Spectroscopic studies and photodynamic actions of hypocrellin B in liposomes

Hypocrellin B (HB), a lipid-soluble natural pigment of perylenequinone deri vative, is considered as potential photosensitizer for photodynamic therapy . Liposomes loaded with HE can constitute a simple model system, appropriat e for better understanding the photodynamic action of HE in vivo. The stead y-state absorption and emission spectra, quantum yield and lifetime of fluo rescence of HE incorporated into egg L-a-phosphatidyl-choline (EPC) liposom e were examined, The photochemical properties (Type I and/or Type II) of HE have also been studied in aqueous dispersions of small unilamellar liposom es of EPC using electron paramagnetic resonance and spectrophotometric meth ods, respectively. The quantum yield of O-1(2) generated by HE is ca 0.76 i n chloroform solution and it did not change upon the incorporation of HE in to liposomes of EPC, The superoxide anion radical was generated by the elec tron transfer from the anion radical of HE (HB.-) to oxygen. The disproport ionation of O-2(.-) can generate H2O2 and ultimately the highly reactive (O H)-O-. via the Fenton reaction. It could be that the disproportionation pro ceeded too fast, so we could not detect O-2(.-) directly in aqueous dispers ions of liposome EPC, Moreover, the self-sensitized photooxygenation of HE embedded in liposomes was studied, and almost fully (87%) inhibiting this r eaction of HE by p-benzoquinone las the quencher of O-2(.-)) in aqueous dis persion of liposome EPC indicated that the radical mechanism (Type I) might be mainly involved in this oxygenation. All these findings suggested that the photodynamic action of HE proceeded via both Type-I and -II mechanisms, but Type-I mechanism might play a more important role in the aqueous dispe rsion.