Effect of structural modifications on photosensitizing activities of hypocrellin dyes: EPR and spectrophotometric studies.

Mono-substituted hypocrellin B (MHB) and di-substituted hypocrellin B (DHB) by mercaptoacetic acid are new photosensitizers synthesized to improve the red absorption and water solubility of the parent hypocrellin B (HB). The photochemistries (Type I and/or Type II) of MHB and DHB have been studied in homogeneous solutions using electron paramagnetic resonance (EPR) and spectrophotometric methods. In anaerobic homogeneous DMSO solution, DHB*- (or MHB*-) was predominantly photoproduced via self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the formation of the reduced form of DHB (or MHB). As compared with hypocrellin B, the efficiencies of DHB*- and MHB*- generation was enhanced obviously. When oxygen-saturated solutions of DHB (or MHB) were illuminated with 532 nm light, singlet oxygen (1O2), superoxide anion radical (O2*-), hydroxyl radical (*OH) and hydrogen peroxide (H2O2) were formed. DHB and MHB generate 1O2 with quantum yields of 0.18 and 0.22, respectively, which are much lower than that of HB (0.76) in chloroform. The superoxide anion radical was significantly enhanced by the presence of electron donors. The rate of O2*- production was also dependent on the concentration of DHB or MHB. Moreover, O2*- upon disproportionation can generate H2O2 and ultimately the highly reactive *OH via the Fenton reaction and other pathway with the involvement of DHB*- (or MHB*-). As in the case of DHB*- (or MHB*-), the efficiencies of O2*- and *OH generation by DHB and MHB were also enhanced obviously, consistent with the fact that DHB*- (or MHB*-) acts as the precursor of O2* and thus *OH. These findings suggest that the photodynamic actions of DHB and MHB may proceed via enhanced Type I mechanism and reduced Type II mechanism as compared with that of HB.