Photosensitized reactions of poly(U) with tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio.