Photoinduced electron transfer from nucleotides to DNA intercalating viologens. A study by laser-flash photolysis and spectroelectrochemistry.

Fluorescent DNA-binding N,N'-dialkyl 6-(2-pyridinium)phenanthridinium dications (where dialkyl stands for -(CH2)2-or-(CH2)3-, abbreviated dq2pyp and dq3pyp, respectively) associate with GMP (guanosine-5'-monophosphate) in 0.1-mol l-1, pH 3.5-5.5, phosphate buffer solution to yield 1:1 and 1:2 non-emissive complexes, the formation constants of which range from 197-63 and 19-11 l mol-1, respectively. In addition to the strong static quenching, dynamic deactivation of their excited state occurs at diffusion-controlled rate ki = 5.2 x 10(9) l mol-1 s-1). Illumination of the GMP-containing solutions of the dyes with a 355 nm laser pulse produces a transient, with strong absorbance at 510 and 720 nm for dq2pyp, and 420 and 560 nm for dq3pyp. An identical transient is produced in the presence of ascorbic acid instead of the mononucleotide. By comparison to the electrochemically generated absorption spectra of the monoreduced dyes, the photogenerated transients have been assigned unequivocally to their corresponding radical-cations, formed by electron transfer to the anglet excited state. The back redox reaction between the oxidized quencher and dq2pyp+ proceeds at a rate of 1-2 x 10(9) l mol-1 s-1. The same transient has been observed also for the DNA intercalated viologens; this result, together with the little ability of these dyes to sensitize the formation of singlet dioxygen or to produce superoxide anion, demonstrate that their DNA photocleavaging activity is initiated by an efficient light-induced electron transfer from the nucleobases.