Self-assembly and energy transfer in artificial light-harvesting complexes of bacteriochlorophyll c with astaxanthin
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Chlorosomes, the light-harvesting antennae of green photosynthetic bacteria, are based on large aggregates of bacteriochlorophyll molecules. Aggregates with similar properties to those in chlorosomes can also be prepared in vitro. Several agents were shown to induce aggregation of bacteriochlorophyll c in aqueous environments, including certain lipids, carotenes, and quinones. A key distinguishing feature of bacteriochlorophyll c aggregates, both in vitro and in chlorosomes, is a large (>60 nm) red shift of their Qy absorption band compared with that of the monomers. In this study, we investigate the self-assembly of bacteriochlorophyll c with the xanthophyll astaxanthin, which leads to the formation of a new type of complexes. Our results indicate that, due to its specific structure, astaxanthin molecules competes with bacteriochlorophylls for the bonds involved in the aggregation, thus preventing the formation of any significant red shift compared with pure bacteriochlorophyll c in aqueous buffer. A strong interaction between both the types of pigments in the developed assemblies, is manifested by a rather efficient (~40%) excitation energy transfer from astaxanthin to bacteriochlorophyll c, as revealed by fluorescence excitation spectroscopy. Results of transient absorption spectroscopy show that the energy transfer is very fast (<500 fs) and proceeds through the S2 state of astaxanthin.
KeywordsLight-harvesting Chlorosomes Self-assembly Bacteriochlorophyll aggregates Astaxanthin
This study was supported by the Czech Ministry of Education, Youth and Sports (projects MSM0021620835, MSM6007665808, AV0Z50510513), Czech Science Foundation (projects 206/09/0375, 202/09/H041, 202/09/1330), and Spanish Ministry of Science and Innovation (AVCR-CSIC joint project 2008CZ0004). The authors would like to thank Ivana Hunalova, Frantisek Matousek, and Anita Zupcanova for their help with pigment isolation.
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