RELATIVE RATES AND POTENTIALS OF COMPETING REDOX PROCESSES DURING DNA CLEAVAGE - OXIDATION MECHANISMS AND SEQUENCE-SPECIFIC CATALYSIS OF THE SELF-INACTIVATION OF OXOMETAL OXIDANTS BY DNA

The redox reactions of the isostructural complexes Ru(tpy)(bpyO2+, Ru(tpy)(bpy)OH2+, and Os(tpy)(bpyO2+ with DNA have been investigated (tpy = 2,2 ''-terpyridine, bpy = 2,2'-bipyridine). The Ru(IV) complex, which is a two-electron oxidant, cleaves DNA by sugar oxidation at the 1' position, which is indicated by the termini formed with and without piperidine treatment and by the production of free bases and 5-methylene-2(5H)-furanone. This sugar oxidation occurs in the minor groove, as indicated by the inhibition of the reaction by distamycin. The Ru(IV) complex also oxidizes guanine bases to produce piperidine-labile cleavages. Densitometry and product analysis indicate that about 20% of the metal complex is reduced via the sugar oxidation pathway and about 30% via the base oxidation pathway. The Ru(III) complex is a one-electron oxidant but can access a two-electron pathway via an unfavorable disproportionation to Ru(IV). The Ru(III) complex cleaves DNA only by guanine oxidation, which is consistent with the higher yield of base oxidation relative to sugar oxidation observed for Ru(IV). The Os(IV) complex is a weaker one-electron oxidant. As a result, the Os(IV) complex cleaves DNA in supercoiled plasmids, but no cleavages have been detected in single- or double-stranded oligomers. Nonetheless, the reduction of the Os(IV) complex is significantly faster in the presence of DNA than in buffer, suggesting that the DNA is catalyzing a self-inactivation reaction of the oxometal oxidant. These self-reduction pathways are known for related oxidants and presumably account for the remainder of the Ru(IV) oxidant not apparent on sequencing gels. Further, the DNA catalysis is sequence-specific, which may have profound implications for understanding the cleavage patterns of many oxometal oxidants.