Low-Energy, Low-Yield Photoionization, and Production of 8-Oxo-2‘-deoxyguanosine and Guanine from 2‘-Deoxyguanosine

Experiments employing electron scavenging methods and high performance liquid chromatography with mass spectrometry detection indicate that electrons, formed via one-photon ionization, guanine (G) and small amounts of 8-oxo-2‘-deoxyguanosine (8-oxo-dG) are formed during 254 nm irradiation of deaerated alkaline 2‘-deoxyguanosine (dG) solutions containing N2O. The G, 8-oxo-dG and electron yields vary in a similar way when the pH changes. At a dG concentration of 1.2 × 10-4 M, the 254 nm photoionization quantum yield is in the range 0.01 to 0.02 at pH 11.4. The low sensitivity with which photoionization can be monitored in electron scavenging experiments does not permit the direct measurement of dG photoionization at near neutral pH. At pH 6.3, the 254 nm photoionization quantum yield for dG is no larger than 0.003. At pH 11.4, the yields of G and 8-oxo-dG formed from 254 nm irradiation of 1.2 × 10-4 M dG for 40 min are more than 5× larger than that at pH 6.3. The similar pH dependence of the G, 8-oxo-dG and photoelectron yields, and earlier reports linking G and 8-oxo-dG formation to dG photoionization provide evidence that, under the present conditions, G and 8-oxo-dG quantum yields, which can be monitored with higher sensitivity than hydrated electrons in electron scavenging experiments, parallel the photoionization quantum yield. From this perspective, the wavelength dependence of G and 8-oxo-dG quantum yields indicates that dG ionization threshold wavelengths at pH 11.4 and 6.3 are 266 ± 16 and 260 ± 16 nm, respectively. When the 260 nm threshold of dG at pH 6.3 is adjusted to account for an approximately 0.5 eV lowering of the ionization energy associated with incorporation of dG into native B DNA sequences containing multiple stacked guanines (Sugiyama, H.; Saito, I. J. Am. Chem. Soc. 1996, 118, 7063−7068. Zhu, Q.; LeBreton, P. R. J. Am. Chem. Soc. 2000, 122, 12 824−12 834.), the present results provide evidence that low quantum-yield DNA photoionization occurs near the short-wavelength cutoff (290 nm) of the ground-level solar spectrum.