Energy dissipative photo-protective mechanism of carotenoid spheroidene from the photoreaction center of purple bacteria Rhodobacter sphaeroides
Energy dissipative photo-protective mechanism of carotenoid spheroidene from the photoreaction center of purple bacteria Rhodobacter sphaeroides
复制标题
紫色细菌球红杆菌光反应中心类胡萝卜素球状蛋白的耗能光保护机制
DOI：
10.1039/c5cp03089g
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发表时间：
2015
影响因子：
3.3
通讯作者：
and J. Hasegawa
中科院分区：
文献类型：
--
作者：
S. Arulmozhiraja;N. Nakatani;A. Nakayama;and J. Hasegawa
类胡萝卜素类球蛋白（SPO）在光合反应中心（RC）中起光保护作用，并有效地耗散其三重态激发能。15，15 ′-cis-SPO的敏化顺反异构化被认为是单重态-三重态能量交叉的可能机制，但最近受到质疑。为了了解这一重要的SPO的耗散光保护机制，并克服现有的争议，在这个问题上，我们进行了理论研究，利用密度泛函理论的可能的三重态能量弛豫机制，通过顺反异构化。结合实验结果，讨论了15，15 ′-cis-SPO三重态能量弛豫的可能机制。结果表明，完全的顺反异构化是不必要的。扭转C15-C15 ′键导致单重态-三重态能量交叉，在π（14，15，15 ′，14 ′）= 77 °，能量比T1 15，15 ′-cis最小值高32.5 kJ mol − 1（7.7 kcal mol − 1）。对最小能量系间穿越（MEISC）点的进一步探索表明，三重态弛豫可以发生在畸变较小的结构（ε = 58.4 °），能量高度为26.5 KJ mol − 1（6.3 kcal mol − 1）。达到MEISC点的另一个重要反应坐标是键长交替。研究了模型截断效应、溶剂效应和自旋轨道耦合效应。还研究了13，14-顺式立体异构体和锁定-13，14-顺式-SPO的单重态-三重态交叉。我们还讨论了反式异构体顺式自然选择的起源。
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 ϕ(14,15,15′,14′) = 77° with an energy 32.5 kJ mol−1 (7.7 kcal mol−1) higher than that of the T1 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 (ϕ = 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.