Decay of Guanine Radical Produced by the Flash Quench Technique: Dependence Upon Quencher
Authors:Evelyn Carillo, Jenny Garcia, Veronica Torres
Mentor:Eric Stemp, Department Chair, Physical Sciences, Mount St. Mary's College
Oxidative damage to DNA is a causative factor in cancer and other diseases. Guanine is the DNA base that is most susceptible to 1-electron oxidation and this is readily accomplished using the flash quench technique, where a photoexcitedintercalator is quenched by an electron acceptor to produce a strong oxidant bound to DNA. Here, we compared the guanine radical decay in the presence of two different quenchers, Ru and Co. The guanine radical has broad absorption bands near 390 nm and 550 nm. Using nanosecond transient absorption spectroscopy, the 373 nm decay of the guanine radical is nearly complete within 100 µs with Ru(NH3)63+ as quencher, whereas the 373 nm signal persists for longer than one millisecond with Co(NH3)5Cl2+ as the quencher. On a longer timescale, excitation with a camera flash produces no detectable transient species with Ru(NH3)63+ , but a species with a broad absorption maximum near 380 nm, as seen for guanine radical, is present for several seconds after analogous treatment in the presence of Co(NH3)5Cl2+. These results can be explained by the lability of the Co(II) complex found formed upon reduction, and formation of a guanine radical that decays into another related radical species on a longer timescale.