Time-Resolved Vibrational Spectroscopic Investigation of Ultrafast Protein Dynamics Coupled with Photoreaction

在许多生物系统中，局部的小结构变化在空间上延伸到介观维度以实现生理功能，并且蛋白质的高阶结构变化对该过程至关重要。肌红蛋白（Mb）是研究蛋白质这些特征的最佳分子之一，因为CO从一氧化碳Mb（Mb CO）的光解离是一个局部化反应，在50 fs内发生，像一个阶跃函数，量子产率几乎为1。从X射线研究中得知，铁原子移出卟啉平面约0.3埃，核心尺寸扩大了0.05埃，CO的deligation. To探讨蛋白质动力学参与这一过程中，我们已经研究了皮秒时间分辨共振拉曼光谱的光解MbCO。仔细检查的光谱脱氧Mb CO光解后，在1300-1650 cm ↑ [2]<-1>区域显示，核心尺寸扩大的卟啉环内完成的仪器响应时间（102 ps）。与此相反，Fe-His伸缩模式（VFe-His）的强度和频率的变化发生在皮秒，这表明从卟啉平面的Fe位移明显的时间演化。对于没有蛋白质基质的血红素组的模型化合物，观察到相同的行为。因此，这反映了血红素本身的内在属性。另一方面，VFe-His模的频率随着时间常数100 ps而变化。对于不含蛋白质基质的模型化合物，未观察到该频率变化。因此，这一定是由蛋白质的三级结构变化引起的。V_4和V_7带的反斯托克斯拉曼强度随时间的变化表明，振动激发态的血红素在光解后立即产生，随后振动激发态的布居数衰减，时间常数分别为1.1±0.6和1.9±0.6 ps。

In many biological systems a localized small structural change extends spatially to mesoscopic dimensions to achieve a physiological function, and higher-order structural changes of proteins are essential to this process. Myoglobin (Mb) is one of the best molecules for studying such features of proteins, because photodissociation of CO from carbonmonoxyMb (MbCO), a localized reaction, takes place within 50 fs like a step-function with a quantum yield of nearly unity. It is know from x-ray studies that the iron atom moves out of the porphyrin plane by 〜0.3 A and the core-size expands by 0.05A upon deligation of CO. To explore the protein dynamics involved in this process, we have investigated picosecond time-resolved resonance Raman spectra of photodissociated MbCO.Close inspection of the spectra of deoxyMb following CO photolysis in the 1300-1650 cm^<-1> region revealed that the core-size expansion of porphyrin ring is completed within the instrumental response time (〜2 ps). In contrast, changes in the intensity and frequency of the Fe-His stretching mode (VFe-His) occurred in picoseconds, suggesting appreciable time evolution for the Fe displacement from the porphyrin plane. The same behaviors were observed for the model compound of the heme group without protein matrix. Therefore, this reflects an intrinsic property of heme itself. On the other hand, the frequency of the VFe-His mode changed with a time constant of 〜100 ps. This frequency change was not seen for the model compound without the protein matrix. Therefore, this must be caused by tertiary structural changes of the protein. Temporal changes of the anti-Stokes Raman intensity of the V_4 and V_7 bands demonstrated immediate generation of the vibrationally excited heme upon photolysis and subsequent decay of the vibrationally excited population with the time constants of 1.1±0.6 and 1.9±0.6 ps, respectively.