Femtosecond excited-state dynamics in chlorosomal carotenoids of the photosynthetic bacterium Chloroflexus aurantiacus revealed by near infrared pump-probe spectroscopy

Efficient two-step excitation energy transfer in artificial light-harvesting antenna based on bacteriochlorophyll aggregates

Chlorosomes of green photosynthetic bacteria are large light-harvesting complexes enabling these organisms to survive at extremely low-light conditions. Bacteriochlorophylls found in chlorosomes self-organize and are ideal candidates for use in biomimetic light-harvesting in artificial photosynthesis and other applications for solar energy utilization. Here we report on the construction and characterization of an artificial antenna consisting of bacteriochlorophyll c co-aggregated with β-carotene, which is used to extend the light-harvesting spectral range, and bacteriochlorophyll a, which acts as a final acceptor for excitation energy. Efficient energy transfer between all three components was observed by means of fluorescence spectroscopy. The efficiency varies with the β-carotene content, which increases the average distance between the donor and acceptor in both energy transfer steps. The efficiency ranges from 89 to 37% for the transfer from β-carotene to bacteriochlorophyll c, and from 93 to 69% for the bacteriochlorophyll c to bacteriochlorophyll a step. A significant part of this study was dedicated to a development of methods for determination of energy transfer efficiency. These methods may be applied also for study of chlorosomes and other pigment complexes.

Quenching of bacteriochlorophyll a triplet state by carotenoids in the chlorosome baseplate of green bacterium Chloroflexus aurantiacus

To capture weak light fluxes, green photosynthetic bacteria have unique structures - chlorosomes, consisting of 104-5 molecules of bacteriochlorophyll (BChl) c, d, e. Chlorosomes are attached to the cytoplasmic membrane through the baseplate, a paracrystalline protein structure containing BChl a and carotenoids (Car). The most important function of Car is the quenching of triplet states of BChl, which prevents the formation of singlet oxygen and thereby provides photoprotection. In our work, we studied the dynamics of the triplet states of BChl a and Car in the baseplate of Chloroflexus aurantiacus chlorosomes using picosecond differential spectroscopy. BChl a of the baseplate was excited into the Qy band at 810 nm, and the corresponding absorption changes were recorded in the range of 420-880 nm. It was found that the formation of the Car triplet state occurs in ∼1.3 ns, which is ∼3 times faster than the formation of this state in the peripheral antenna of C. aurantiacus according to literature data. The Car triplet state was recorded by the characteristic absorption band T1 → Tn at ∼550 nm. Simultaneously with the appearance of absorption T1 → Tn, there was a bleaching of the singlet absorption of Car in the region of 400-500 nm. Theoretical modeling made it possible to estimate the characteristic time of formation of the triplet state of BChl a as ∼0.5 ns. It is shown that the experimental data are well described by the sequential scheme of formation and quenching of the BChl a triplet state: BChl a* → BChl aT → CarT. Thus, carotenoids from green bacteria effectively protect the baseplate from possible damage by singlet oxygen.