Simultaneous photoconjugation of methylene blue and cis-Rh(phen)(2)Cl-2(+)to DNA via a synergistic effect

Irradiation of the red-light absorbing dye, methylene blue (MB), in the pre sence of the metal complex, cis-Rh(phen)(2)Cl-2(+) (BISPHEN), leads to irre versible photobinding of both reagents to DNA, Evidence from absorption and emission spectroscopy indicates that the dye is strongly complexed to the DNA at the concentrations used in the experiments and that this complex is unaffected by the presence of BISPHEN. The level of covalent binding is pro portional to the absorbed light dose, with the quantum efficiency for coval ent binding of BISPHEN to the DNA with 633 nm light equal to 3.5 x 10(-4). Electrospray ionization mass spectrum of a mixture of DNA fragments created by enzymatic degradation of DNA isolated following irradiation indicates t hat purine adducts are formed with both BISPHEN and the dye. In addition, U V-Vis and high-performance liquid chromatography analyses of the irradiated MB/BISPHEN/DNA mixture and isolated adducts show extensive conversion of t he dye and metal complex to the corresponding N-demethylated and aquated de rivatives, respectively. Triplet quenchers for MB, for example oxygen and b enzoquinone, inhibit both the photoconjugation and the photochemistry of BI SPHEN. A mechanism for the synergistic interaction is proposed that involve s photoconjugation of both partners to the DNA following oxidation and redu ction via electron transfer between (MB)-M-1*/DNA and (MB)-M-3*/BISPHEN.