A novel S = 7/2 configuration of the Mn cluster of photosystem II
A novel S = 7/2 configuration of the Mn cluster of photosystem II
复制标题
DOI：
10.1021/ja016418u
复制
发表时间：
2001-10-31
影响因子：
15
通讯作者：
Petrouleas, V
中科院分区：
文献类型：
--
作者：
Sanakis, Y;Ioannidis, N;Petrouleas, V
光系统II（PSII）的放氧复合物（OEC）由一个四核Mn簇、一个具有氧化还原活性的酪氨酸Z和辅因子Ca ~（2+）和Cl ~-组成。1 OEC循环通过五个氧化还原状态表示为Si，i）0，1，2，3，4，因为它经历了连续的单电子氧化的叶绿素物种称为P680。在积累四个氧化当量（S4状态）时，两个水分子被氧化，分子氧被排出，循环再次开始（S 0状态）。EPR光谱在各种S态的表征中至关重要。最近的进展是从S3状态检测垂直和平行模式的EPR信号。有趣的是，在这种状态下的OEC被发现对近红外（NIR）光敏感，其方式与S2状态相似。S3的NIR激发在其它变化中产生在约g/cm 5处的显著EPR信号。3如图1A所示。光谱A1和A2表示在50 K下NIR照射之前和之后的S3-态。图1A中所示的它们的差异清楚地表明在g）4.65处诱导导数信号，以及在g）3处诱导更宽的导数。g）4.65信号与Nugent等人5在77 K下长时间孵育经历多次S1状态转换的样品后获得的所谓g）5信号显示出有趣的相似性。我们已经证实了Nugent等人的观察结果。5与样品在S3状态。图1中的光谱B1显示了在S3处保持平衡并在77 K下孵育数周的样品的光谱（初始光谱与光谱A1相似）。光谱在g = 5处显示出明显的导数信号，部分地被背景贡献所扭曲。在升温至-50 ℃ 2分钟后，光谱B2，信号失去相当大的强度。通过将两个光谱相减，如图1B所示，可获得在g）4.75处具有零交叉点的导数信号。图1中的光谱A和B在g = 5处包含类似的信号，除了在光谱A中信号更宽并且g值移动到更小的值。[6]光谱A还包含在约g）3处的宽导数贡献。将光谱A中的样品在77 K下退火几分钟导致g β 3特征的降低和g β 5信号的变窄，即向光谱B的移动（未示出）。诱导（图A2）或减弱（图B2）g β 5信号的处理不可避免地影响g β 10处的S3状态信号。结果，光谱A和B的低场区域失真并被省略。在与下面的理论光谱进行比较时，应记住这一点。
The oxygen evolving complex (OEC) of Photosystem II (PSII) comprises a tetranuclear Mn cluster, a redox-active tyrosine, Z, and the cofactors Ca2+ and Cl-. 1 The OEC cycles through five redox states denoted Si, i) 0, 1, 2, 3, 4, as it undergoes consecutive one-electron oxidations by a photooxidizable chlorophyll species termed P680. Upon accumulation of four oxidizing equivalents (S4 state), two water molecules are oxidized, molecular oxygen is expelled, and the cycle starts again (S0 state). EPR spectroscopy has been crucial in the characterization of the various S-states. A recent advance is the detection of EPR signals in both perpendicular and parallel mode from the S3 state. 2, 3 Interestingly, the OEC poised at this state was found to be sensitive to nearinfrared (NIR) light3 in a fashion similar to that of the S2-state. 4 NIR excitation of S3 produces among other changes a prominent EPR signal at about g∼ 5. 3 This is illustrated in Figure 1A. Spectra A1 and A2 represent the S3-state prior and after the NIR illumination at 50 K. Their difference shown in Figure 1A clearly demonstrates the induction of a derivative signal at g) 4.65, as well as a broader derivative at g) 3. The g) 4.65 signal shows an interesting similarity to the, so-called, g) 5 signal obtained earlier by Nugent et al. 5 after prolonged incubation at 77 K of samples that had undergone multiple turnovers from the S1 state. We have confirmed the observations of Nugent et al. 5 with samples poised at the S3 state. Spectrum B1 in Figure 1 shows the spectrum of a sample poised at S3 (initial spectrum similar to spectrum A1) and incubated at 77 K for several weeks. The spectrum shows a pronounced derivative signal at g∼ 5, partially distorted by background contributions. Upon warming to-50 C for 2 min, spectrum B2, the signal loses considerable intensity. By subtracting the two spectra, Figure 1B, a derivative signal with a zero crossing point at g) 4.75 is obtained. Spectra A and B in Figure 1 contain similar signals at g∼ 5 except that in spectrum A the signal is broader and the g value is shifted to smaller values. 6 Spectrum A contains in addition a broad derivative contribution at about g) 3. Annealing of the sample in spectrum A for a few minutes at 77 K results in a decrease of the g) 3 feature and a narrowing of the g∼ 5 signal, ie, a shift toward spectrum B (not shown). The treatments that induce (Figure A2) or diminish (Figure B2) the g∼ 5 signal inevitably affect the S3-state signal at g∼ 10. As a result, the low-field region of spectra A and B is distorted and was omitted. This should be kept in mind when comparing with the theoretical spectra below.