SPINACH PLASTOCYANIN AS AN ELECTRON ACCEPTOR FROM MEMBRANE‐BOUND CHLOROPHYLL TRIPLET STATE IN POSITIVELY CHARGED LIPID BILAYER VESICLES*

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Abstract
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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