Najafi Mehdi, Herascu Nicoleta, Shafiei Golia, Picorel Rafael, Zazubovich Valter
†Department of Physics, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada.
‡Estacion Experimental de Aula Dei (CSIC), Avda. Montañana 1005, 50059 Zaragoza, Spain.
J Phys Chem B. 2015 Jun 11;119(23):6930-40. doi: 10.1021/acs.jpcb.5b02845. Epub 2015 May 29.
We employed nonphotochemical hole burning (NPHB) and fluorescence line narrowing (FLN) spectroscopies to explore protein energy landscapes and energy transfer processes in dimeric Cytochrome b6f, containing one chlorophyll molecule per protein monomer. The parameters of the energy landscape barrier distributions quantitatively agree with those reported for other pigment-protein complexes involved in photosynthesis. Qualitatively, the distributions of barriers between protein substates involved in the light-induced conformational changes (i.e., -NPHB) are close to glass-like ∼1/√V (V is the barrier height) and not to Gaussian. There is a high degree of correlation between the heights of the barriers in the ground and excited states in individual pigment-protein systems, as well as nearly perfect spectral memory. Both NPHB and hole recovery are due to phonon-assisted tunneling associated with the increase of the energy of a scattered phonon. As the latter is unlikely for simultaneously both the hole burning and the hole recovery, proteins must exhibit a NPHB mechanism involving diffusion of the free volume toward the pigment. Entities involved in the light-induced conformational changes are characterized by md(2) value of about 1.0 × 10(-46) kg·m(2). Thus, these entities are protons or, alternatively, small groups of atoms experiencing sub-Å shifts. However, explaining all spectral hole burning and recovery data simultaneously, employing just one barrier distribution, requires a drastic decrease in the attempt frequency to about 100 MHz. This decrease may occur due to cooperative effects. Evidence is presented for excitation energy transfer between the chlorophyll molecules of the adjacent monomers. The magnitude of the dipole-dipole coupling deduced from the Δ-FLN spectra is in good agreement with the structural data, indicating that the explored protein was intact.
我们采用非光化学空穴烧蚀（NPHB）和荧光线窄化（FLN）光谱技术，来探索二聚体细胞色素b6f中的蛋白质能量景观和能量转移过程，该二聚体细胞色素b6f每个蛋白质单体含有一个叶绿素分子。能量景观势垒分布的参数与报道的参与光合作用的其他色素 - 蛋白质复合物的参数在数量上一致。定性地说，参与光诱导构象变化（即 -NPHB）的蛋白质亚态之间的势垒分布接近类玻璃状的 ∼1/√V（V 是势垒高度），而不是高斯分布。在单个色素 - 蛋白质系统的基态和激发态中，势垒高度之间存在高度相关性，以及近乎完美的光谱记忆。NPHB和空穴恢复都是由于与散射声子能量增加相关的声子辅助隧穿。由于空穴烧蚀和空穴恢复同时发生时后者不太可能，蛋白质必须表现出一种涉及自由体积向色素扩散的NPHB机制。参与光诱导构象变化的实体的特征是md(2)值约为1.0×10(-46) kg·m(2)。因此，这些实体是质子，或者是经历亚埃位移的小原子团。然而，仅使用一种势垒分布来同时解释所有光谱空穴烧蚀和恢复数据，需要将尝试频率大幅降低到约100 MHz。这种降低可能是由于协同效应。文中给出了相邻单体的叶绿素分子之间激发能量转移的证据。从Δ - FLN光谱推导出的偶极 - 偶极耦合大小与结构数据很好地吻合，表明所研究的蛋白质是完整的。
