BINDING AND KINETICS STUDIES OF OXIDATION OF DNA BY OXORUTHENIUM(IV)

The binding to DNA of complexes based on Ru(tpy)(L)OH22+ (tpy = 2,2',2 ''-terpyridine; L = bpy, 2,2'-bipyridine; phen, 1,10-phenanthroline; o r dppz, dipyridophenazine) has been studied by viscometry,thermal dena turation, and absorbance hypochromism along with the kinetics of oxida tion of DNA by the analogous Ru(tpy)(L)O2+ complexes. These studies sh ow that very weak binding occurs when L = bpy; however, when L = dppz, DELTAT(m) is larger than that for ethidium bromide. Viscometry studie s of the dppz complex show that the dppz complex does lengthen DNA, as occurs with intercalative binding. The slope of the viscometry plot i s identical to that for ethidium bromide, and neighbor exclusion bindi ng is observed for both, with saturation occurring between 0.2 and 0.2 5 small molecules per nucleotide phosphate. The Ru(tpy)(dppz)OH22+ com plex also unwinds DNA by 17 +/- 2., as determined using a topoisomeras e assay. For L = bpy, no evidence of DNA lengthening was obtained. The kinetics of the oxidation of DNA by the Ru(IV)O2+ forms of these comp lexes occurs in two phases. The first phase involves oxidation of DNA by Ru(IV)O2+ that is bound at time zero. The second phase occurs when a reduced Ru(II)OH22+ complex dissociates and another Ru(IV)O2+ comple x binds from solution. The kinetics of this phase are governed by diss ociation of the reduced complex, which allows the relative dissociatio n rates of the L = bpy, phen, and dppz complexes to be determined. The se experiments show that the dissociation rate for the dppz complex is an order of magnitude slower than those for bpy and phen, which is al so consistent with an intercalative interaction for dppz. The cleavage reaction is shown to lead to the release of nucleic acid bases, impli cating sugar oxidation as the reaction pathway.