ENERGY TRANSFER FROM CAROTENOIDS TO CHLOROPHYLL a IN BLACK LIPID MEMBRANES

Abstract— Black lipid membranes (BLM) were prepared from extracts of Chlorella and spinach chloroplasts. Excitation spectra of the 730 nm fluorescence of chlorophyll a in the BLM contained peaks identified as due to carotenoids and which therefore indicate sensitization of the chlorophyll fluorescence by them. The efficiency of this energy transfer was evaluated by comparison of the actual excitation spectra with those corresponding to 0 and 100 per cent transfer efficiency. Efficiencies were of the order of 40–50 per cent in BLM, but only 10 per cent in pigment solutions, when the mean distance between pigment molecules was 23 Å in both systems. The fluorescence quantum yield of chlorophyll a in such solutions was only 2 per cent of that found in BLM. Enhancement of energy transfer in BLM is considered to be mainly due to suppression of competing deactivation processes of excited carotenoid states, such as diffusional quenching by ground-state molecules and internal conversion. Favorable orientation of pigment molecules in the BLM constitutes a further enhancement factor.