tuning the mechanism of dna cleavage photosensitized by ruthenium dipyridophenazine complexes by varying the structure of the two non intercalating ligands the influence of the nature of ligands on the efficiency of ruthenium complexes for photosensitizing dna cleavage was investigated rubipy2dppz2 and rubpz2dppz2 were selected as dna breakers on the basis of their high affinity for dna due to the presence of a dppz ligand which can partially intercalate in the major groove of dna their photosensitizing properties were compared to those of rubipy32 a complex which binds to dna with a far lower constant upon irradiation these complexes promoted the formation of single strand breaks in supercoiled phi x 174 dna unexpectedly rubipy2dppz2 was found to be less efficient than rubipy32 whatever the dye concentration or the base pairru molar ratio r scavenging experiments have shown that the oxidative dna cleavage induced by rubipy2dppz2 mainly results from a type ii mechanism the behavior of rubipy2dppz2 was different this compound was clearly more efficient than rubipy32 as dna breaker and its efficiency was not modified by the presence of oxygen or by addition of scavengers of reactive oxygen species in this case a mechanism involving electron transfer between the excited state of the ruthenium complex and the guanine residue was proposed in agreement emission lifetime measurements the change in mechanism observed between rubipy2dppz2 and ru bipy2dppz2 results from an increase of the reduction potential of the ruthenium complexes in the excited state which appears to be the main factor controlling the efficiency