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

时间分辨共振拉曼光谱特别适合于研究有色分子在溶液中的振动态动力学。反斯托克斯和斯托克斯散射的强度比有望提供关于相对振动布居的最直接信息。在这里，我们应用这项技术来探索血红素在珠蛋白基质中的振动能量在一氧化碳肌红蛋白(MbCO)光解过程中的重新分配。对反斯托克斯带强度随时间变化的分析表明，反斯托克斯带强度随仪器响应时间的变化而变化。利用对互相关信号的高斯拟合，从反斯托克斯强度的衰减中解卷积仪器的响应。分析得到V_4波段的衰减常数为1.1±0.6 ps，V_7波段的衰减常数为1.9±0.6 ps。由于在3-50ps的时间范围内，Stokes v_4和v_7带的强度没有变化，因此反Stokes v_4和v_7带的强度衰减可以归因于振动能弛豫。面内模式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，两个平面外的振动模式显示出偏离该线的情况。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。铁的细胞色素c也观察到了反斯托克斯v_5模的缓慢强度衰减，我们将这种偏离归因于振动模之间的弱非谐耦合。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，对于血红素来说，分子内和分子间的过程在时间上不是分开的。

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.