The electronic structure of platinum-guanine complexes

Molecular orbital calculations using the SC-MEH method have been carried out on guanine and its mode of interaction with the anti cancer drug, cis-diamminodichloroplatinum(II) (DDP), in various geometrical bonding conformations. It is shown that if indeed a monomeric complex between DDP and guanine is formed, the N-7 and O-6 positions of guanine bond most strongly with DDP in both monodentate and bidentate chelation models. The influence of this bonding on the N-1-H-1 bond is studied in terms of atomic charge variations and orbital overlap population densities to determine if misspairing in DNA might occur via the loss of the guanine H-1 proton, as proposed in the mechanism of O-6 methylation. Preliminary findings do not support one of the proposed mechanisms for miss-pairing in DNA if a bidentated N-7, O-6 DNA-DDP chelate is formed. Interest in the cellular effects of square-planar platinum complexes began with Rosenberg's discovery in 1965 that DDP, inhibited cell division with inhibiting growth rate in Escherichia coli bacteria [1]. Shortly thereafter, the anti-cancer activity of DDP and a number of related platinum complexes was demonstrated [2]. Since then, DDP has been used effectively for a number of types of cancers including lung, head and neck, prostate, bladder, ovaran, and testicular cancers [3], and has recently been released in the United States for distribution as a prescribed drug in chemotherapy.