RELATIVE RATES AND POTENTIALS OF COMPETING REDOX PROCESSES DURING DNACLEAVAGE - 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, whi ch is a two-electron oxidant, cleaves DNA by sugar oxidation at the 1' position, which is indicated by the termini formed with and without p iperidine treatment and by the production of free bases and 5-methylen e-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 cleav ages. 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-electro n oxidant but can access a two-electron pathway via an unfavorable dis proportionation to Ru(IV). The Ru(III) complex cleaves DNA only by gua nine oxidation, which is consistent with the higher yield of base oxid ation relative to sugar oxidation observed for Ru(IV). The Os(IV) comp lex is a weaker one-electron oxidant. As a result, the Os(IV) complex cleaves DNA in supercoiled plasmids, but no cleavages have been detect ed 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-inactivat ion reaction of the oxometal oxidant. These self-reduction pathways ar e known for related oxidants and presumably account for the remainder of the Ru(IV) oxidant not apparent on sequencing gels. Further, the DN A catalysis is sequence-specific, which may have profound implications for understanding the cleavage patterns of many oxometal oxidants.