Nickel, copper, and zinc centered ruthenium-substituted porphyrins: effect of transition metals on photoinduced DNA cleavage and photoinduced melanoma cell toxicity

Abstract

Coordination of two [Ru(bipy)₂Cl]⁺ moieties (where bipy = 2,2′-bipyridine) to the pyridyl nitrogens in the 5,10-positions of meso-5,10,15-(4-pyridyl)-20-(pentafluorophenyl)porphyrin gives the diruthenium porphyrin complex I. Insertion of nickel(II), copper(II), and zinc(II) into the porphyrin center gives the complexes II–IV, respectively. Electronic transitions associated with the ruthenium porphyrin include an intense Soret band and four less intense Q-bands in the visible region of the spectrum. An intense π–π* transition in the UV region associated with the bipyridyl groups and a metal-to-ligand charge transfer (MLCT) band appearing as a shoulder to the Soret band are also observed. A shift of the Soret band and collapse of the Q-bands into one band is observed upon insertion of the metal ions into the porphyrin center. Electrochemical properties associated with the complexes include a redox couple in the cathodic region attributed to the porphyrin and a redox couple in the anodic region due to the Ru^III/II couple. DNA titrations of the complexes indicate that they interact strongly with DNA potentially through an intercalation mechanism. Irradiation of aqueous solutions of the complexes and supercoiled DNA at a 5 : 1 base pair to complex ratio with visible light above 400 nm shows nicking of DNA for the nickel(II) and copper(II) complexes and photocleavage of DNA for the zinc(II) complex. Cell studies with dermal skin (normal) fibroblast and melanoma cells indicate that the free base porphyrin(I) is toxic to both normal and melanoma cells, while the nickel(II) and copper(II) complexes, II and III, are non-toxic to both cell lines when irradiated with a tungsten lamp. The zinc(II) complex, IV, is non-toxic to normal cells but toxic to melanoma cells when irradiated under the same conditions.