Enantioselectively generating imidazolone dIz by the chiral DNA intercalating and "light-switching" Ru(II) polypyridyl complex via a novel flash-quench method.

The 2-aminoimidazolone is a major and ubiquitous in vitro product of guanine oxidation. The flash-quench method, combining spectroscopy and product analysis, offers a novel and tunable approach to study guanine oxidation on double helical DNA. Herein we found that imidazolone dIz (2-amino-5-[(2-deoxy-β-D-erythro-pentofuranosyl)amino]-4H-imidazole-4-one) and dZ (2,2-diamino-5-[2-deoxy-β-D-erythro-pentofuranosyl)amino]-5(2H)-oxazolone) were the major oxidation products of double-strand DNA from the visible-light irradiation of the well-known DNA intercalating and light-switching Ru(OP)dppz (OP = 1,10-phenanthroline, dppz = dipyrido [3,2-a:2',3'-c]phenazine) in the presence of a typical quencher methyl viologen (MV). Using ESR spin-trapping method, the radical intermediate MV with typical hyperfine pattern was detected which indicated the successful formation of the corresponding Ru intercalated oxidant. The formation of dIz and dZ decreased markedly with the addition of nitrotetrazolium blue chloride (NBT), a typical O reactant. With a more specific and highly sensitive O probe CT02-H, its ESR signal decayed rapidly in the presence of Ru(OP)dppz and MV, suggesting that O was indeed produced. More interestingly, enantio-selective generation of oxidation products from dsDNA was observed with the two chiral forms of Ru(OP)dppz. This represents the first report that the flash-quench technique with MV as the quencher can oxidize dsDNA effectively to form dIz and dZ via the Ru/O mediated mechanism. Our new findings provide a novel method to generate two radicals simultaneously, G (-H) and O, in close proximity to one another in dsDNA.