Effects of DNA on driving force dependence of photoinduced electron transfer from the excited state of tris(2,2-bipyridine)ruthenium(II) to intercalators in DNA: Distinction between intra-and inter-DNA pathways

Photoinduced electron transfer (ET) dynamics from the excited state of a ruthenium complex [Ru(bpy)₃²⁺ (bpy = 2,2′-bipyridine)] to a series of intercalators in DNA, 9-substituted-10-methylacridinium ions (AcrR⁺, R = H, CH₂Ph, Prⁱ and Ph), 3-substituted-1-methylquinolinium ions (RQuH⁺, R = H, Me, CN and Br) and 4- and 5-methylphenanthridinium ions (4- and 5-MePhen⁺), were examined from the emission decay profiles of Ru(bpy)₃²⁺ in the absence and presence of DNA in an aqueous solution. Intercalation of AcrH⁺ to DNA is found to result in inhibition of hydride transfer from an NADH model compound, 1-benzyl-1,4-dihydronicotinamide, to AcrH⁺. In contrast, the rate constants of photoinduced ET of intercalated molecules to DNA become much larger than those of free intercalators in solution due to the positive shift in the one-electron reduction potentials by the intercalation into DNA. The intramolecular pathway of photoinduced ET from Ru(bpy)₃^2+* bound electrostatically to DNA to intercalators bound to the same DNA molecule has been distinguished from the intermolecular pathway of photoinduced ET of intercalators bound to a different DNA molecule. The occurrence of photoinduced ET is examined by laser flash photolysis experiments which show the transient absorption spectra of the one-electron reduced intercalator when the ET is exergonic. The resulting data were analyzed in light of the Marcus theory of ET to determine reorganization energies of ET in DNA as well as in an aqueous solution.