Interaction between the Guanine−Cytosine Watson−Crick DNA Base Pair and Hydrated Group IIa (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) and Group IIb (Zn²⁺, Cd²⁺, Hg²⁺) Metal Cations

Abstract

Structures and energetics of complexes between the guanine−cytosine Watson−Crick DNA base pair and pentahydrated Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, and Hg2+ metal cations were studied. Comparison has been made with the data for the unsolvated cations. The complexes were fully optimized within the Hartree−Fock approximation applying the 6-31G* basis set of atomic orbitals, while relativistic pseudopotentials were used for the cations except magnesium. The energetics have been studied with the inclusion of electron correlation using the full second-order Møller−Plesset perturbation theory. The cation with its hydration sphere has been considered as one subsystem in the calculations of interaction energy. Thus, the complete system for a calculation would include the hydrated cation−guanine−cytosine trimer. The interaction between hydrated cation and guanine is significantly reduced compared to the guanine−unsolvated cation interaction. Though the stabilizing three-body contribution has been reduced by almost 50% by hydration, it still remains significant. The stability of the guanine−cytosine Watson−Crick base pair is enhanced by ca. 20−30% due to the coordination of the hydrated cation. All the transition metal and Mg2+ cations are tightly bound to the N7 atom of guanine, constituting an octahedral coordination sphere. The Ca2+, Sr2+, and Ba2+ cations are coordinated simultaneously to the N7 and O6 atoms of guanine and the base−cation distance increases with the row number in this series. However, the energy difference between the N7 and N7−O6 types of coordination is rather small. The calculations show a different balance between the transition metal and alkaline earth cations with respect to the cation−base and cation−water interactions. Zn2+ compared to Mg2+ is bound more tightly to the base, and the hydration shell around Zn2+ is more flexible. The replacement of Mg2+ by Zn2+ can be viewed, to some extent, as a shift from the interaction between nucleobase and a hydrated cation toward hydration of a metalated base. This is likely to contribute to the different biological role of Zn2+ and Mg2+.