Carotenoid Single-Molecular Singlet Fission and the Photoprotection of a Bacteriochlorophyll b‑Type Core Light-Harvesting Antenna

• Published in: The journal of physical chemistry letters 2022-04, Vol.13 (16), p.3534-3541
• Main Authors: Zhang, Yan, Qi, Chen-Hui, Yamano, Nami, Wang, Peng, Yu, Long-Jiang, Wang-Otomo, Zheng-Yu, Zhang, Jian-Ping
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - metabolism; Bacteriochlorophylls - metabolism; Carotenoids; Light-Harvesting Protein Complexes - metabolism; Photosynthesis; Physical Insights into Light Interacting with Matter
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.2c00519

Carotenoid (Car) in photosynthesis plays the major roles of accessary light harvesting and photoprotection, and the underlying structure–function relationship attracts continuing research interests. We have attempted to explore the dynamics of Car triplet excitation (3Car*) in the bacteriochlorophyll b (BChl b)-type light harvesting reaction center complex (LH1-RC) of photosynthetic bacterium Halorhodospira halochloris. We show that the LH1 antenna binds a single Car that was identified as a lycopene derivative. Although the Car is hardly visible in the LH1-RC stationary absorption, it shows up conspicuously in the triplet excitation profile with distinct vibronic features. This and the ultrafast formation of 3Car* on direct photoexcitation of Car unequivocally manifest the unimolecular singlet fission reaction of the Car. Moreover, the Car with even one molecule per complex is found to be rather effective in quenching 3BChl b*. The implications of different 3Car* formation mechanisms are discussed, and the self-photoprotection role of BChl b are proposed for this extremophilic species.