An ESR study of the visible light photochemistry of gilvocarcin V.

Photolysis of gilvocarcin (GV) at 405 nm in argon saturated dimethylsulfoxide (DMSO) or 50% DMSO-water solutions in the presence of the sodium salt of 3,5-dibromo-2,6-dideutero-4-nitrosobenzene sulfonic acid (DBNBS-d2) generates the CH3-DBNBS-d2.spin adduct. It is postulated that this spin adduct is produced by photoreduction of DMSO by GV and the consequent formation and trapping of the generated methyl radicals. Gilvocarcin V also photoreduces oxygen and methyl viologen with quantum yields of 0.019 and 0.0012 respectively. The quantum yield for singlet oxygen formation by GV in DMSO, determined by measuring the rate of production of the nitroxyl radical produced by the reaction of 2,2,6,6-tetramethylpiperidinol with singlet oxygen, was found to be 0.15. Thus, GV photochemistry proceeds by both Type I and Type II pathways which could contribute to the reported GV phototoxicity in biological systems.