Molecular-Orbital Studies of Intermolecular Interaction Energies. I. On the Role of π Electrons in Intermolecular Interactions, with Some Application to DNA

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Abstract
A semiemphirical method, based on molecular orbitals, is presented for the treatment of intermolecular interactions which involve π-electron systems, as is common in biopolymers. In the application of the method, a numerical analysis is performed on the interaction energy between the bases of a guanine—cytosine homo-dinucleotide. The computed values of the exchange, the electrostatic, the polarization, and dispersion energies are given for an interaction between base pairs in configurations corresponding to some of the possible occurences in DNA. The following conclusions are based on the numerical results. It is incorrect to assume that the π-electron systems of the two monomers are separable. It seems acceptable, for the purpose of a perturbation treatment, to confine the set of wavefunctions to the π orbitals. The interaction energy between more distant monomers is not negligible relative to the interaction between nearest neighbors. While the energy of attraction between π-electron systems is significant, it cannot constitute the dominant part of the energy required for the stabilization of the double-helix structure.