Complete set of H-bonded homoassociates of 9-methylguanine with participation of mutagenic tautomers: quantum-mechanical study

Abstract

For the first time on the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) theory level, the complete family of m⁹Gua·m⁹Gua homoassociates in vacuum, which consists of 57 structures the interval 0–17.69 kcal/mol of relative Gibbs free energies under standard conditions, is obtained. Homoassociates are stabilized through classical (NH...N, NH...O, OH...N, OH...O) and weak (CH...N, CH...O) H-bonds, and van-der-Waals contacts. The structures of 11 m⁹Gua·m⁹Gua homoassociates are planar (3 of them are centrosymmetric), 7 structures are U-shaped, 12 structures have L-shaped noncanonical geometry, 2 structures have T-shaped geometry, 20 structures — spiral, 3 — cross shaped, 2 — significantly non-planar. It is proved that the methyl group of m⁹Gua in position 9 is a donor of H-bonding and influences the energy distribution of homoassociates. It is demonstrated that, during the self-association of m⁹Gua, its amino-group can be simultaneously a donor and an acceptor of an H-bond. A linear relation between the energy of CH...O/N H-bonds and their electron density in corresponding critical bonds is established.