Direct spectroscopic observation of 8-oxo-7,8-dihydro-2′-deoxyguanosine radicals in double-stranded DNA generated by one-electron oxidation at a distance by 2-aminopurine radicals

Oligonucleotide duplexes (15-mers) containing 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) and the base analogue 2-aminopurine (2AP) separated from one another by zero, two, or four thymidine residues on the same strand, were synthesized. Selective two-photon excitation of 2AP with intense 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ∼80 mJ pulse^-1 cm^-2) gives rise to 2AP radical cations that, in turn, oxidize 8-oxo-dG at a distance. Transient absorption techniques allowed for the direct monitoring of the oxidant and product radical intermediates as a function of time on the 100 ns-1 ms time scale and revealed the existence of two-component oxidation kinetics. The evolution in time of the rapid component (<100 ns), attributed to the oxidation of 8-oxo-dG by the radical cation 2AP·⁺, was not resolved. However, this component was observed only in the case of zero or two intervening thymidine bases and was not evident when the 2AP and 8-oxo-dG residues were separated by four thymidines. A slower kinetic component (> 100 ns) is attributed to the oxidation of 8-oxo-dG by the deprotonation product of 2AP·⁺, the neutral radical 2AP(-H)·. Only the slow, microsecond kinetic component was observed when the 8-oxo-dG and 2AP are separated by four thymidines. The rate constants of electron transfer are (3.8 ± 0.5) × 10⁴ s^-11 and (3.0 ± 0.5) × 10³ s^-1 in the case of two and four intervening thymidines, respectively. Because of the pK_a properties of the donor/acceptor couples, this slow, 0.1 - 1000 μs time scale electron transfer at a distance from 8-oxo-dG to the 2AP(-H)· radicals, is likely to occur via a proton-coupled electron-transfer mechanism.