Interaction of the adenine-thymine Watson-Crick and adenine-adenine reverse-Hoogsteen DNA base pairs with hydrated group IIa (Mg2+, Ca2+, Sr2+, Ba2+)and IIb (Zn2+, Cd2+, Hg2+) metal cations: Absence of the base pair stabilization by metal-induced polarization effects

Structures and energetics of complexes between the adenine-thymine Watson-C rick (AT WC) and adenine-adenine reverse-Hoogsteen (AA rH) DNA base pairs a nd hydrated (five water molecules) Mg2+, Ca2+, Sr2+, Ba2+, Zn2+, Cd2+, and Hg2+ metal cations were studied using high-level quantum chemical technique s. Binding of the cations to N7 of adenine does not enhance the strength of the base pairing through polarization effects. This is in stark contrast w ith the results obtained for the GG and GC base pairs. This finding and oth er recently published data indicate a qualitative difference between adenin e-containing (AA,AT) and guanine-containing (GC,GG) base pairs. There are s ignificant changes in the electronic structure of the guanine aromatic syst em upon cation binding to N7 which propagate toward the H-bonded partner. N o such effect has been observed for any adenine-containing pair. The intera ction between hydrated cations and adenine is much weaker than that between hydrated cations and guanine due to the low dipole moment of adenine. Furt hermore, the cation and its surrounding polarized water molecules interact with the nitrogen atom of the adenine amino group which then acts as an H-b ond acceptor. This can lead to destabilization of the base pairing. The zin c and magnesium groups of divalent cations have a different balance of the water-cation and base-cation interactions. Binding of the zinc-group elemen ts to nucleobases is more efficient. Interaction of large IIa group divalen t cations (Ca2+, Sr2+, and mainly Ba2+) with the N7 site of adenine is not likely unless the amino group nitrogen atom serves as a coordination center which may disrupt the base pairing. The complexes were optimized within th e Hartree-Fock approximation with the 6-31 G* basis set of atomic orbitals and relativistic pseudopotentials for the cations. All atoms of the base pa irs were kept coplanar. No other constraints were applied. The interaction energies have been calculated with inclusion of the electron correlation by means of the full second-order Moeller-Plesset perturbational theory.