Interactions of platinum compounds with heterocyclic bases.

It is generally accepted that platinum antitumoar drugs bind, preferentially, to Guanine N7 in DNA. Thus the kinetics of formation and the energetics of dissociation of platinum-nitrogen bonding have been investigated, using simple model bases, namely pyridine. pyrimidine. purine and some derivatives. Six complexes of the type cis-PtCl2L2, where L is a N-heterocycle, have been successfully prepared and characterized. The kinetics of the reaction between K2PtCl4 and the N-heterocyclic bases in aqueous solution have beeD studied. This was best achieved by pre-aquating the PtCl4(2-) and measuring the decrease in concentration of platinum or ligand by AA or UV spectroscopy respectively. It was found that the rate of reaction is second order, the rate being dependent on the concentration of both platinum complex and the ligand. The substitution of ligand takes place by direct replacement, involving an associative S.2 mechanism. It was found that the activation entropy and the activation energy for the reactions of platinum complexes in solution are strongly correlated. The energetics of platinum-nitrogen bond dissociation were followed using thermogravimetric analysis. Quantitative data, which enabled calculation of the activation energy for the decomposition process involving the loss of one molecule of ligand was obtained from isothermal studies. No correlation was found between the solid state studies, involving bond breaking, and the syntheses of the complexes, probably because in solution the reaction is complicated by solvent effects. Theoretical studies, which involved Molecular Orbital calculations on the N-heterocyclic bases themselves, were also carried out in an attempt to correlate the kinetic and energetic parameters with the electronic structure of the ligand. Several correlations were attempted but only one was found. This was the relationship between the solid state decomposition energy and the LUMO energy of the quarternized base