Electron transfer at a distance induced by site-selective photoionization of 2-aminopurine in oligonucleotides and investigated by transient absorption techniques

Electron transfer from guanine donor to 2-aminopurine radical acceptor at v arious distances from one another in oligonucleotides was investigated usin g transient absorption techniques that allow for the direct monitoring of b oth the acceptor and oxidized donor short-lived species. The oligonucleotid es (15-mers) containing one GG doublet and a 2-aminopurine (2AP) base analo g, separated by 0-4 thymine bases, were studied either in the single-strand ed form (ss), or in the double-stranded form (ds) with T opposite the 2AP r esidue in the complementary strand. The 2AP residues were selectively photo ionized by a two-photon excitation with intense 308 nm XeCl excimer laser p ulses (FWHM=12 ns, similar to 60 mJ pulse(-1) cm(-2)). The oxidation of gua nine by the 2AP radicals was monitored by the evolution of the transient ab sorption spectra of 2AP radicals (absorption band at 360 nm, bleaching at 3 10 nm) and guanine radicals (narrow absorption band at 312 nm). The fast (< 100 ns) and slow (> 100 ns) kinetic components of guanine radical formatio n were observed. The time dependence of the fast component, attributed to t he oxidation of guanines by the radical cation 2AP(.+), was not resolved. A t neutral pH, 2AP(.+) rapidly (similar to 30 ns) deprotonates to the neutra l radical 2AP(-H)(.), which oxidizes the guanines on the 0.1-500 mu s times cale and gives rise to the neutral G(-H)(.) radical (the slow component). B oth the prompt (< 100 ns) relative yield of the guanine radicals, Phi(G), a nd the rate constant of the slow electron transfer decrease with the number of bridging thymine bases on the strand bearing both the 2AP and GG units (the attenuation parameter beta in dsDNA is 0.75 +/- 0.20 Angstrom(-1)). Wh en four thymine bridging bases between the 2AP and GG units are replaced by four adenine bases (with normal complementary strands in both cases), the rate of the electron transfer is increased from < 500 s(-1) (Phi(G)similar to 0) to 1.7x10(6) s(-1) (Phi(G)=0.36), indicating that the electron transf er rates can strongly dependent on the base sequence. The slow kinetic comp onent is discussed in terms of a proton-coupled electron transfer mechanism in which electron transfer from G to 2AP(-H)(.) is coupled with deprotonat ion of G(.+) and protonation of 2AP(-H)(-).