Time-dependent thermodynamics during early electron transfer in reaction centers from Rhodobacter sphaeroides.
Time-dependent thermodynamics during early electron transfer in reaction centers from Rhodobacter sphaeroides.
复制标题
球形红细菌反应中心早期电子转移过程中的时间依赖性热力学。
DOI：
10.1021/bi00192a014
复制
发表时间：
1994
期刊：
影响因子：
2.9
通讯作者：
Woodbury,NW
中科院分区：
文献类型：
--
作者：
Peloquin,JM;Williams,JC;Lin,X;Alden,RG;Taguchi,AK;Allen,JP;Woodbury,NW
摘要：用皮秒时间分辨率测量了球形红杆菌R-26菌株和两个P/P+中点电位升高的突变体的反应中心荧光在皮秒到纳秒时间尺度上的温度依赖性。在所有三个样品中，荧光衰减的动力学都很复杂，只能用四个或更多的指数衰减项来很好地描述，时间跨度从皮秒到纳秒。在295-20K的所有温度范围内都需要多指数拟合，用动态溶剂化模型解释了复杂的衰变动力学，该模型通过蛋白质构象的变化来稳定形成后的电荷分离态。这些运动中的许多还没有时间在初始电子转移的时间尺度上发生和/或在低温下冻结。这导致了激发单重态和电荷分离态之间依赖于时间和温度的热变，这是这些态之间自由能差的主导项。即使在20K，仍然可以观察到长寿命的荧光，特别是对于高潜力突变体。这意味着，在R-26反应中心，低温下纳秒时间尺度上的电子转移驱动力小于200 cm-1(25 MeV)，而在皮秒时间尺度或高电势突变体中，电子转移的驱动力更小。考虑了这一令人惊讶的结果的机制含义，并提出，至少在某些条件下，反应中心的电子转移可能最好被描述为绝热，发生在强耦合极限附近，而不是振动平衡状态之间的非绝热反应。紫色非硫细菌最初的光合作用电子转移反应所涉及的热力学参数在过去十年中一直是相当不同的主题(Schenck等人，1982；Woodbury&Parson，1984；Horber等人，1986；Goldstein等人，1988；Ogrodniketal，1988)。以前的测量结果导致了与初始电子转移反应(S)相关的标准自由能变化值相对较小，特别是与模型体系中大多数快速电子转移反应相比[例如，克洛斯和米勒(1988)]。初始激发单重态和第一全布居电子转移标准自由能带隙的不同测量之间的变化
Revised Manuscript Received April 19, 1994* abstract: The temperature dependence of fluorescence on the picosecond to nanosecond time scale from the reaction centers of Rhodobacter sphaeroides strain R-26 and two mutants with elevated P/P+ midpoint potentials has been measured with picosecond time resolution. In all three samples, the kinetics of the fluorescence decay is complex and can only be well described with four or more exponential decay terms spanning the picosecond to nanosecond time range. Multiexponential fits are needed at all temperatures between 295 and 20 K. The complex decay kinetics are explained in terms of a dynamic solvation model in which the charge-separated state is stabilized after formation by protein conformational changes. Many of these motions have not had time to occur on the time scale of initial electron transfer and/or are frozen out at low temperature. This results in a time-and temperature-dependent enthalpy change between the excited singlet state and the charge-separated state that is the dominant term in the free energy difference between these states. Long-lived fluorescence is still observed even at 20 K, particularly for the highpotential mutants. This implies that the driving force for electron transfer on the nanosecond time scale at low temperature is less than 200 cm-1 (25 meV) in R-26 reaction centers and even smaller on the picosecond time scale or in the high-potential mutants. The mechanistic implications of this surprising result are considered, and it is suggested that, at least under certain conditions, electron transfer in the reaction center may be best described as adiabatic, occurring near the strong coupling limit, rather than as a nonadiabatic reaction between vibronically equilibrated states.The thermodynamic parameters involved in the initial photosynthetic electron-transfer reactions of purple nonsulfur bacteria have been the subject of considerable disagreement over the past decade (Schenck et al., 1982; Woodbury & Parson, 1984; Horber et al., 1986; Goldstein et al., 1988; Ogrodniketal., 1988). Previous measurements have resulted in relatively small values for the standard free energy change associated with the initial electron-transfer reaction (s), especially when compared to most fast electron-transfer reactions in model systems [eg, Closs and Miller (1988)]. The variation between different measurements of the value of the standard free energy gap for electron transfer between the initial excited singlet state and the first fully populated