Hasson K C, Gai F, Anfinrud P A
Department of Physics, Harvard University, Cambridge, MA 02138, USA.
Proc Natl Acad Sci U S A. 1996 Dec 24;93(26):15124-9. doi: 10.1073/pnas.93.26.15124.
The primary events in the all-trans to 13-cis photoisomerization of retinal in bacteriorhodopsin have been investigated with femtosecond time-resolved absorbance spectroscopy. Spectra measured over a broad range extending from 7000 to 22,400 cm-1 reveal features whose dynamics are inconsistent with a model proposed earlier to account for the highly efficient photoisomerization process. Emerging from this work is a new three-state model. Photoexcitation of retinal with visible light accesses a shallow well on the excited state potential energy surface. This well is bounded by a small barrier, arising from an avoided crossing that separates the Franck-Condon region from the nearby reactive region of the photoisomerization coordinate. At ambient temperatures, the reactive region is accessed with a time constant of approximately 500 fs, whereupon the retinal rapidly twists and encounters a second avoided crossing region. The protein mediates the passage into the second avoided crossing region and thereby exerts control over the quantum yield for forming 13-cis retinal. The driving force for photoisomerization resides in the retinal, not in the surrounding protein. This view contrasts with an earlier model where photoexcitation was thought to access directly a reactive region of the excited-state potential and thereby drive the retinal to a twisted conformation within 100-200 fs.
利用飞秒时间分辨吸收光谱研究了细菌视紫红质中视黄醛全反式到13-顺式光异构化的主要过程。在7000至22400 cm-1的宽范围内测量的光谱揭示了一些特征，其动力学与先前提出的用于解释高效光异构化过程的模型不一致。这项工作中出现了一个新的三态模型。用可见光对视黄醛进行光激发会进入激发态势能面上的一个浅阱。这个阱由一个小势垒界定，该势垒源于一个避免交叉，它将弗兰克-康登区域与光异构化坐标附近的反应区域分隔开。在环境温度下，以大约500 fs的时间常数进入反应区域，随后视黄醛迅速扭转并遇到第二个避免交叉区域。蛋白质介导进入第二个避免交叉区域，从而控制形成13-顺式视黄醛的量子产率。光异构化的驱动力存在于视黄醛中，而非周围的蛋白质中。这一观点与早期模型形成对比，早期模型认为光激发直接进入激发态势能的反应区域，从而在100 - 200 fs内将视黄醛驱动到扭曲构象。
