Spectroscopic studies of green-bacterial reaction center

用闪光吸收光谱研究了绿色硫杆菌Chlorobium tepidum与两个细胞色素c(Cytc)结合的反应中心(Re)中的电子转移动力学。光氧化后的P84O+在1ms后被cytc还原了95%和80%，第一次和第二次闪光后的t^lt；1/2和gt；分别约为65µs和100µs。观察到的速率、每次闪光后氧化还原平衡的建立以及光氧化Cytc的总量与每个光氧化P840中存在2个当量的Cytc是一致的。假设一个Cytc到P8404^+的电子转移的中点电势差为-53 mV，*Em与另一个cytc的氧化状态无关，这些数据被很好地拟合。这些观察结果支持一个模型，即两个单血红素Cytc对称地排列在RC核心周围。纯化的RC络合物与0.6-1.2孟喹酮-7(MQ)结合。然而，在10 ns到4亩S的时间窗口内，在360~450 nm范围内没有观察到由MQ引起的瞬变信号。在受主一侧，纯化的RC在亩S的时间范围内可以稳定地保持多达3个电子，但在第4次闪光后，首次观察到了20 ns的t^&lt；1/2&gt；的电荷复合反应。根据我们的数据，没有证据表明MQ参与了绿色硫磺细菌的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.