Mechanistic Aspects of Hydration of Guanine Radical Cations in DNA

The mechanistic aspects of hydration of guanine radical cations, G(•+) in double- and single-stranded oligonucleotides were investigated by direct time-resolved spectroscopic monitoring methods. The G(•+) radical one-electron oxidation products were generated by SO4(•-) radical anions derived from the photolysis of S2O8(2-) anions by 308 nm laser pulses. In neutral aqueous solutions (pH 7.0), after the complete decay of SO4(•-) radicals (∼5 μs after the actinic laser flash) the transient absorbance of neutral guanine radicals, G(-H)(•) with maximum at 312 nm, is dominant. The kinetics of decay of G(-H)(•) radicals depend strongly on the DNA secondary structure. In double-stranded DNA, the G(-H)(•) decay is biphasic with one component decaying with a lifetime of ∼2.2 ms and the other with a lifetime of ∼0.18 s. By contrast, in single-stranded DNA the G(-H)(•) radicals decay monophasically with a ∼ 0.28 s lifetime. The ms decay component in double-stranded DNA is correlated with the enhancement of 8-oxo-7,8-dihydroguanine (8-oxoG) yields which are ∼7 greater than in single-stranded DNA. In double-stranded DNA, it is proposed that the G(-H)(•) radicals retain radical cation character by sharing the N1-proton with the N3-site of C in the [G(•+):C] base pair. This [G(-H)(•):H(+)C ⇆ G(•+):C] equilibrium allows for the hydration of G(•+) followed by formation of 8-oxoG. By contrast, in single-stranded DNA, deprotonation of G(•+) and the irreversible escape of the proton into the aqueous phase competes more effectively with the hydration mechanism, thus diminishing the yield of 8-oxoG, as observed experimentally.