Cation-pi and amino-acceptor interactions between hydrated metal cations and DNA bases. A quantum-chemical view.

Cation - pi interactions between cytosine and hexahydrated cations have bee n characterized using ab initio method with inclusion of electron correlati on effects, assuming idealized and crystal geometries of the interacting sp ecies. Hydrated metal cations can interact with nucleobases in a cation - p i manner. The stabilization energy of such complexes would be large and com parable to the one for cation - a complex with benzene. Further, polarized water molecules belonging to the hydration shell of the cation are capable to form a strong hydrogen bond interaction with the nitrogen lone electron pair of the amino groups of bases and enforce a pronounced sp(3) pyramidali zation of the nucleobase amino groups. However, in contrast to the benzene - cation complexes, the cation - pi configurations are highly unstable for a nucleobase since the conventional in plane binding of hydrated cations to the acceptor sites on the nucleobase is strongly preferred. Thus, a cation - pi interaction with a nucleobase can occur only if the position of the c ation is locked above the nucleobase plane by another strong interaction. T his indeed can occur in biopolymers and may have an effect on the local DNA architecture. Nevertheless, nucleobases have no intrinsic propensity to fo rm cation - pi interactions.