Vibronic coupling effect on circular dichroism spectrum: Carotenoid-retinal interaction in xanthorhodopsin
Vibronic coupling effect on circular dichroism spectrum: Carotenoid-retinal interaction in xanthorhodopsin
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DOI：
10.1063/1.4977045
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发表时间：
2017-03-07
影响因子：
4.4
通讯作者：
Balashov, Sergei P.
中科院分区：
文献类型：
--
作者：
Fujimoto, Kazuhiro J.;Balashov, Sergei P.
通过计算检查黄视紫红质 (XR) 中天线类胡萝卜素和视网膜的振动耦合在其圆二色性 (CD) 光谱中的作用。开发了与跃迁密度碎片相互作用（TDFI）方法相结合的电子振动激子模型，并将其应用于XR的吸收和CD光谱计算。 TDFI 方法基于电子库仑和单个发色团跃迁密度之间的交换相互作用 [K. J.藤本，J.化学。物理。 137, 034101 (2012)]，它提供了电子耦合能量的定量描述。 TDFI计算揭示了库仑相互作用对电子耦合能量的主要贡献和交换相互作用的贡献可以忽略不计，表明类胡萝卜素的天线功能是由福斯特型激发能量转移产生的，而不是由德克斯特型激发能量转移产生的。计算出的吸收和CD光谱成功地再现了实验结果的主要特征，这使我们能够研究XR中观察到的双相CD光谱的机制。结果表明，类胡萝卜素和视网膜之间的电子振动耦合在 CD 光谱的形状中起着重要作用。进一步分析表明，电子耦合的负值直接影响圆二色谱的双相形状。这项研究还表明，盐黄质的 C6-C7 键旋转并不是双相 CD 光谱的主要因素，尽管它对光谱位移有不可忽视的贡献。本方法可用于分析生物系统中发色团-发色团相互作用的分子机制。
The role of vibronic coupling of antenna carotenoid and retinal in xanthorhodopsin (XR) in its circular dichroism (CD) spectrum is examined computationally. A vibronic exciton model combined with a transition-density-fragment interaction (TDFI) method is developed, and applied to absorption and CD spectral calculations of XR. The TDFI method is based on the electronic Coulomb and exchange interactions between transition densities for individual chromophores [K. J. Fujimoto, J. Chem. Phys. 137, 034101 (2012)], which provides a quantitative description of electronic coupling energy. The TDFI calculation reveals a dominant contribution of the Coulomb interaction to the electronic coupling energy and a negligible contribution of the exchange interaction, indicating that the antenna function of carotenoid results from the Forster type of excitation-energy transfer, not from the Dexter one. The calculated absorption and CD spectra successfully reproduce the main features of the experimental results, which allow us to investigate the mechanism of biphasic CD spectrum observed in XR. The results indicate that vibronic coupling between carotenoid and retinal plays a significant role in the shape of the CD spectrum. Further analysis reveals that the negative value of electronic coupling directly contributes to the biphasic shape of CD spectrum. This study also reveals that the C6-C7 bond rotation of salinixanthin is not the main factor for the biphasic CD spectrum although it gives a non-negligible contribution to the spectral shift. The present method is useful for analyzing the molecular mechanisms underlying the chromophore-chromophore interactions in biological systems.