Effects of lysophosphatidylcholine micelle and phosphatidylcholine liposome on photoreduction of methylviologen

Abstract

Photoinduced reduction of methylviologen (MV 2+) by ethylenediaminetetraacetate (EDTA 3−), which was sensitized by thiacarbocyanine dyes having long alkyl chains (C m−n +) embedded in palmitoyl lysophosphatidylcholine micelle and dipalmitoyl phosphatidylcholine liposomal membrane, was carried out. The formation rate of reduced methylviologen cation radical (MV +.) decreased with the time of irradiation with visible light, and the deceleration was more pronounced in the micellar solution. In kinetic studies, we found that the sensitizer divalent cation radical (C m−n 2+.) is formed through the reaction of photoexcited sensitizer (C m−n 2+∗) with MV 2+ as an intermediate in this reaction, and that the reduction of C m−n 2+. with EDTA 3− inhibits the back reaction of MV +. with C m−n 2+.. The inhibition was greater in the liposomal solution than in the micellar solution. This was ascribed to a higher concentration of EDTA 3− on the liposomal surfaces through the electrostatic interaction between EDTA 3− and the liposomal surfaces, the charge of which is attributed to the univalent cation sensitizer embedded in the liposomal membrane. The difference in the positive charge density of the surface of these lipid aggregates was due to the difference in the curvature of the micelle and the liposome. These results suggest that the dipalmitoyl phosphatidylcholine liposome is a more effective carrier than the palmitoyl lysophosphatidylcholine micelle for the production of MV +. in the photoreduction studied here.