The efficiency of photosensitized DNA cleavage and quantum yields of singlet oxygen production (phi(Delta)) have been determined for the metal complexes [Ru(bpy)(2)(ddz)](2+), [Ru(phen)(2)(ddz)](2+), [Ru(phen)(3)](2+), and [Ru(dsdp)(3)](4-) (bpy = 2,2'-bipyridine, ddz = dibenzo[hj]dipyrido[3,2-a:2',3'-c]phenazine, phen = 1,10-phenanthroline, and dsdp = 1,10-phenanthroline-4,7-diphenylsulfonate). phi(Delta) values (0.19-0.52) have been measured in O-2-saturated D2O solution, both in the absence and presence of DNA. Protection of the photoexcited Ru(II) complexes from O-2 quenching imparted by the polynucleotide was evaluated from their emission lifetimes (0.13-3.7 mus) measured at three different O-2 concentrations. Our results show the highest photocleavage efficiency for those sensitizers that display strong affinity for DNA (binding constant K-b > 10(6) M-1), despite their lower phi(Delta) values (0.08-0.10 for the three cationic complexes vs 0.40 for the tris-dsdp, in H2O/air/ DNA). Kinetic analysis (agarose gel electrophoresis) of the photocleavage reaction of plasmid DNA reveals that single-strand breaks are produced with the four photosensitizers, as expected for a singlet oxygen-mediated mechanism. Moreover, an intercalative binding of the metal complex, compared to groove-bound or unbound sensitizers of the same family, is shown to accelerate double-strand break that occurs by accumulation of single-strand breaks.