ESR Spectroscopy of Singlet Oxygen Generated by Protoporphyrin IX in Aqueous Surfactant Solutions.

Abstract

Generation of singlet oxygen in protoporphyrin IX (PpIX) solubilizing surfactant solutions irradiated with light was investigated by electron spin resonance (ESR) spectroscopy using 2,2,6,6-tetramethyl-4-piperidone hydrochloride (TMPD) as a spin label probe. The surfactants employed in this study were sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dodecyl hexaethylene glycol ether (C12EO6). When PpIX solubilizing solutions were irradiated with light, the ESR signal of TMPD nitroxide radical was observed in SDS and DTAB solutions, whereas no ESR signal appeared in C12EO6 solutions. The ESR signal of TMPD nitroxide radical disappeared in SDS and DTAB solutions when histidine, a quencher of singlet oxygen, was added, which indicates that the ESR signal is assigned to TMPD oxidized by singlet oxygen. Taking into account the solubility of PpIX in water and the distribution of TMPD between micelle and water phases, moreover, we consider that singlet oxygen generated in SDS and DTAB micellar phase is released to water bulk phase. It was also found that ESR signal of TMPD nitroxide radical in SDS system was gradually reduced at a longer period of irradiation. These results show that DTAB system is very useful to determine singlet oxygen generated by water-insoluble photosensitizing agents in water media by ESR spectroscopy with TMPD. In addition, light irradiation ranging from 350 to 450 nm generated singlet oxygen more than that from 550 to 750 nm in DTAB solutions.