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有望提供有关相对振动群体的最直接信息。在这里，我们应用这种技术来探索碳单氧肌红蛋白（MbCO）光解后血红素在球蛋白基质中的振动能量重新分布。对反斯托克斯带强度随时间变化的分析表明，反斯托克斯带强度在仪器响应时间内发展。使用对互相关信号的高斯拟合，将仪器响应从反斯托克斯强度的衰减中解卷积。该分析获得了 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 至 50 ps 时间范围内斯托克斯 v_4 和 v_7 频带没有强度变化，因此在反斯托克斯 v_4 和 v_7 频带中观察到的强度衰减可归因于振动能量弛豫。 v_3、v_4、v_6 和 v_7 等面内模式的衰减率与统计分布假设下计算的衰减率非常吻合。这表明这些模式之间的振动能量交换非常快。相反，两种面外振动模式表现出与直线的偏差。此外，在面内振动中，v_5模式表现出非常缓慢的反斯托克斯强度衰减。还观察到铁细胞色素 c 的反斯托克斯 v_5 模式的缓慢强度衰减。我们将这些偏差归因于振动模式之间的弱非谐波耦合。目前的结果不能用单一的振动温度来解释。换句话说，结果表明血红素的分子内和分子间过程在时间上没有分离。

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.