Self-assembly and energy transfer in artificial light-harvesting complexes of bacteriochlorophyll c with astaxanthin

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.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).Peer reviewe

β-carotene to bacteriochlorophyll c energy transfer in self-assembled aggregates

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 β-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, approx. 100-fs energy transfer from the S2 state of β-carotene to bacteriochlorophyll c was revealed, while no evidence for significant energy transfer from the S1 state was found. Picosecond formation of the carotenoid triplet state (T1) was observed, which was likely generated by singlet homo-fission from the S1 state of β-carotene.This study was supported by Czech Ministry of Education, Youth and Sports (projects MSM0021620835, MSM6007665808 and AV0Z50510513), Czech Science Foundation (projects 206/09/0375, 202/09/1330 and 202/09/H041), Spanish Ministry of Education and Science (project BF2007-68107-C02-02/BMC), and AVCR-CSIC joint project (project 2008CZ0004). The authors would like to thank Marcel Fuciman and Petr Hribek for technical assistance with femtosecond spectroscopy measurements, and Ivana Hunalova, Frantisek Matousek and Anita Zupcanova for their help with pigment isolation.Peer reviewe

Excited state properties of aryl carotenoids

Excited-state properties of aryl carotenoids, important components of light harvesting antennae of green sulfur bacteria, have been studied by femtosecond transient absorption spectroscopy. To explore effects of the conjugated aryl group, we have studied a series of aryl carotenoids with conjugated phi-ring, chlorobactene, β-isorenieratene and isorenieratene, and compared them with their non-aryl counterparts γ-carotene and β-carotene, which contain β-ring. Changing β-ring to phi-ring did not reveal any changes in absorption spectra, indicating negligible effect of the phi-ring on the effective conjugation length. This observation is further supported by the carotenoid S1 lifetimes. In n-hexane, the S1 lifetime of chlorobactene having one phi-ring is 6.7 ps, while the S1 lifetime of the β-ring analog, γ-carotene is 5.4 ps. The same effect is observed for the series β-carotene (two β-rings), β-isorenieratene (one β- and one phi-ring) and isorenieratene (two phi-rings) whose S1 lifetimes in n-hexane are 8.2, 10.3 and 12.7 ps, respectively. The systematically longer lifetimes of aryl carotenoids show that the additional conjugated C=C bonds at the -ring do not contribute to the conjugation length. The S1 lifetimes of aryl carotenoids were slightly shortened in benzene, indicating π–π stacking interaction between the phi-ring and benzene.This research was supported by grants from the Czech Ministry of Education (MSM6007665808, MSM0021620835 and AV0Z50510513), and the Grant Agency of the Czech Academy of Sciences (IAA608170604). JBA thanks the Spanish Ministry of Science and Innovation (Ref. BF2007-68107-C02-02/BMC) and the AVCR-CSIC joint program(Ref. 2008CZ0004) for financial support.Peer reviewe

Self-assembly and energy transfer in artificial light-harvesting complexes of bacteriochlorophyll c with astaxanthin

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.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).Peer reviewe