Petrie Simon, Stranger Robert, Pace Ron J
Research School of Chemistry, College of Physical and Mathematical Sciences, The Australian National University, Acton ACT, Australia, 2601.
Chemphyschem. 2018 Oct 5. doi: 10.1002/cphc.201800686.
Recently two atomic resolution crystal structures of Photosystem II, in the double flashed, nominal S intermediate state of its Mn Ca water oxidising complex (WOC), have been presented (Young et al., Nature 2016, 540, 453; Suga et al., Nature 2017, 543, 131). These structures are at 2.25 Å and 2.35 Å resolution, respectively. Although highly similar in most respects, the structures differ in a key region within the WOC catalytic site. In the 2.25 Å structure, one oxy species (O5) is observed within the WOC cavity, weakly associated with the Mn centres, similar to that seen earlier in the 1.95 Å XRD structure of the S intermediate (Suga et al., Nature, 2015, 517, 99). In the 2.35 Å structure, two such species are seen (O5, O6), with the Mn centres and O5 positioned as in the 2.25 Å structure and an O5-O6 separation of ∼1.5 Å, consistent with peroxo formation. This suggests O5 and O6 are substrate water derived species in this double flashed form. Recently we have presented (Petrie, et al., Chem. Phys. Chem., 2017) a large scale (220 atom) quantum chemical model of the Young et al. 2.25 Å structure, which quantitatively explains all significant features within the WOC region of that structure, particularly the positions of the metal centres and O5 group. Critical to this was our assumption of a 'low' Mn oxidation paradigm (mean S Mn oxidation level of +3.0, Petrie et al., Angew. Chem. Int. Ed., 2015), rather than a 'high' oxidation model (mean S oxidation level of +3.5), widely assumed in the literature. Here we show that our same oxidation state model predicts two classes of energetically close S structural forms, analogous to the S state, one with the metal centres and O5 positioned as in the 2.25 Å structure, and the other with the metals similarly placed, but with O5 located in the O6 position of the 2.35 Å structure. We show that the Suga et al. 2.35 Å structure is likely a superposition of two such forms, one from each class, which is consistent with reported atomic occupancies for that structure and the relative total energies we calculate for the two structural forms.
最近，有人给出了光系统II处于其锰钙水氧化复合物（WOC）双闪光、名义S中间态的两个原子分辨率晶体结构（Young等人，《自然》，2016年，540卷，453页；Suga等人，《自然》，2017年，543卷，131页）。这些结构的分辨率分别为2.25 Å和2.35 Å。尽管在大多数方面高度相似，但这些结构在WOC催化位点内的一个关键区域有所不同。在2.25 Å结构中，在WOC腔内观察到一种氧物种（O5），与锰中心弱相关，这与之前在S中间态的1.95 Å X射线衍射结构中看到的情况类似（Suga等人，《自然》，2015年，517卷，99页）。在2.35 Å结构中，可以看到两种这样的物种（O5、O6），锰中心和O5的位置与2.25 Å结构中的相同，且O5 - O6间距约为1.5 Å，这与过氧形成一致。这表明O5和O6是这种双闪光形式中源自底物水的物种。最近我们给出了（Petrie等人，《化学物理化学》，2017年）Young等人2.25 Å结构的一个大规模（220个原子）量子化学模型，该模型定量解释了该结构WOC区域内的所有重要特征，特别是金属中心和O5基团的位置。对此至关重要的是我们假设的“低”锰氧化模式（S态锰的平均氧化水平为 +3.0，Petrie等人，《德国应用化学》，2015年），而不是文献中广泛假设的“高”氧化模型（平均S氧化水平为 +3.5）。在这里我们表明，我们相同的氧化态模型预测了两类能量相近的S结构形式，类似于S态，一种金属中心和O5的位置与2.25 Å结构中的相同，另一种金属位置相似，但O5位于2.35 Å结构的O6位置。我们表明，Suga等人的2.35 Å结构可能是两种这样的形式的叠加，每种形式各一个，这与该结构报道的原子占有率以及我们计算的两种结构形式的相对总能量一致。
