ENERGY TRANSFER AND ENERGY LOSSES IN BILAYER MEMBRANE VESICLES (LIPOSOMES)

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Abstract
The efficiency of singlet‐singlet energy transfer was studied in bilayer lipid membrane vesicles (liposomes) for the following donor‐acceptor systems: (1) p‐terphenyl (TP) and diphenyloctatetraene (DPO); (2) DPO and chlorophyll a (Chl a); and (3) β‐carotene and Chl a. The energy transfer efficiency φDA was measured by sensitized fluorescence of the acceptor. Fractional quenching of the donor φQ was found from the donor fluorescence in absence and presence of the acceptor. For TP‐DPO and for DPO‐Chl a, the transfer efficiency increased with increasing acceptor concentration but was essentially independent of the donor concentration. No energy transfer from β‐carotene to Chl a could be detected. In liposomes, φDA differed only slightly from φQ at all donor and acceptor concentrations, thus demonstrating the absence of any appreciable energy losses. For solutions of the same donor‐acceptor pairs in cyclohexane φQ was considerably larger than φDA. The difference represents energy lost, principally by internal conversion, due to collisional quenching. The principal function of the lipid membrane appears to be the suppression of such losses. In addition, the rate of energy transfer in lipid membranes is about double that in solutions (at the same intermolecular distance) due to more favorable orientation.

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