Guanine, 6-thioguanine and 6-selenoguanine: ab initio HF/DZP and MP2/DZP comparative studies

Abstract

The molecular geometries of four major tautomers of guanine (Gua), 6-thioguanine (6TG) and 6-selenoguanine (6SeG) were studied using the ab initio LCAO-MO method at the Hartree—Fock level with valence double-zeta basis sets augmented by d and p polarization functions (DZP). Only nonplanar C 1 symmetry species are minimum structures on the HF/DZP potential energy surfaces (PES) of Gua, 6TG and 6SeG as it was concluded from harmonic vibrational frequency calculations carried out at the same level. The similarities between the molecular structures of guanine and its heavier analogs are revealed. The calculated molecular parameters of 6TG and 6SeG tautomers are similar to those of corresponding guanine forms, which suggests the possibility of replacing guanine by its heavier analogs in nucleotides, which could rationalize the observed biological activity of these species. Our best estimation of the relative stabilities of the tautomers includes electron correlation contributions calculated at the MP2/DZP approximation and the zero-point-energy contributions scaled by 0.9. The global minimum species on the PESs of Gua and 6SeG are those with a hydrogen atom bonded to N7 ( 1), while for 6TG the most stable form at this level is that with a hydrogen atom bonded to N9 ( 3); in the latter, 3 has a relative energy 1.8 kJ mol −1 lower than that of form 1.