Interaction of cis- and trans-dichlorodiammineplatinum(II) with synthetic polydeoxyribonucleotides: Effect on equilibrium buoyant densities

Equilibrium buoyant densities in Cs₂SO₄ of four synthetic double-stranded polydeoxyribonucleotides were compared following reaction with either cis-Pt(NH₃)₂Cl₂, trans-Pt(NH₃)₂Cl₂ or PtCl₄. While both Pt(NH₃)₂Cl₂ isomers caused significant increases in the buoyant densities (ρ{variant}) of the two G + C polymers, the cis- isomer increased the ρ{variant} of poly[d(G-C)] and the trans- isomer increased the ρ{variant} of poly(dG) · poly(dC) more sharply than the corresponding isomer at low molar ratios of metal ion to polymer. Chloroplatinic acid was without effect. Only cis-Pt(NH₃)₂Cl₂ produced substantial increases in the ρ{variant} of poly[d(A-T)] and poly(dA) · poly(dT) with increasing metal ion concentrations. These increases were accompanied by a proportional decrease in the thermal transition temperature (T_m) of poly(dA)-poly(dT) and a sharp increase in the T_m of poly[d(A-T)]. Our results indicate that the cis- isomer reacts with all four polymers to produce ρ{variant} changes clearly distinct from those values observed after reaction with either the trans- isomer or PtCl₄. These results help explain the observation by others that the cis- isomer produces greater ρ{variant} changes in native DNA than the trans- isomer. On the basis of our data and the published results of others, one may rationalize the quantitative differences between the cis- and trans- compounds on the buoyant densities of naturally occurring DNA; however, additional studies are clearly needed.