Chem. Pharm. Bull. 54(7) 1004—1009 (2006)

ers used in photodynamic therapy. Upon light irradiation, porphyrins produce singlet oxygen, which causes cytotoxic activity at specific subcellular sites, leading to cell death. The quantum yield of singlet oxygen depends on the porphyrin employed, being significantly lowered by dimerization or aggregation of the porphyrin. The aggregation of porphyrins also affects the transfer of electrons from porphyrins to accepter molecules, and the efficiency of a monomer was greater than that of a dimer by a factor of about two. We are interested in examining the influence of the aggregation of porphyrins in terms of the ability to photogenerate hydrogen peroxide (H2O2). Although this approach has provided significant information about the photodynamic actions of porphyrins involving the electron transfer process, few attempts have been made to clarify structure–activity relationships, partly due to the lack of an appropriate analytical method for determining H2O2. Although H2O2 has usually been determined with a colorimetric method based on an enzymatic reaction using peroxidase coupled with phenol and 4-aminoantipyrine, there is a large degree of interference from strong absorbance due to porphyrin itself. Thus, we employed a luminol chemiluminescence method making it possible to detect down to 1 mM of H2O2 even in the presence of a high concentration of porphyrin. Using this method, we found that highly aggregated porphyrin was much more active in producing H2O2 than the dimer and concluded that a highly aggregated form of porphyrin had a semiconductorlike role, causing effective charge separation, leading to a larger amount of H2O2. 8)