Spectroscopic studies of green-bacterial reaction center

采用闪光吸收光谱法研究了绿硫菌（Chlorobium tepidum）结合两种细胞色素c （cyt c）的反应中心（Re）配合物的电子转移动力学。第1次和第2次闪光后，t <1/2>的cyt c分别在65和100 muz s左右照射1 ms后，光氧化P84O^+的还原率分别为95%和80%。观察到的速率、每次闪光后氧化还原平衡的建立以及可光氧化cyt - c的总量与每个可光氧化P840存在2个等效的cyt - c一致。假设电子从一个cyt c转移到P8404^+的中点电位差为-53 mV， Em与另一个cyt c的氧化态无关，这些数据拟合得很好。这些观察结果支持一个模型，即两个单血红素cyt c对称地排列在RC核心周围。纯化的RC复合物结合0.6-1.2甲基萘醌-7 （MQ）。然而，在从10 ns到4 μ s的时间窗口内，在360 ~ 450 nm之间没有观察到由于MQ引起的瞬态信号。在受体侧，纯化的RC在μ s时间范围内可以稳定地容纳多达3个电子。然而，在第4次闪光后，首次观察到t^<1/2>为20 ns的电荷复合反应。从我们的数据来看，没有证据表明MQ参与了绿硫菌RC中的电荷分离。

Electron transfer kinetics in the reaction center (Re) complex from the green sulfur bacterium Chlorobium tepidum which binds two cytochrome c (cyt c) were studied by flash-absorption spectroscopy. Photooxidized P84O^+ was reduced 95% and 80% after 1 ms by cyt c with t^<1/2> of about 65 mus and 100 muz s after the 1st and the 2nd flashes, respectively . The observed rates, the establishment of redox equilibrium after each flash and the total amount of photooxidizable cyt c are consistent with the presence of 2 equivalent cyt c per photooxidizable P840. The data were well fitted assuming a midpoint potential difference of -53 mV for electron transfer from one cyt c to P8404^+, * Em being independent of the oxidation state of the other cyt c. These observations support a model with 2 monoheme cyt c which are symmetrically arranged around the RC core. The purified RC complex binds 0.6-1.2 menaquinone-7 (MQ) . However, no transient signal due to MQ could be observed between 360 and 450 nm in the time window from 10 ns to 4 mu s. On the acceptor side, purified RC can stably hold up to 3 electrons in the mu s time range.However, after the 4th flash, a charge recombination reaction with t^<1/2> of 20 ns was first observed. From our data, there is no evidence for involvement of MQ in charge separation in the RC of green sulfur bacteria.