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)-, abbrevi ated dq2pyp and dq3pyp, respectively) associate with GMP (guanosine-5' -monophosphate) in 0.1-mol 1(-1), pH 3.5-5.5, phosphate buffer solutio n to yield 1:1 and 1:2 non-emissive complexes, the formation constants of which range from 197-63 and 19-11 1 mol(-1), respectively. In addi tion to the strong static quenching, dynamic deactivation of their exc ited state occurs at diffusion-controlled rate (k(q) = 5.2 x 10(9) 1 m ol(-1) s(-1)). Illumination of the GMP-containing solutions of the dye s with a 355 nm laser pulse produces a transient, with strong absorban ce at 510 and 720 nm for dq2pyp, and 420, 460 and 560 nm for dq3pyp. A n identical transient is produced in the presence of ascorbic acid ins tead of the mononucleotide. By comparison to the electrochemically gen erated absorption spectra of the monoreduced dyes, the photogenerated transients have been assigned unequivocally to their corresponding rad ical-cations, formed by electron transfer to the singlet excited state . The back redox reaction between the oxidized quencher and dq2pyp(.+) proceeds at a rate of 1-2 x 10(9) 1 mol(-1) s(-1). The same transient has been observed also for the DNA intercalated viologens; this resul t, together with the little ability of these dyes to sensitize the for mation 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.