Energy dissipative photoprotective mechanism of carotenoid spheroidene from the photoreaction center of purple bacteria Rhodobacter sphaeroidesElectronic supplementary information (ESI) available: Optimized structures of singlet-triplet crossing and MEISC points, convergence behaviour of singlet-triplet energy gap during MEISC optimization, some additional singlet-triplet crossing potential energy surfaces. Complete lists of the authors for ref. 58 and 59. See DOI: 10.1039/c5cp03089g

• Main Authors: Arulmozhiraja, Sundaram, Nakatani, Naoki, Nakayama, Akira, Hasegawa, Jun-ya
• Format: Journal Article
• Language: English
• Published: 02.09.2015
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c5cp03089g

Carotenoid spheroidene (SPO) functions for photoprotection in the photosynthetic reaction centers (RCs) and effectively dissipates its triplet excitation energy. Sensitized cis -to- trans isomerization was proposed as a possible mechanism for a singlet-triplet energy crossing for the 15,15′- cis -SPO; however, it has been questioned recently. To understand the dissipative photoprotective mechanism of this important SPO and to overcome the existing controversies on this issue, we carried out a theoretical investigation using density functional theory on the possible triplet energy relaxation mechanism through the cis -to- trans isomerization. Together with the earlier experimental observations, the possible mechanism was discussed for the triplet energy relaxation of the 15,15′- cis -SPO. The result shows that complete cis -to- trans isomerization is not necessary. Twisting the C15-C15′ bond leads to singlet-triplet energy crossing at (14,15,15′,14′) = 77° with an energy 32.5 kJ mol −1 (7.7 kcal mol −1 ) higher than that of the T 1 15,15′- cis minimum. Further exploration of the minimum-energy intersystem crossing (MEISC) point shows that triplet relaxation could occur at a less distorted structure ( = 58.4°) with the energy height of 26.5 KJ mol −1 (6.3 kcal mol −1 ). Another important reaction coordinate to reach the MEISC point is the bond-length alternation. The model truncation effect, solvent effect, and spin-orbit coupling were also investigated. The singlet-triplet crossing was also investigated for the 13,14- cis stereoisomer and locked-13,14- cis -SPO. We also discussed the origin of the natural selection of the cis over trans isomer in the RC. Triplet energy dissipation mechanism of a carotenoid: just bond twisting and stretching lead to minimum energy intersystem crossing point.