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.
The effects of electrostatic interactions on the dynamics of photoinduced electron transfer reactions involving chlorophyll and electrically charged acceptors (either positively charged methyl viologen or negatively charged sulfonated quinones) have been investigated by laser flash photolysis in lipid bilayer vesicles into which varying amounts (0-30 mole percent) of positively or negatively charged surfactants were incorporated. Chorophyll triplet decay kinetics were modified due both to vesicle expansion caused by charge repulsion effects, and to changes in the local concentration of quenchers resulting from attractive or repulsive interactions with the vesicle surface. Radical yields were either increased or decreased as a result of electrostatic interactions which occurred between the radical products of triplet quenching and the charged surface, and which acted upon the radical ion-pair separation process. In some cases, these effects were quite large. Radical decay halftimes were also changed by large amounts as a consequence of either attractive or repulsive forces acting upon the acceptor ion-radical species, which influenced its ability to undergo reverse electron transfer to oxidized chorophyll. In the most favorable case, approximately 100% of the chlorophyll triplets produced by pulsed laser excitation were converted into radicals, which decayed by reverse electron transfer with a halftime of 70 ms.
Spinach plastocyanin is bound to egg phosphatidylcholine vesicles containing 5–25 mole percent dioctadecyldimethylammonium chloride (DODAC) via electrostatic interactions in a 50 mM betaine medium (pH=6.5). This was demonstrated by both gel filtration experiments and kinetic results using laser flash photolysis. Under those conditions, oxidized plastocyanin can function as a direct electron acceptor from membrane‐bound triplet chlorophyll to produce chlorophyll cation radical and reduced plastocyanin. The fraction of chlorophyll triplet which is quenched by oxidized plastocyanin increases, and the yield of electron transfer products also increases, with an increase in the magnitude of the positive charge on the vesicles. Product decay and rise halftimes decrease with an increase in the mole percent of DODAC+ incorporated into egg phosphatidylcholine vesicles. However, both of these halftimes are independent of oxidized plastocyanin concentration. Even though ∼50% of the Chi triplets were quenched, no electron transfer product formation was observed in 5 mM phosphate buffer (pH=7.0). Under similar conditions in betaine, approximately 13% of the Chi triplets could be converted into products.
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