Proton-Coupled Electron-Transfer Reactions at a Distance in DNA Duplexes:  Kinetic Deuterium Isotope Effect

Kinetic solvent isotope effects on the electron-transfer kinetics associated with the oxidation of guanine by a 2-aminopurine (2AP) neutral radical separated by 2 or 3 thymine, or 6 adenine residues on the same strand in 15-mer double-stranded oligonucleotides in H 2 O or D 2 O were measured. The evolution in time of the oxidized form of guanine, the neutral radical G(−H) •, and the electron acceptor, the neutral 2AP(−H) • radical, were followed directly by a spectroscopic laser pulse-induced transient absorption technique on a 1−200 μs time scale. About 70−100% of this one-electron-transfer reaction occurs on this relatively slow time scale in the three oligonucleotide duplexes studied. The rate constants of formation of G(−H) • in the oligonucleotides are larger in H 2 O than in D 2 O by a factor of 1.3−1.7. This kinetic isotope effect suggests that the electron-transfer reaction from G to 2AP(−H) • is coupled to a deprotonation of G •+, and a protonation of 2AP -, the primary products of the electron-transfer reaction. Thus, electron-transfer reactions occurring at a distance in these DNA duplexes can be considered in terms of proton-coupled electron-transfer reactions.