Asymmetric photogeneration of the magnesium octaethylporphyrin cation in vesicular bilayers

Abstract

Three examples of inside/outside asymmetric behavior result from the flash photolysis of aqueous suspensions of small ({approx} 250 {angstrom} in diameter) egg yolk phosphatidylcholine (PC) vesicles containing magnesium octaethylporphyrin in the presence of water-soluble electron acceptors (either methylviologen or ferricyanide): (1) Many more porphyrin cations are produced by reaction with the electron acceptor on the inside of the vesicle than by reaction with the electron acceptor on the outside. (2) The lifetime of the porphyrin triplet is much shorter when the electron acceptor is on the inside of the vesicle than when it is present only on the outside of the vesicle. (3) The lifetime of a porphyrin cation produced by reaction with an electron acceptor on the inside of a vesicle is much shorter than that of a porphyrin cation produced by reaction with this same electron acceptor on the outside. All of these observations are shown to be consistent with a model which presumes that there are two populations of the porphyrin within the vesicle bilayer on or near each surface and that the concentration of porphyrin associated with the inside vesicle surface is much higher than that associated with the outside vesicle surface. This model agrees withmore » the one proposed by Tollin and co-workers for the distribution of chlorophyll a molecules in small PC vesicles.« less