Photosensitization with anticancer agents 19. EPR studies of photodynamic action of calphostin C: Formation of semiquinone radical and activated oxygen on illumination with visible light

Abstract

When calphostin C was illuminated with visible light, the semiquinone radical, singlet oxygen, and superoxide anion radical were detected. The formation of the semiquinone radical and activated oxygen species and the transformations and competitions between them depend upon the quinone and oxygen concentrations, time and intensity of illumination, and the nature of the substrate. In anaerobic solution, the semiquinone radical was predominantly photoproduced via the self-electron transfer between the excited and ground species. In aerobic solution, singlet oxygen is the principal product in the photosensitization of calphostin C. In addition to singlet oxygen, superoxide anion radical is also generated by the quinones upon illumination in aerobic solution, but to a lesser extent than singlet oxygen. The superoxide anion is produced via the reduction of oxygen by the semiquinone radical, and this process is significantly enhanced by the presence of electron donors.