Boussac A, Un S, Horner O, Rutherford A W
Section de Bioénergétique, URA CNRS 2096, Gif sur Yvette, France.
Biochemistry. 1998 Mar 24;37(12):4001-7. doi: 10.1021/bi9728710.
The Mn4 complex which is involved in water oxidation in photosystem II (PSII) is known to exhibit two types of EPR signals in the S2 state, one of the five redox states of the enzyme cycle: either a multiline signal (S = 1/2) or a signal at g = 4.1 (S = 3/2 or S= 5/2). The S = 1/2 state can be converted to that responsible for the g = 4.1 signal upon the absorption of near-infrared (IR) light [Boussac, A., Girerd, J.-J., and Rutherford, A.W. (1996) Biochemistry 35, 6984-6989]. It is shown here that a third state gives rise to signals at g = 10 and 6. This state is formed by IR illumination of the S = 1/2 state at 65 K, a temperature where IR illumination leads to the loss of the S = 1/2 signal but to no formation of the g = 4.1 state. On the basis of the corresponding decrease of the S = 1/2 state, the new state can be trapped in approximately 40% of the PSII centers. Warming of the sample above 65 K, in the dark, leads to the loss of the g = 10 and 6 resonances with the corresponding appearance of the g = 4.1 signal. It is suggested that the IR-induced conversion of the S = 1/2 state into the g = 4.1 state at 150 K involves the transient formation of the new state. The new state is attributed to a S = 5/2 state of the Mn4 complex (although a S value > 5/2 is also a possibility). Spectral simulations indicate an E/D ratio of -0.05 with D </= 1 cm-1. The resonances at g = 10 and 6 correspond to the gz of the +/-5/2 and +/-3/2 transition, respectively. The temperature-dependent conversion of this S = 5/2 state into the g = 4.1 state is proposed to be due to relaxation of the ligand environment around the Mn4 cluster that leads to a change in the zero field splitting parameters, assuming an S = 5/2 value for the g = 4.1 state. The new form of the S2 state reported here may explain some earlier data where the S2 state was present and yet not detectable as either a S = 1/2 or a g = 4.1 EPR signal.
已知参与光系统II（PSII）中水氧化的Mn4配合物在酶循环的五个氧化还原状态之一的S2状态下表现出两种类型的电子顺磁共振（EPR）信号：要么是多线信号（S = 1/2），要么是g = 4.1处的信号（S = 3/2或S = 5/2）。在吸收近红外（IR）光后，S = 1/2状态可转变为产生g = 4.1信号的状态[布萨克，A.，吉尔德，J.-J.，和卢瑟福，A.W.（1996年）《生物化学》35，6984 - 6989]。本文表明，第三种状态会产生g = 10和6处的信号。这种状态是通过在65 K下对S = 1/2状态进行红外照射形成的，在该温度下，红外照射会导致S = 1/2信号消失，但不会形成g = 4.1状态。基于S = 1/2状态的相应减少，新状态可在约40%的PSII中心中被捕获。在黑暗中将样品加热到65 K以上，会导致g = 10和6处的共振消失，同时出现g = 4.1信号。有人提出，在150 K下红外诱导的S = 1/2状态向g = 4.1状态的转变涉及新状态的瞬时形成。新状态归因于Mn4配合物的S = 5/2状态（尽管S值> 5/2也是一种可能性）。光谱模拟表明E/D比为 -0.05，D≤1 cm-1。g = 10和6处的共振分别对应于±5/2和±3/2跃迁的gz。假设g = 4.1状态的S = 5/2值，这种S = 5/2状态随温度向g = 4.1状态的转变被认为是由于Mn4簇周围配体环境的弛豫导致零场分裂参数发生变化。本文报道的S2状态的新形式可能解释了一些早期数据，即在存在S2状态但无法检测为S = 1/2或g = 4.1 EPR信号的情况下的数据。
