Rose-bengal-photosensitized formation of hydrogen peroxide and hydroxyl radicals

Abstract

Electron transfer reactions between several electron donors, including common antioxidants such as ascorbate, reduced glutathione and trolox, and the lowest triplet excited state of Rose Bengal (RB) lead to the formation of free radicals that are capable of reducing molecular oxygen and redox-active metal ions. We studied RB-photosensitized redox reactions and the formation of hydroxyl radicals by direct electron spin resonance (ESR) and ESR-spin-trapping techniques respectively. Our data indicate that the free-radical pathway of RB-photosensitized oxidation reactions (PORs) can, under certain conditions, complete efficiently with the singlet oxygen route. The protective action of antioxidants in PORs may be compromised by their ability to serve as primary electron donors in photosensitized electron transfer. Owing to its metal ion sequestering ability and free-radical scavenging efficiency, melanin exhibits powerful antioxidant action that may reduce the damaging efficacy of photodynamic phenomena in pigmented cells.