β-Carotene to bacteriochlorophyll c energy transfer in self-assembled aggregates mimicking chlorosomes

Carotenoids are together with bacteriochlorophylls important constituents of chlorosomes, the light-harvesting antennae of green photosynthetic bacteria. Majority of bacteriochlorophyll molecules form self-assembling aggregates inside the chlorosomes. Aggregates of bacteriochlorophylls with optical properties similar to those of chlorosomes can also be prepared in non-polar organic solvents or in aqueous environments when a suitable non-polar molecule is added. In this work, the ability of beta-carotene to induce aggregation of bacteriochlorophyll c in aqueous buffer was studied. Excitation relaxation and energy transfer in the carotenoid-bacteriochlorophyll assemblies were measured using femtosecond and nanosecond transient absorption spectroscopy. A fast, similar to 100-fs energy transfer from the S-2 state of beta-carotene to bacteriochlorophyll c was revealed, while no evidence for significant energy transfer from the S-1 state was found. Picosecond formation of the carotenoid triplet state (T-1) was observed, which was likely generated by singlet homo-fission from the S-1 state of beta-carotene. (C) 2010 Elsevier B. V. All rights reserved.