Durbeej Bo
Department of Chemistry, University of Siena, Via Aldo Moro 2, I-53100, Siena, Italy.
Phys Chem Chem Phys. 2009 Mar 7;11(9):1354-61. doi: 10.1039/b811813b. Epub 2009 Jan 14.
Phytochromes are widespread photoreceptors responsive to red and far-red light that exist in two photochromic forms Pr (inactive) and Pfr (active). The Pr --> Pfr conversion proceeds through a series of events initiated by Z-->E photoisomerization of the tetrapyrrole chromophore, believed to occur at C15 of the methine bridge between rings C and D. Recent crystal structures show that ring D in Pr is less tightly packed by the protein than rings A, B and C, which should favor the C15 reaction over reactions at C4 (AB methine bridge) and C10 (BC). In the present work, quantum chemical methods are used to establish the intrinsic reactivity of the chromophore towards all three possible Z-->E isomerization events in the absence of steric effects and specific interactions with the protein. Using a level of theory that reproduces spectroscopic data with an accuracy of approximately 0.2 eV, it is demonstrated that isolated conditions allow the C10 photoreaction to substantially dominate. This finding suggests that the different degrees of ring-packing observed in the protein are crucial not only to facilitate a reaction at C15, but also to prevent an intrinsically more favorable reaction at C10 from taking place.
光敏色素是广泛存在的对红光和远红光有反应的光感受器，以两种光致变色形式存在，即Pr（无活性）和Pfr（有活性）。Pr向Pfr的转化通过一系列由四吡咯发色团的Z→E光异构化引发的事件进行，据信该异构化发生在环C和D之间的次甲基桥的C15处。最近的晶体结构表明，Pr中的环D与蛋白质的堆积不如环A、B和C紧密，这应该有利于C15处的反应，而不是C4（AB次甲基桥）和C10（BC）处的反应。在本工作中，使用量子化学方法来确定发色团在不存在空间效应和与蛋白质的特定相互作用的情况下对所有三种可能的Z→E异构化事件的内在反应活性。使用一种能以约0.2 eV的精度再现光谱数据的理论水平，证明在孤立条件下C10光反应基本占主导。这一发现表明，在蛋白质中观察到的不同程度的环堆积不仅对于促进C15处的反应至关重要，而且对于防止在C10处本质上更有利的反应发生也至关重要。
