Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations are reported on three sets of isomeric models of the {Mn(4)/Ca} water-oxidizing complex of photosystem II (PS II), which share the general formula [CaMn(4)O(4)(N(2)C(3)H(4))(RCOO)(6)](q)⋅(H(2)O)(n) (R=H, CH(3); q=-1, 0, +1, +2; n=3, 4, 5, 6, 7). Comparison with the full range of available data on Mn K-edge X-ray absorption energy values determined for the photosystem allows us to validate the structures that correspond to the particular S states and to determine their Mn oxidation patterns. By using a new TDDFT procedure, it is shown that variations in the absolute K-edge energy values for a particular S state, reported by different research groups, can be quantitatively explained by different geometries adopted by the Mn cluster, which demonstrates flexibility in the position of the fourth 'dangling' Mn atom in relation to a cubane structure created by the Ca atom and the three other Mn atoms. Computational results show that each step of the S cycle occurs by removal of one electron directly from the Mn cluster. This Mn-centered oxidation still agrees with the small difference observed experimentally between the K-edge energy values of the S(2) and S(3) states of the photosystem, thus resolving a controversy as to whether this represents ligand-centered or metal-centered oxidation. The overall oxidation state of Mn atoms in the tetramanganese cluster during functional turnover changes from 2.75 for S(0), 3.00 for S(1), and 3.25 for S(2) up to 3.50 for the S(3) state, which is systematically 0.50 lower than the previously proposed oxidation states of the cluster. The calculations give insight into why these earlier, purely empirical, assignments of the Mn oxidation levels in PS II could be in error.

Great progress has been made in characterizing the water-oxidizing complex (WOC) in photosystem II (PSII) with the publication of a 1.9 Å resolution X-ray diffraction (XRD) and recently a 1.95 Å X-ray free-electron laser (XFEL) structure. However, these achievements are under threat because of perceived conflicts with other experimental data. For the earlier 1.9 Å structure, lack of agreement with extended X-ray absorption fine structure (EXAFS) data led to the notion that the WOC suffered from X-ray photoreduction. In the recent 1.95 Å structure, Mn photoreduction is not an issue, but poor agreement with computational models which adopt the 'high' oxidation state paradigm, has again resulted in criticism of the structure on the basis of contamination with lower S states of the WOC. Here we use DFT modeling to show that the distinct WOC geometries in the 1.9 and 1.95 Å structures can be straightforwardly accounted for when the Mn oxidation states are consistent with the 'low' oxidation state paradigm. Remarkably, our calculations show that the two structures are tautomers, related by a single proton relocation.

International audiencetrans-[(η5-C5H5)Fe(η5-C5H4-η1-C≡C)Ru(C≡C-4-C6H4NPh2)(dppe)2] [4; dppe = 1,2-bis(diphenylphosphanyl)ethane] and trans,trans,trans-[₍η5-C5H5)Fe(η5-C5H4-η1-C≡C)Ru(dppe)2(C≡C-4-C6H4)₃N] (7) have been synthesized from trans-[Ru(C≡C-4-C6H4NPh2)Cl(dppe)2] (3) and trans,trans,trans-[₍dppe)2ClRu(C≡C-4-C6H4)₃N] (6), respectively, and the identities of trans-[Ru(C=CH-4-C6H4NPh2)Cl(dppe)2][PF6] (2, precursor to 3), 3, and 4 have been confirmed crystallographically. Chemical oxidation of 4 and 7 afforded the isolable mixed-valence species 4[PF6] and 7[PF6]3. The CV of 4 reveals sequential loss of three electrons in fully reversible oxidation steps, whereas the CV of 7 shows five reversible redox waves; in contrast, oxidation of the precursor amines HC≡C-4-C6H4NPh2 and (HC≡C-4-C6H4)3N are irreversible processes. All oxidation processes afford reversible changes in the linear optical properties. Complementary time-dependent density functional theory (TD-DFT) studies suggest that the initial oxidation process for 4 and 7 is iron-centered and is followed by one (for 4) or three (for 7) ruthenium-centered oxidations. The final reversible oxidation is assigned by TD-DFT as delocalized along the metalla-ethynylarylamine moiety. The intense optical changes consequent on reversible oxidation, together with their charge-transfer character, suggest that 4 and 7 have potential as nonlinear as well as linear optical multistate switches

