Elucidation of Reaction Mechanism of Photochemistry in Solution Phase Studied with Quantitative Measurements of Excess Energy

时间分辨共振拉曼（TR ^3）光谱特别适合于探测溶液相中有色分子的振动态动力学。反斯托克斯和斯托克斯散射的强度比预计提供最直接的信息，相对振动的人口。在这里，我们应用这种技术来探索血红素在球蛋白矩阵光解一氧化碳肌红蛋白（MbCO）后的振动能量重新分配。反斯托克斯带强度的时间变化的分析表明，反斯托克斯强度的仪器响应时间内的发展。使用高斯拟合对互相关信号从反斯托克斯强度的衰减去卷积仪器响应。分析得到v_4带的衰减常数为1.1± 0.6ps，v_7带的衰减常数为1.9± 0.6ps。由于在3 - 50 ps时间范围内Stokes v_4和v_7带的强度没有变化，所以在反Stokes v_4和v_7带中观察到的强度衰减可以归因于振动能弛豫。面内模v_3，v_4，v_6和v_7的衰减率与统计分布假设下的衰减率符合得很好。这表明这些模式之间的振动能量交换非常快。相反，两个面外振动模式表现出偏离线。此外，在面内振动中，v_5模的反斯托克斯强度衰减非常缓慢。铁细胞色素c的反斯托克斯v_5模也观察到缓慢的强度衰减。我们将这些偏差归因于振动模式之间的弱非谐耦合。目前的结果不能用单一的振动温度来解释。换句话说，结果表明，分子内和分子间的过程是没有时间分离的血红素。

Time-resolved resonance Raman (TR^3) spectroscopy is particularly suited for probing the vibrational state dynamics of colored molecules in solution phase. The intensity ratio of anti-Stokes and Stokes scattering is expected to provide the most direct information about relative vibrational populations. Here, we applied this technique to explore the vibrational energy redistribution of heme in the globin matrix upon photolysis of carbonmonoxy myoglobin (MbCO). An analysis of the temporal changes of anti-Stokes band intensities showed that the anti-Stokes intensity develops within the instrument response time. The instrument response was deconvoluted from the decay of the anti-Stokes intensity using a Gaussian fit to the cross correlation signal. This analysis obtained the decay constants of 1.1±0.6 ps for the v_4 band and 1.9±0.6 ps for the v_7 band. Because there is no intensity change in the Stokes v_4 and v_7 bands in the 3 to 50-ps time range, the observed intensity decay in the anti-Stokes v_4 and v_7 bands can be ascribed to vibrational energy relaxation. The decay rates of in-plane modes including v_3, v_4, v_6, and v_7 agree well with the rates calculated under the assumption of the statistical distribution. This suggests exchanges of vibrational energy among these modes are very fast. In contrast, two out-of-plane vibrational modes exhibited deviations from the line. Moreover, among the in-plane vibrations, v_5 mode showed very slow decay of the anti-Stokes intensity. Slow intensity decay of the anti-Stokes v_5 mode was also observed for ferric cytochrome c. We ascribed these deviations to weak anharmonic coupling among the vibrational modes. The present results cannot be explained with a single vibrational temperature. In other words, the results show that intramolecular and intermolecular processes are not temporally separated for the heme.