers was noticed. The amino groups are nonsymmetrical pyramids with out of plant hydrogen at oms. For 2-OH-A and 8-oxo-A atom H-62 is turned above and atom H-61 be low the ring plane. The opposite orientation is observed in the case o f fapy-A. However, the 2-OH-A tautomers are characterized by a much sm aller deviation from the strict planar structure then 8-oxo-A. The use of the prefix oxo for C-8-substituted adenine and hydroxyl for the de rivative modified at the C-2 position is justified by the most stable tautomeric forms.