Kim S T, Hartman R F, Rose S D
Department of Chemistry, Arizona State University, Tempe 85287-1604.
Photochem Photobiol. 1990 Oct;52(4):789-94. doi: 10.1111/j.1751-1097.1990.tb08683.x.
Cyclobutadipyrimidines (pyrimidine dimers) undergo splitting that is photosensitized by indole derivatives. We have prepared a compound in which a two-carbon linker connects a dimer to an indolyl group. Indolyl fluorescence quenching indicated that the two portions of the molecule interact in the excited state. Intramolecular photosensitization of dimer splitting was remarkably solvent dependent, ranging from phi spl = 0.06 in water to a high value of phi spl = 0.41 in the least polar solvent mixture examined, 1,4-dioxane-isopentane(5 : 95). A derivative with a 5-methoxy substituent on the indolyl ring behaved similarly. These results have been interpreted in terms of electron transfer from the excited indolyl group to the dimer, which would produce a charge-separated species. The dimer anion within such a species could split or undergo back electron transfer. The possibility that back electron transfer is in the Marcus inverted region can be used to rationalize the observed solvent dependence of splitting. In the inverted region, the high driving force of a charge recombination exceeds the reorganization energy of the solvent, which is less for solvents of low polarity than those of high polarity. If this theory is applicable to the hypothetical charge-separated species, a slower back electron transfer, and consequently higher splitting efficiencies, would be expected in solvents of lower polarity. Photolyases may have evolved in which a low polarity active site retards back transfer of an electron and thereby contributes to the efficiency of the enzymatic dimer splitting.
环丁二嘧啶（嘧啶二聚体）会发生由吲哚衍生物光敏化的裂解反应。我们制备了一种化合物，其中一个双碳连接基团将一个二聚体与一个吲哚基相连。吲哚基荧光猝灭表明分子的这两部分在激发态下相互作用。二聚体裂解的分子内光敏化显著依赖于溶剂，范围从水中的φspl = 0.06到所研究的极性最小的溶剂混合物（1,4 - 二氧六环 - 异戊烷(5 : 95)）中的高值φspl = 0.41。在吲哚环上带有5 - 甲氧基取代基的衍生物表现类似。这些结果已根据从激发的吲哚基到二聚体的电子转移来解释，这会产生一个电荷分离的物种。这种物种中的二聚体阴离子可能会裂解或发生反向电子转移。反向电子转移处于马库斯反转区域的可能性可用于解释观察到的裂解对溶剂的依赖性。在反转区域，电荷复合的高驱动力超过了溶剂的重组能，对于低极性溶剂来说，重组能比高极性溶剂小。如果该理论适用于假设的电荷分离物种，那么在低极性溶剂中预计会有较慢的反向电子转移，从而有更高的裂解效率。光裂合酶可能已经进化，其中一个低极性的活性位点会阻碍电子的反向转移，从而有助于酶促二聚体裂解的效率。
