Analysis of the temperature-dependence of P+HA- charge recombination in the Rhodobacter sphaeroides reaction center suggests nanosecond temperature-independent protein relaxation
Analysis of the temperature-dependence of P+HA- charge recombination in the Rhodobacter sphaeroides reaction center suggests nanosecond temperature-independent protein relaxation
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DOI：
10.1039/c3cp44187c
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发表时间：
2013-01-01
影响因子：
3.3
通讯作者：
Jones, Michael R.
中科院分区：
文献类型：
--
作者：
Gibasiewicz, Krzysztof;Pajzderska, Maria;Jones, Michael R.
用亚纳秒到微秒时间尺度的瞬时吸收法研究了紫色细菌球形红杆菌的分离反应中心中P+HA-对与预还原醌Q(A)电荷复合的温度依赖性。总体而言，从室温降温到接近200K时，动力学大大减慢，然后在77K以下基本保持不变，这表明存在两条竞争的电荷复合路径，一条仅在200K以上才出现的热激活路径和一条温度无关的路径。在我们的模型中，热激活路径包括从H-A到B-A的上行电子转移，导致P+BA-态的瞬时形成，而与温度无关的路径是由于从H-A(-)到P+的直接下坡电子转移。在所研究的所有温度下，动力学可以用四组分的衰减来近似。对特定相在温度范围内的寿命和振幅的详细分析表明，动力学分解的相揭示了三种构象状态的连续出现，其特征是P+BA-和P+HA-状态之间的自由能隙增加。初始能隙约为8 meV，中间能隙约为92 meV，最终能隙约为135 meV，与温度无关。通过一个非常简单的方法计算出，从初始到中间态的驰豫过程在0.6±0.1 ns内发生，而从中间态到终态的第二步弛豫过程需要11±2 ns。蛋白质松弛过程的两个阶段基本上都与温度无关。还讨论了描述不能明确排除的实验数据的可能的替代模型。
The temperature dependence of charge recombination of the pair P+HA- in isolated reaction centers from the purple bacterium Rhodobacter sphaeroides with prereduced quinone Q(A) was studied by sub-nanosecond to microsecond time-scale transient absorption. Overall, the kinetics slowed down substantially upon cooling from room temperature to similar to 200 K, and then remained virtually unchanged down to 77 K, indicating the coexistence of two competitive pathways of charge recombination, a thermally-activated pathway appearing only above similar to 200 K and a temperature-independent pathway. In our modelling, the thermally activated pathway includes an uphill electron transfer from H-A to B-A leading to transient formation of the state P+BA-, whereas the temperature-independent pathway is due to direct downhill electron transfer from H-A(-) to P+. At all temperatures studied, the kinetics could be approximated by a four-component decay. Detailed analysis of the lifetimes and amplitudes of particular phases over the range of temperatures suggests that the kinetically resolved phases reveal the consecutive appearance of three conformational states characterized by an increasing free energy gap between the states P+BA- and P+HA-. The initial gap between these states was estimated to be only similar to 8 meV, the intermediate gap being similar to 92 meV, and the final gap similar to 135 meV, with no dependence on temperature. It was also calculated through a very straightforward approach that the relaxation process from the initial to the intermediate state occurs within 0.6 +/- 0.1 ns, whereas the second step of relaxation from the intermediate to the final state takes 11 +/- 2 ns. Both phases of the protein relaxation process are essentially temperature-independent. Possible alternative models to describe the experimental data that cannot be definitely excluded are also discussed.