A COMPREHENSIVE INVESTIGATION OF THE MECHANISM AND PHOTOPHYSICS OF ISOMERIZATION OF A PROTONATED AND UNPROTONATED SCHIFF-BASE OF 11-CIS-RETINAL
A COMPREHENSIVE INVESTIGATION OF THE MECHANISM AND PHOTOPHYSICS OF ISOMERIZATION OF A PROTONATED AND UNPROTONATED SCHIFF-BASE OF 11-CIS-RETINAL
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DOI：
10.1021/ja00292a005
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发表时间：
1985-01-01
影响因子：
15
通讯作者：
FREEDMAN, K
中科院分区：
文献类型：
--
作者：
BECKER, RS;FREEDMAN, K
以11 - 顺式视黄醛正丁胺席夫碱和质子化席夫碱为探针，完成了一项关于其光异构化量子产率（φ）随溶剂性质变化的综合研究。采用了激光闪光和稳态技术。对于席夫碱，量子产率在己烷中低至≤0.01，在乙腈中为0.34。在己烷中一种特定的氢键复合物显示出0.31的高φ值。除了光异构化，还观察到一种非激发态瞬态，其量子产率φₓ，特别是寿命τₓ，随溶剂性质而变化。φ和φₓ的值不受氧气存在的影响。一种固定的12 - 13 - s - 反式构象体（13 - 去甲基 - 11 - 顺式席夫碱的）所呈现的φ值仅等于或小于11 - 顺式席夫碱的φ值。结果根据在最低激发态中随着溶剂极性增加，**图形**混合到**图形**的增加以及源于弗兰克 - 康登或平衡态的异构化活化能随之降低来解释。很明显，关于**图形**态对光异构化的促进作用并没有什么特别之处，事实上有证据表明是**图形**态促进了异构化。对于质子化席夫碱，与席夫碱形成鲜明对比的是，φ作为溶剂性质的函数大致恒定（约0.20），并且没有观察到任何种类的瞬态。与未质子化的情况相比，质子化情况下的φ仅在非极性溶剂中有显著增加。然而，在极性溶剂中，φ并不更大，实际上在一些极性溶剂中比席夫碱的更小或相等。除了在非极性溶剂中，质子化对于显著的光异构化不是必需的。在5种溶剂中未发现质子化席夫碱的φ对激发波长的依赖性，这与之前在一种溶剂中的发现形成鲜明对比。对于席夫碱和质子化席夫碱，都证明了异构化为全反式异构体。给出了光异构化的光物理机制提议，并且重要的是，这些提议扩展到对视紫红质中异构化的光物理机制以及质子化（或氢键）在视觉过程中的作用的考虑。
A comprehensive study of the photoisomerization quantum yield (.vphi.) of 11-cis-retinal n-butylamine Schiff base and the protonated Schiff base as a function of the nature of the solvent as probes was accomplished. Both laser-flash and steady-state techniques were employed. For the Schiff base, the yield varies from as low as .ltoreq. 0.01 in hexane to 0.34 in acetonitrile. A specifically H-bonded complex in hexane shows a high .vphi. value of 0.31. In addition to photoisomerization, a non-excited-state transient was seen for which the quantum yield, .vphi.x, and particularly the lifetime, .tau.x, vary as a function of the nature of the solvent. The values of .vphi. and .vphi.x are not affected by the presence of oxygen. A fixed 12-13-s-trans conformer (of the 13-demethyl-11-cis Schiff base) exhibits .vphi. values only equal to or less than those of the 11-cis Schiff base. The results are interpreted in terms of increased .**GRAPHIC**. mixing into .**GRAPHIC**. in the lowest excited state as a function of increased polarity of the solvent and a concomitant lowering of the activation energy for isomerization which originates in the Franck-Condon or equilibrium state. It is clear that there is nothing special regarding the facilitation of photoisomerization by the .**GRAPHIC**. state, and in fact the evidence is that it is the .**GRAPHIC**. state that facilitates isomerization. In the case of the protonated Schiff base, the .vphi. is approximately constant (.apprx. 0.20) as a function of the nature of the solvent in marked contrast to the Schiff base, and no transient of any kind is observed. There is a notable increase of .vphi. for the protonated case, compared to the unprotonated case, only in nonpolar solvents. However, in polar solvents, .vphi. is not larger but in fact is smaller or equal to that of the Schiff base in a number of polar solvents. Protonation is not a requirement for significant photoisomerization except in nonpolar solvents. No excitation wavelength dependence of .vphi. for the protonated Schiff base was found in 5 solvents in marked contrast to a previous finding in one solvent. For both the Schiff base and protonated Schiff base, proof of isomerization to the all-trans isomer was made. Photophysical mechanistic proposals for photoisomerization are given and, importantly, these are extended into a consideration of the photophysical mechanism for isomerization in rhodopsin as well as the role of protonation (or H bonding) in the visual process.