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

夏威夷大学马诺阿分校化学系的罗伯特·H·H·刘教授在化学系有机与高分子化学计划的支持下，将研究1985年提出的体积守恒的光化学顺反异构化反应机理，以解释视觉色素视紫红质中视黄基生色团的快速光异构化反应。这种机制被称为呼啦扭转(Hula-Twist，HT)。其显着的立体化学特征是两个相邻的双键和单键同时发生构象和构象转换。为区分Hula-Twist(HT)和传统的单键翻转(OBF)机制而设计的模型化合物将被制备，并将检测它们的光化学和光物理性质。该介质有望在抑制OBF过程方面发挥重要作用，从而允许检测到通常更困难的HT过程。然而，可能导致分子从以OBF方式反应到HT方式的其他条件也被提出和讨论。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(参与黄疸及其光疗的黄色胆色素)的异构化可以通过HT参与它们的光化学反应来合理化。现在提出了实验，以明确证明它的参与。对于视觉色素视紫红质和能量储存系统细菌视紫红质，羟色胺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)的初级光产物进行的X射线晶体结构研究表明，它的结构不是曾经提出的HT-I4过程的结构。现在提出了一种改进的BR和视紫红质的途径。在最初的光化学过程中，高温被保留下来。然而，在其完成之前，该过程可能会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顺或全反式产物。为检验这些想法而设计的新实验也被提出。拟议的实验将由刘教授的小组与国际专家网络(时间分辨UV-Vis和FT-IR以及其他光谱和生物物理研究)合作开展化学合成和低温光化学-光谱研究。共同的目标是了解在自然体系和实验室中发生的非常基本的光化学顺反异构化的性质，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，即多烯发色团在高度特定的蛋白质结合部位的看似需要体积的光异构化过程如何在不到一皮秒的时间内发生，对于蛋白质重组来说，这个时间尺度太快了。

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.