HORSE HEART CYTOCHROME c AS AN ELECTRON ACCEPTOR FROM CHLOROPHYLL TRIPLET IN NEGATIVELY CHARGED LIPID BILAYER VESICLES*

Abstract

Abstract— Cytochrome c has been shown to bind via electrostatic interactions to egg phosphatidylcholine vesicles which contain 5–30 mol percent of negatively‐charged surfactant (dihexadecylphosphate) in a low ionic strength medium. Under these conditions the oxidized cytochrome can function as a direct one‐electron acceptor from membrane‐bound triplet state chlorophyll to produce chlorophyll cation radical and reduced cytochrome. Kinetic experiments using laser flash photolysis have demonstrated that triplet quenching and the yield of electron transfer products increase, and product lifetime decreases, with an increase in the magnitude of the negative charge on the vesicles, and with a decrease in the ionic strength of the medium. Both triplet quenching and product formation rates and yields showed saturation behavior as the cytochrome concentration was increased, and reached limiting values at 20–30 μM cytochrome when the vesicle contained 20 mol percent of the negatively‐charged surfactant. This behavior is interpreted in terms of saturation of the vesicle surface binding sites. Under optimum conditions in this system, approximately 20% of the chlorophyll triplet molecules could be converted to electron transfer products which had a halftime for the reverse reaction of approximately 1.5 ms.