PHOTOBLEACHING OF MEROCYANINE 540: INVOLVEMENT OF SINGLET MOLECULAR OXYGEN

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Abstract
Abstract— The purpose of this study was to assess the mechanism of merocyanine 540 (MC540) photobleaching in a liposomal system. Broad based visible irradiation of MC540 in unilamellar dilauroylphosphatidylcholine (DLPC) vesicles resulted in dye bleaching that was strictly O2 dependent. The rate of self‐sensitized photobleaching was enhanced in D2O and inhibited by both azide and histidine, consistent with 1O2 intermediacy (Type II chemistry). Supportive evidence for this mechanism was obtained by using a Type II sensitizer, aluminum phthalocyanine tetrasulfonate (AIPcS Λmax, = 678 nm). Irradiation of AIPcS and MC540 in DLPC with Λ > 630 nm (absorbed only by AIPcS) light resulted in rapid bleaching of MC540, which was stimulated by D2O and inhibited by azide. A rate constant of 107M‐1 s‐1 was determined for the chemical quenching of 1O2 by MC540. The rate constant for physical quenching of 1O2 by MC540 was estimated to be ca 109M‐1 s‐1.