Interaction of cis- and trans-dichlorodiammine-platinum(II) with synthetic polydeoxyribonucleotides: effect of 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 (rho) of the two G + C polymers, the cis- isomer increased the rho of poly [d(G-C)] and the trans- isomer increased the rho 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 rho 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 (Tm) of poly(dA)-poly(dT) and a sharp increase in the Tm of poly[d(A-T)]. Our results indicate that the cis- isomer reacts with all four polymers to produce rho 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 rho 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 of the buoyant densities of naturally occurring DNA; however, additional studies are clearly needed.