Binding of cis- and trans-Dichlorodiammineplatinum(II) to DNA: Evidence for Unwinding and Shortening of the Double Helix

The antitumor drug cis-dichlorodiammineplatinum(II) (cis-DDP) and the inactive trans isomer bind and produce cooperative changes in closed and nicked circular duplex DNA's. Covalent binding of both platinum complexes to the closed circular DNA alters the degree of supercoiling, presumably by disrupting and unwinding the double helix. Electron micrographs show the platinated DNA's to be shortened by up to 50 percent of their original length. At similar ratios of bound platinum per nucleotide, the electrophoretic mobilities of the DNA's in gels containing the dye ethidium bromide are the same for both isomers. The only detectable difference in the binding of the two platinum isomers is an increase in the electrophoretic mobility in nondye gels of closed circular DNA having small amounts of bound cis-DDP that is not apparent for the trans complex.