Solvation of Al−Guanine Complexes with NH3: A Theoretical Study

Abstract

Geometry optimizations on complexes composed of an Al atom, a guanine molecule, and an ammonia molecule have been performed with density functional methods. In the most stable structures, the ammonia molecule forms hydrogen bonds with previously studied Al-guanine complexes. The two lowest structures correspond to an unusual tautomer of guanine in which both N atoms of the five-membered ring, N7 and N9, are protonated. Within 3 kcal/mol in energy lie two additional structures in which a proton is shifted from N9 to N3. Ionization energies calculated with ab initio, electron-propagator methods for the two latter structures are in close agreement with the experimentally observed ionization threshold. Higher ionization energies are obtained for the two lowest structures. Dyson orbitals for the lowest ionization energies are guanine π* functions. The order of isomers in the cationic species is different from that of the neutrals. Energies of ammonia dissociation are approximately equal for all forms of the Al-guanine-NH 3 complex, except for a structure with an Al-NH 3 dative bond.