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 various distances from one another in oligonucleotides was investigated using transient absorption techniques that allow for the direct monitoring of both the acceptor and oxidized donor short-lived species. The oligonucleotides (15-mers) containing one GG doublet and a 2-aminopurine (2AP) base analog, separated by 0-4 thymine bases, were studied either in the single-stranded form (ss), or in the double-stranded form (ds) with T opposite the 2AP residue in the complementary strand. The 2AP residues were selectively photoionized by a two-photon excitation with intense 308 nm XeCl excimer laser pulses (FWHM = 12 ns, ~ 60 mJ pulse^-1 cm^{- 2}). The oxidation of guanine by the 2AP radicals was monitored by the evolution of the transient absorption spectra of 2AP radicals (absorption band at 360 nm, bleaching at 310 nm) and guanine radicals (narrow absorption band at 312 nm). The fast (< 100 ns) and slow (> 100 ns) kinetic components of guanine radical formation were observed. The time dependence of the fast component, attributed to the oxidation of guanines by the radical cation 2AP^.+, was not resolved. At neutral pH, 2AP^.+ rapidly (~ 30 ns) deprotonates to the neutral radical 2AP(-H)^., which oxidizes the guanines on the 0.1-500 μs timescale and gives rise to the neutral G(-H)^. radical (the slow component). Both the prompt (< 100 ns) relative yield of the guanine radicals, Φ(G), and 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 β in dsDNA is 0.75 ± 0.20 Å^-1). When 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 (Φ(G) ~ 0) to 1.7 x 10⁶ s^-1 (Φ(G) = 0.36), indicating that the electron transfer rates can strongly dependent on the base sequence. The slow kinetic component is discussed in terms of a proton-coupled electron transfer mechanism in which electron transfer from G to 2AP(-H)^. is coupled with deprotonation of G^.+ and protonation of 2AP(-H)^-.