Generation of 8-oxo-7,8-dihydroguanine in G-Quadruplexes Models of Human Telomere Sequences by One-electron Oxidation

The mechanistic aspects of one-electron oxidation of G-quadruplexes in the basket (Na+ ions) and hybrid (K+ ions) conformations were investigated by transient absorption laser kinetic spectroscopy and HPLC detection of the 8-oxo-7,8-dihydroguanine (8-oxoG) oxidation product. The photo-induced one-electron abstraction from G-quadruplexes was initiated by sulfate radical anions (SO4-) derived from the photolysis of persulfate ions by 308 nm excimer laser pulses. In neutral aqueous solutions (pH 7.0), the transient absorbance of neutral guanine radicals, G(-H), is observed following the complete decay of SO4- radicals (similar to 10 mu s after the actinic laser flash). In both basket and hybrid conformations, the G(-H) decay is biphasic with one component decaying with a lifetime of similar to 0.1 ms, and the other with a lifetime of 20-30 ms. The fast decay component (similar to 0.1 ms) in G-quadruplexes is correlated with the formation of 8-oxoG lesions. We propose that in G-quadruplexes, G(-H) radicals retain radical cation character by sharing the N1-proton with the O-6-atom of G in the [G(+): G] Hoogsteen base pair; this [G(-H): H(+)G reversible arrow G(+): G] leads to the hydration of G(+) radical cation within the millisecond time domain, and is followed by the formation of the 8-oxoG lesions.