ESR studies of the photodynamic properties of a long-wavelength and water-soluble hypocrellin B derivative: photogeneration of semiquinone radical anion and activated oxygen

Abstract

15-Deacetyl-13-glycine hypocrellin B (13-Gly-DHB) is a new type of long-wavelength perylenequinone with enhanced water solubility. Its photophysical and photochemical properties were investigated by electron spin resonance (ESR) spectrum associated with spin trapping techniques. When 13-Gly-DHB in DMSO or DMSO-buffer (pH 7.4) were illuminated with visible light >470 nm, semiquinone radical anion, superoxide anion radical, hydroxyl radical and singlet oxygen were formed depending on different conditions. In anaerobic solution, semiquinone radical anion of 13-Gly-DHB was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of 13-Gly-DHB. In aerobic solution, superoxide anion radical (O ⋅− 2) was generated by the 13-Gly-DHB anion radical (13-Gly-DHB ⋅−) via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. In aqueous solution, the superoxide radical anion formed can rapidly transform to a hydroxyl radical via the Fenton reaction and via the reaction of the semiquinone radical anion with H 2O 2. Singlet oxygen ( 1O 2) was also formed in the photosensitization of 13-Gly-DHB in an aerobic solution. These findings suggest that the photodynamic action of 13-Gly-DHB may proceed via both type I and type II mechanisms.