Molecular orbital theory of the hydrogen bond: Part XXXI. Water with the purine bases adenine and guanine

Abstract

Ab initio SCF and SCF-CI calculations with the STO-3G basis set have been performed to investigate the structures and energies of adenine—water and guanine—water complexes, and the intermolecular surfaces in the molecular plane of these bases. Three distinguishable dimers have been found on the adenine—water surface. The most stable is dimer M″ in which the water molecule assumes a bridging position between N 7 and the amino NH which is s- trans to N 1C 6. The dimer stabilization energy is -11.8 kcal mol −1. Next in stability is dimer L, in which N 9H is the proton donor to water, with the water hydrogens cis to N 3 with respect to the intermolecular NO line. This dimer is 0.3 kcal mol −1 less stable than dimer M″. The least stable dimer is the wobble dimer ( I1″ J) in the N 1 and amino NH′ region of the surface. The cyclic form of this dimer, with a stabilization energy of -9.3 kcal mol −1, is slightly preferred over the open form. The guanine—water surface is quite distinct from the adenine—water surface. Of the four guanine—water dimers, the most stable is an amide wobble dimer ( A″ E) in the N 1H and O 6 region, with a stabilization energy of -10.7 kcal mol −1. Dimer L, which is only slightly less stable at -10.6 kcal mol −1, has guanine as the proton donor to water with a linear N 9 H⋯O hydrogen bond. Another wobble dimer ( I″ J) with a stabilization energy of -10.0 kcal mol −1, is found in the amino NH′ and N 3 region of the surface. The least stable dimer B, in which the water molecule is a double proton donor to guanine at O 6 and at N 7, has a stabilization energy of -6.8 kcal mol −1. No stable dimer was found with the amino NH s- cis to N 1C 2 as the proton donor. Significant structural and energetic changes may occur in particular adenine—water and guanine—water dimers upon excitation of these purine bases to each of the first three n → π* states. The changes which occur depend on the nature of the n → π* excitation, and the hydrogen bonding scheme in the dimer. However, all adenine—water and guanine—water dimers remain bound in the excited n → π* states.