Spin-coupled study of hydrogen-bonded systems: The nucleic acid pairs

Nucleic base]base interactions play a determinant role in DNA and RNA structure. In the present work, applications to such systems of the SCspin-coupled. theory are presented. The studies regard the hydrogen bond in adenine]thymine and guanine]cytosine pairs in the Watson]Crick configuration. Equilibrium geometries and binding energies were always calculated via basis set superposition errorBSSE.free algorithms, without artificial geometrical constraints. The structures of the systems were optimized using the analytical gradient method in the framework of the SCF-MI self- consistent field for molecular interactions. formalism. The SC method was used to study the effect of correlation in the specific region of the hydrogen bonds. This was effected via localization of the appropriate SCF-MI molecular orbitals. Four electrons per hydrogen bond were explicitly correlated: 8 for adenine]thymine and 12 for guanine]cytosine. Also at the SC level, the algorithm remains BSSE free. Interaction density, evaluated at the SCF-MI and at the correlated level, was used to interpret the nature of the interactions involved in the hydrogen bond formation. The importance of covalent forces was investigated by considering the spin recoupling effects, as highlighted by the spin-coupled wave function.