Institute of Biology and Technology Saclay, CEA, UMR 8221 CNRS, University Paris Sud , CEA Saclay 91191 Gif sur Yvette, France.
J Phys Chem B. 2013 Sep 26;117(38):11015-21. doi: 10.1021/jp309908r. Epub 2013 Jan 15.
Predicting the complete electronic structure of carotenoid molecules remains an extremely complex problem, particularly in anisotropic media such as proteins. In this paper, we address the electronic properties of nine relatively simple carotenoids by the combined use of electronic absorption and resonance Raman spectroscopies. Linear carotenoids exhibit an excellent correlation between (i) the inverse of their conjugation chain length N, (ii) the energy of their S0 → S2 electronic transition, and (iii) the position of their ν1 Raman band (corresponding to the stretching mode of their conjugated C═C bonds). For cyclic carotenoids such as β-carotene, this correlation is also observed between the latter two parameters (S0 → S2 energy and ν1 frequency), whereas their "nominal" conjugation length N does not follow the same relationship. We conclude that β-carotene and cyclic carotenoids in general exhibit a shorter effective conjugation length than that expected from their chemical structure. In addition, the effect of solvent polarizability on these molecular parameters was investigated for four of the carotenoids used in this study. We demonstrate that resonance Raman spectroscopy can discriminate between the different effects underlying shifts in the S0 → S2 transition of carotenoid molecules.
预测类胡萝卜素分子的完整电子结构仍然是一个极其复杂的问题，特别是在蛋白质等各向异性介质中。在本文中，我们通过电子吸收和共振拉曼光谱的联合使用来研究九种相对简单的类胡萝卜素的电子性质。线性类胡萝卜素在以下三个方面表现出很好的相关性：（i）它们的共轭链长度 N 的倒数，（ii）它们的 S0→S2 电子跃迁的能量，以及（iii）它们的 ν1 拉曼带（对应于它们共轭 C═C 键的伸缩模式）的位置。对于β-胡萝卜素等环状类胡萝卜素，后两个参数（S0→S2 能量和 ν1 频率）之间也存在这种相关性，而它们的“标称”共轭长度 N 则不遵循相同的关系。我们得出结论，β-胡萝卜素和一般的环状类胡萝卜素的有效共轭长度比其化学结构所预期的要短。此外，我们还研究了在这项研究中使用的四种类胡萝卜素的溶剂极化性对这些分子参数的影响。我们证明，共振拉曼光谱可以区分导致类胡萝卜素分子 S0→S2 跃迁位移的不同效应。
