EPR STUDIES OF THE PHOTODYNAMIC-ACTION OF MERCAPTO-SUBSTITUTED HYPOCRELLIN-B DERIVATIVES - FORMATION OF SEMIQUINONE RADICAL-ANION AND ACTIVATED OXYGEN ON ILLUMINATION WITH VISIBLE-LIGHT

Perylenequinonoid pigments exhibit several photodynamic therapeutic ad vantages over the commonly used haematoporphyrin derivatives. Two type s of mercapto-substituted hypocrellin B derivatives (5,8-RS-HB, 5-RS-H B) were prepared. The photophysical and photochemical properties of th e mercapto-substituted hypocrellin B derivatives were investigated. Wh en 5,8-RS-HB or 5-RS-HB was illuminated with visible light, semiquinon e radical anion, superoxide anion radical, singlet oxygen and hydroxyl radical were detected. The formation of semiquinone radical anion and activated oxygen and the transformation and competition between them depend on the sample and oxygen concentrations, the time and intensity of illumination and the nature of the substrate. In anaerobic solutio n, the semiquinone radical anion was predominantly photoproduced via t he self-electron transfer between the excited and ground species. In a erobic solution, singlet oxygen was generated. In addition, the supero xide anion radical was also generated by 5,8-RS-HB or 5-RS-HB on illum ination in aerobic solution. The superoxide anion radical was produced via the reduction of oxygen by the semiquinone radical anion, and thi s process was significantly enhanced by the presence of electron donor s. In DMSO-H2O (1 : 1, v/v) solution, the hydroxyl radical was observe d via the Fenton reaction. On the basis of these results, both electro n transfer (type I) and O-1(2) (type II) paths were found to be involv ed in 5,8-RS-HB and 5-RS-HB photosensitization to different extents. ( C) 1997 Elsevier Science S.A.