EPR and spectrophotometric studies on free radicals (O2.-, Cysa-HB.-) and singlet oxygen (1O2) generated by irradiation of cysteamine substituted hypocrellin B.

PURPOSE: The aim of this work was to investigate the photodynamic action of cysteamine substituted hypocrellin B (Cysa-HB) photochemically and survey the effect of oxygen on its reaction mechanism. MATERIALS AND METHODS: The EPR method is used to detect the generation of the semiquinone anion radical of Cysa-HB (Cysa-HB.-), and active oxygen species using DMPO and TEMP as spin traps. DPA-bleaching method is used to determine the quantum yield of 1O2 generated from Cysa-HB photosensitization. RESULTS: In anaerobic solution, Cysa-HB.- was predominantly photoproduced via the self-electron transfer between the excited and ground state species. The presence of electron donors significantly promotes the reduced form of Cysa-HB. When oxygen-saturated solutions of Cysa-HB were illuminated with visible light, superoxide anion radical (O2.-) and singlet oxygen (1O2) were formed. The superoxide anion radical was generated by Cysa-HB.- via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The 1O2 quantum yield is estimated to be 0.71. Moreover, the accumulation of 1O2 will be replaced by that of Cysa-HB.- with the depletion of oxygen. CONCLUSIONS: These observations suggest that the photodynamic action of Cysa-HB may proceed via both a Type I and Type II mechanism, and that a Type II mechanism will transform into a Type I mechanism as oxygen gets depleted.