Luminescence quenching by DNA-bound viologens: effect of reactant identityon efficiency and dynamics of electron transfer in DNA

Photoinduced electron transfer from two intercalating photoactive donors, R u(phen)(2)dppz(2+) and ethidium, to intercalating viologen accepters of the N,N'-dialkyl-6-(2'-pyridiniumyl)phenanthridinium family has been investiga ted through steady-state and time-resolved luminescence quenching measureme nts. Efficient quenching of the emission from these donors bound to DNA is observed at low concentrations of acceptor (1-10 eq.), and in time-resolved emission experiments it is determined that electron transfer occurs on the nanosecond time scale. Furthermore, transient absorption measurements conf irm that the quenching is the result of a charge-transfer process; upon pho toreaction of intercalated Ru(phen)(2)dppz(2+) with a viologen acceptor, an intermediate with spectral properties resembling the expected charge-separ ated pair is observed. The quenching yields and kinetics obtained with this quencher are in marked contrast to those observed with these same donors p aired with Rh(phi)(2)bpy(3+) as an acceptor. The differing efficiencies of electron transfer for these donor/acceptor pairs bound to DNA as compared t o others previously described are discussed qualitatively in terms of the s tructural and electronic properties of the different reactants. (C) 2000 El sevier Science B.V. All rights reserved.