Proton-coupled electron-transfer reactions at a distance in DNA duplexes: Kinetic deuterium isotope effect

Kinetic solvent isotope effects on the electron-transfer kinetics associate d with the oxidation of guanine by a 2-aminopurine (2AP) neutral radical se parated by 2 or 3 thymine, or 6 adenine residues on the same. strand in 15- mer double-stranded oligonucleotides in H2O or D2O were measured. The evolu tion in time of the oxidized form of guanine, the neutral radical G(-H)(.), and the electron acceptor, the neutral 2AP(-H)(.) radical, were followed d irectly by a spectroscopic laser pulse-induced transient absorption techniq ue on a 1-200 mus time scale. About 70-100% of this one-electron-transfer r eaction occurs on this relatively slow time scale in the three oligonucleot ide duplexes studied. The rate constants of formation of G(-H)(.) in the ol igonucleotides are larger in H2O than in D2O by a factor of 1.3-1.7. This k inetic isotope effect suggests that the electron-transfer reaction from G t o 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 con sidered in terms of proton-coupled electron-transfer reactions.