Department of Chemistry and Biochemistry, Montclair State University, 1 Normal Avenue, Montclair, New Jersey 07043, USA.
J Phys Chem A. 2010 Oct 14;114(40):10897-905. doi: 10.1021/jp107295p.
The aromatic amino acid tryptophan plays an important role in protein electron-transfer and in enzyme catalysis. Tryptophan is also used as a probe of its local protein environment and of dynamic changes in this environment. Raman spectroscopy of tryptophan has been an important tool to monitor tryptophan, its radicals, and its protein environment. The proper interpretation of the Raman spectra requires not only the correct assignment of Raman bands to vibrational normal modes but also the correct identification of the Raman bands in the spectrum. A significant amount of experimental and computational work has been devoted to this problem, but inconsistencies still persist. In this work, the Raman spectra of indole, 3-methylindole (3MI), tryptophan, and several of their isotopomers have been measured to determine the isotope shifts of the Raman bands. Density functional theory calculations with the B3LYP functional and the 6-311+G(d,p) basis set have been performed on indole, 3MI, 3-ethylindole (3EI), and several of their isotopomers to predict isotope shifts of the vibrational normal modes. Comparison of the observed and predicted isotope shifts results in a consistent assignment of Raman bands to vibrational normal modes that can be used for both assignment and identification of the Raman bands. For correct assignments, it is important to determine force field scaling factors for each molecule separately, and scaling factors of 0.9824, 0.9843, and 0.9857 are determined for indole, 3MI, and 3EI, respectively. It is also important to use more than one parameter to assign vibrational normal modes to Raman bands, for example, the inclusion of isotope shifts other than those obtained from H/D-exchange. Finally, the results indicate that the Fermi doublet of indole may consist of just two fundamentals, whereas one fundamental and one combination band are identified for the Fermi resonance that gives rise to the doublet in 3MI and tryptophan.
芳香族氨基酸色氨酸在蛋白质电子转移和酶催化中起着重要作用。色氨酸也被用作其局部蛋白质环境和环境动态变化的探针。色氨酸的拉曼光谱一直是监测色氨酸、其自由基及其蛋白质环境的重要工具。正确解释拉曼光谱不仅需要将拉曼带正确分配给振动正则模式，还需要正确识别光谱中的拉曼带。已经进行了大量的实验和计算工作来解决这个问题，但仍然存在不一致的情况。在这项工作中，测量了吲哚、3-甲基吲哚（3MI）、色氨酸及其几种同位素的拉曼光谱，以确定拉曼带的同位素位移。使用 B3LYP 函数和 6-311+G(d,p)基组对吲哚、3MI、3-乙基吲哚（3EI）及其几种同位素进行了密度泛函理论计算，以预测振动正则模式的同位素位移。将观察到的和预测的同位素位移进行比较，得出了将拉曼带分配给振动正则模式的一致结果，可用于拉曼带的分配和识别。正确分配时，重要的是要分别确定每个分子的力场缩放因子，并且确定吲哚、3MI 和 3EI 的缩放因子分别为 0.9824、0.9843 和 0.9857。使用多个参数将振动正则模式分配给拉曼带也很重要，例如，包括除 H/D 交换获得的同位素位移之外的同位素位移。最后，结果表明，吲哚的费米二聚体可能仅由两个基频组成，而 3MI 和色氨酸中产生二聚体的费米共振则确定了一个基频和一个组合频。
