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)₂dppz²⁺ (OP = 1,10-phenanthroline, dppz = dipyrido [3,2-a:2',3'-c]phenazine) in the presence of a typical quencher methyl viologen (MV²⁺). Using ESR spin-trapping method, the radical intermediate MV^•+ with typical hyperfine pattern was detected which indicated the successful formation of the corresponding Ru³⁺ intercalated oxidant. The formation of dIz and dZ decreased markedly with the addition of nitrotetrazolium blue chloride (NBT), a typical O₂^•- reactant. With a more specific and highly sensitive O₂^•- probe CT02-H, its ESR signal decayed rapidly in the presence of Ru(OP)₂dppz²⁺ and MV²⁺, suggesting that O₂^•- was indeed produced. More interestingly, enantio-selective generation of oxidation products from dsDNA was observed with the two chiral forms of Ru(OP)₂dppz²⁺. This represents the first report that the flash-quench technique with MV²⁺ as the quencher can oxidize dsDNA effectively to form dIz and dZ via the Ru³⁺/O₂^•- mediated mechanism. Our new findings provide a novel method to generate two radicals simultaneously, G (-H)^• and O₂^•-, in close proximity to one another in dsDNA.