Chu H A, Sackett H, Babcock G T
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824, USA.
Biochemistry. 2000 Nov 28;39(47):14371-6. doi: 10.1021/bi001751g.
We have developed conditions for recording the low-frequency S(2)/S(1) Fourier transform infrared difference spectrum of hydrated PSII samples. By exchanging PSII samples with buffered (18)O water, we found that a positive band at 606 cm(-)(1) in the S(2)/S(1) spectrum in (16)O water is clearly downshifted to 596 cm(-)(1) in (18)O water. By taking double-difference (S(2)/S(1) and (16)O minus (18)O) spectra, we assign the 606 cm(-)(1) mode to an S(2) mode and also identify a corresponding S(1) mode at about 625 cm(-)(1). In addition, by Sr and (44)Ca substitution experiments, we found that the 606 cm(-)(1) mode is upshifted to about 618 cm(-)(1) by Sr(2+) substitution but that this mode is not affected by substitution with the (44)Ca isotope. On the basis of these results and also on the basis of studies of Mn model compounds, we assign the 625 cm(-)(1) mode in the S(1) state and the 606 cm(-)(1) mode in the S(2) state to a Mn-O-Mn cluster vibration of the oxygen-evolving complex (OEC) in PSII. This structure may include additional bridge(s), which could be another oxo, carboxylato(s), or atoms derived from an amino acid side chain. Our results indicate that the bridged oxygen atom shown in this Mn-O-Mn cluster is exchangeable and accessible by water. The downshift in the Mn-O-Mn cluster vibration as manganese is oxidized during the S(1) --> S(2) transition is counterintuitive; we discuss possible origins of this behavior. Our results also indicate that Sr(2+) substitution in PSII causes a small structural perturbation that affects the bond strength of the Mn-O-Mn cluster in the PSII OEC. This suggests that Sr(2+), and by inference, Ca(2+), communicates with, but is not integral to, the manganese core.
我们已经开发出用于记录水合光系统II（PSII）样品低频S(2)/S(1)傅里叶变换红外差谱的条件。通过用缓冲的(18)O水交换PSII样品，我们发现(16)O水中S(2)/S(1)光谱中606 cm(-)(1)处的一个正峰在(18)O水中明显下移至596 cm(-)(1)。通过获取双差谱（S(2)/S(1)以及(16)O减去(18)O），我们将606 cm(-)(1)模式归为S(2)模式，并在约625 cm(-)(1)处识别出一个相应的S(1)模式。此外，通过Sr和(44)Ca替代实验，我们发现606 cm(-)(1)模式在Sr(2+)替代后上移至约618 cm(-)(1)，但该模式不受(44)Ca同位素替代的影响。基于这些结果以及对锰模型化合物的研究，我们将S(1)态的625 cm(-)(1)模式和S(2)态的606 cm(-)(1)模式归为PSII中放氧复合体（OEC）的Mn-O-Mn簇振动。这种结构可能包括额外的桥连基团，其可能是另一个氧代、羧基或源自氨基酸侧链的原子。我们的结果表明，该Mn-O-Mn簇中所示的桥连氧原子是可交换的且可被水接触到。在S(1)→S(2)转变过程中随着锰被氧化，Mn-O-Mn簇振动的下移是违反直觉的；我们讨论了这种行为可能的起源。我们的结果还表明，PSII中的Sr(2+)替代会引起小的结构扰动，从而影响PSII OEC中Mn-O-Mn簇的键强。这表明Sr(2+)，进而推断Ca(2+)，与锰核心有相互作用，但并非其不可或缺的一部分。
