Photoinduced interaction of Ru(bpy) 32+ with nucleotides and nucleic acids in the presence of S 2O 82−: A transient conductivity study

Abstract

The photochemical reactions of Ru(bpy) 3 2+ with single- and double-stranded DNA, polynucleotides and purine-containing nucleotides in argon-saturated aqueous solution in the presence of S 2O 8 2− were studied using time-resolved absorption and conductivity methods. The conversion of Ru(bpy) 3 3+ to Ru(bpy) 2 3+, monitored spectroscopically either after rapid mixing with substrate or after laser flash excitation (λ exc=353 nm) is quantitative at nucleotide-to-sensitizer ratios [N]/[S] of 1–2 for DNA and other guanine-containing compounds. Conductivity measurements following the laser pulse revealed a fast conductivity increase (rise time, less than 0.1 ms) due to the formation of protons and, to a lesser degree, to charged species of much lower ion mobility. A slower component in the 0.01–1 s range was observed for nucleic acids; its amplitude is markedly reduced at pH 6–9. In buffered neutral solution the signal is replaced by a slight decrease in conductivity. Electronically excited Ru(bpy) 3 2+ bound to DNA reacts with S 2O 8 2− to form Ru(bpy) 3 3+ and SO 4 .− as primary oxidizing species both of which react with bases. The resulting base radicals react subsequently with Ru(bpy) 3 3+ and Ru(bpy) 3 2+ or the ligands in the ruthenium complex, producing protons which give rise to the slower conductivity increase. The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow component. The transient conductivity behaviour is sensitive to the single-or double-stranded nature of DNA.