Photo-Induced Electron Transfer in Chlorophyll Containing Liposomes

Liposomes (lipid, DL-a-dipalmitoyl-phosphatidylcholine) containing chlorophyll a (ratio lipid to chlorophyll 30:1) exhibit an absorption maximum at 670 nm. Upon oxidation with iodine these liposomes yield a chlorophyll radical that shows a ESR signal with a line width (peak to peak) A Y = ~0.1 G and a g-value of 2.0022, consistent with the presence of monomeric chlorophyll. Under anaerobic conditions, in the absence of acceptors, no light induced ESR signal is observed, whereas under aerobic conditions the chlorophyll free radical is generated in the light. Acceptors having access to the lipid membrane, like Fe3+-pyrophosphate and methylviologen, give rise to chlorophyll radical formation, or, like quinones, form a semiquinone radical in the light. Non-permeable ac­ ceptors, such as ferricyanide, NADP, FM N and cytochrome c do not act as acceptors and no chlorophyll radicals can be produced by light. Furthermore, light dependent and completely revers­ ible electron transfer from N,N,N',N'-tetramethyl-p-phenylenediamine to ubiquinone can be de­ monstrated. Liposomes containing chlorophyll, therefore, can serve as a model system for photo­ synthetic electron transport.