Rech T, Durrant J R, Joseph D M, Barber J, Porter G, Klug D R
Department of Biochemistry, Imperial College, U.K.
Biochemistry. 1994 Dec 13;33(49):14768-74. doi: 10.1021/bi00253a015.
We have used spectrally photoselective femtosecond transient absorption spectroscopy on photosystem II reaction centers to show that there are at least two pools of chlorin molecules/states which can transfer excitation energy to P680, the primary electron donor in photosystem II. It has previously been shown that one chlorin pool equilibrates with P680 in 100 fs [Durrant et al. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11632-11636], and we report here the observation of energy transfer from a second more weakly coupled chlorin pool. The effect of the weakly coupled pool is to increase the apparent time constant for radical pair formation from 21 ps when P680 is selectively excited to 27 ps when the accessory chlorins are excited. We conclude that it is possible to observe both radical pair formation somewhat slowed by an energy transfer step and radical pair formation not limited by this slow energy transfer, depending upon which chromophores are initially excited. These observations provide evidence that when using photoselective excitation of P680, the observed 21 ps time constant for radical pair formation is not limited by a slow energy transfer step.
我们利用光谱光选择性飞秒瞬态吸收光谱对光系统II反应中心进行研究，结果表明，至少存在两个叶绿素分子/状态池，它们能够将激发能传递给光系统II中的初级电子供体P680。此前已有研究表明，一个叶绿素池在100飞秒内与P680达到平衡[杜兰特等人（1992年）《美国国家科学院院刊》89, 11632 - 11636]，我们在此报告观察到来自第二个耦合较弱的叶绿素池的能量转移。耦合较弱的池的作用是使自由基对形成的表观时间常数从选择性激发P680时的21皮秒增加到激发辅助叶绿素时的27皮秒。我们得出结论，根据最初被激发的发色团不同，有可能观察到因能量转移步骤而有所减慢的自由基对形成，以及不受这种缓慢能量转移限制的自由基对形成。这些观察结果证明，当使用P680的光选择性激发时，观察到的自由基对形成的21皮秒时间常数不受缓慢能量转移步骤的限制。
