Hula-Twist, A Supramolecular Photochemical Reaction Mechanism. A New Perspective of Photoisomerization

在化学系有机与高分子化学项目的支持下，罗伯特·S·H.夏威夷大学化学系的Liu将研究1985年提出的体积守恒光化学顺反异构化反应机制，以解释视色素视紫红质中视黄基发色团的快速光异构化反应。这种机制被称为Hula-Twist（HT）。其突出的立体化学特征是两个相邻的双键和单键同时发生构型和构象转变。模型化合物的设计，以提供可区分的化学之间的Hula-扭转（HT）和传统的单键翻转（OBF）机制将被制备和它们的光化学和物理性质将被检查。预期该介质在抑制OBF过程中发挥重要作用，从而允许检测通常更困难的HT过程。然而，也存在并讨论了可能导致分子从OBF方式反应重定向到HT方式的其他条件。提出了用于测试实验的有意义的模型化合物。在含有Pollyanna发色团的光敏生物分子中，PYP（使细菌对蓝光产生排斥反应的光活性黄色蛋白）和胆红素（黄疸及其光疗中涉及的黄色胆色素）的异构化可以通过HT参与其光化学反应来合理化。现在有人提出实验来明确地证明它的参与。对于视色素视紫红质和能量储存系统细菌视紫红质，HT不再被认为是负责所观察到的光化学的唯一过程。最近对bR（K）初级光产物的X射线晶体结构研究表明，其结构不是曾经提出的HT-I 4过程的结构。现在提出了bR和视紫红质的修饰途径。HT被保留在初始的光化学过程中。然而，在其完成之前，该过程很可能被刚性蛋白质腔重定向到一系列体积守恒的自行车踏板（BP）过程（在基态），从而产生更稳定的13-顺式或全反式产物。提出了新的实验设计来测试这些想法。拟议的实验将由刘教授的小组进行，重点是化学合成和低温光化学光谱研究，并与国际专家网络合作（时间分辨紫外-可见和FT-IR以及其他光谱和生物物理研究）。共同的目标是了解非常基本的光化学顺反异构化的性质，发生在本地系统以及在实验室，并确定新的双键扭曲机制的范围。也许这项研究最终可以提供一个明确的答案，这个问题是如何在一个高度特异性的蛋白质结合位点上的多烯发色团的表面上要求体积的光异构化过程可以在不到一皮秒的时间内发生的，这个时间尺度对于蛋白质重组来说太快了。

With the support of the Organic and Macromolecular Chemistry Program in the Chemistry Division, Professor Robert S. H. Liu, of the Department of Chemistry at the University of Hawaii- Manoa, will study a volume-conserving photochemical cis-trans isomerization reaction mechanism postulated in 1985 to account for the rapid photoisomerization reaction of the retinyl chromophore in the visual pigment rhodopsin. This mechanism is referred to as the Hula-Twist (HT). Its salient stereochemical feature is the simultaneous configurational and conformational transformation of two adjacent double and single bonds. Model compounds designed to provide distinguishable chemistry between Hula-Twist (HT) and the conventional one-bond-flip (OBF) mechanism will be prepared and their photochemical and photophysical properties will be examined. The medium is expected to play an important role to inhibit the OBF process allowing detection of the normally more difficult HT process. However, other conditions that might result in re-directing molecules from reacting in an OBF manner to the HT manner are also present and discussed. Meaningful model compounds used for test experiments are proposed. Among photosensitive bio-molecules containing a Pollyanna chromophore, the isomerization of PYP (photoactive yellow protein that makes bacteria response repulsively toward blue light) and bilirubin (the yellow bile pigment involved in jaundice and its phototherapy) can be rationalized by the involvement of HT in their photochemical reactions. Experiments are now proposed to prove unambiguously of its involvement. For the visual pigment rhodopsin and the energy storage system bacteriorhodopsin, HT is no longer considered likely to be the sole process responsible for the observed photochemistry. Recent X-ray crystal structural work on the primary photoproduct of bR (K) showed that the structure is not that from the once proposed HT -I 4 process. A modified pathway for both bR and rhodopsin is now proposed. HT has been retained in the initial photochemical process. However, before its completion, the process is likely to be re-directed by the rigid protein cavity to a series of volume-conserving bicycle pedal (BP) processes (in the ground state) leading to the more stable 13-cis or all-trans products. New experiments designed to test these ideas are proposed. The proposed experiments will be carried out by Professor Liu's group, concentrating on chemical synthesis and low temperature photochemical-spectroscopic studies in collaboration with a network of international specialists (time-resolved UV-Vis and FT-IR and other spectroscopic and biophysical studies). The common goal is to understand the nature of the very fundamental photochemical cis-trans isomerization that happens in native systems as well as in laboratories and to determine the scope of the novel two-bond twist mechanism. Perhaps this research can finally provide a definitive answer to the question of how the seemingly volume-demanding photoisomerization process of a polyene chromophore in a highly specific protein binding site can take place in less than a picosecond, a time scale much too fast for protein reorganization.