Time-dependent density functional theory (TDDFT) has been applied to study core excitations from 1s and 2p Mn orbitals in a series of manganese complexes with oxygen and nitrogen donor ligands. The effect of basis set and functional on the excitation energy was evaluated in detail for one complex, Mn(acac)2 x (H2O)2. The results obtained for a range of compounds, namely, [Mn(Im)6]Cl2, Mn(CH3COO)2 x 4 H2O, Mn(acac)3, Mn(SALADHP)2 and [Mn(SALPN)O]2, show good consistency with the data from X-ray absorption spectroscopy (XAS), confirming the relation between the Mn K-edge energy and the oxidation state of the Mn atom. The energies predicted for 2p core excitations show a dependence on the metal oxidation state very similar to that determined experimentally by 1s2p resonant inelastic X-ray scattering (RIXS) studies for Mn(acac)2 x (H2O)2, Mn(acac)3, and Mn(sal)2(bipy). The reliability of the K-edge energies obtained in the present study indicates that TDDFT can be used in determining the oxidation states of Mn atoms in different computational models of the manganese cluster of photosystem II (PSII).

Time-dependent density functional theory (TDDFT) calculations have been performed on a series of manganese dimers with averaged metal oxidation states of 2.0, 2.5, 3.0, 3.5 and 4.0. The excitation energies and oscillator strengths of transitions within the Mn K-core edges have been determined. The theoretical edge energies reproduce the experimental correlation between the relative position of the Mn K-edge and the averaged Mn oxidation state extremely well. A comparison with the results obtained previously for Mn complexes with different ligand environments shows that TDDFT can be successfully applied to determine the relative edge energy differences between Mn systems, taking into account the various oxidation states of the metal and differences in ligand environment in a self-consistent manner. The accuracy of the calculated edge energies indicates that the methodology employed in the current study can be used to determine the oxidation states of Mn atoms in the Mn4Ca cluster of photosystem II (PSII).

Green plants and algae oxidize water to molecular oxygen in photosystem II (PS II) within a calcium/tetramanganese site known as the water-oxidizing complex (WOC). Oxygen is generated by the WOC in a four-electron process involving a series of intermediate states (S states, labeled S 0 …S 4 ) of increasingly higher mean oxidation level. [1] Over the past decade, X-ray crystallographic (XRD) structures of PS II at progressively improved resolution [2] have revealed much detail of the WOC. At present, only PS II from thermophilic cyanobacteria has been crystallized for XRD study and the enzyme is presumed to be in the dark stable S 1 state. The first PS II structure (at 3.5 resolution) to resolve side chain positions was presented by Barber and co-workers. [2c] Consistent with subsequent studies at higher resolution, it revealed the compact Mn 3 Ca "cube" structure of the WOC connected more distantly to a single Mn, referred to as the "dangler". More recent improved structures at 3.0 and 2.9 , [2d,e] substantially clarified the metal-and proteinsupplied ligand positions within the WOC, but were still of insufficient resolution to reveal the positions of bridging oxo groups and water molecules (including the substrate water molecules). Finally, Umena et al., [2f] using a new crystallization method, produced an atomic resolution structure at 1.9 , the most resolved to date. Despite this remarkable achievement, revealing, for the first time, the positions of bridging O atoms within the Mn 4 Ca core of the WOC, aspects of the new structure have been met with scepticism. [3,4] Central concerns over this structure involve 1) the identity and unexpected placement of the O(5) moiety (Figure 1), which appears to be either a weakly bound oxo, hydroxo, or water ligand at distances of 2.4-2.7 from four of the metal atoms in the WOC, and 2) the disparity in some key metalmetal distances when compared with earlier, high-precision extended X-ray absorption fine structure (EXAFS) results [5,6] and the previous lower-resolution XRD structures (see Table 1). Although the Mn EXAFS data do not unambiguously assign the individual near (less than 3 ) metal-metal distances within the cluster, they clearly indicate that two Mn-Mn vectors of a magnitude of approximately 2.7 exist within the functional WOC in the S 1 and S 2 states. These are totally consistent with the Mn1-Mn2 and Mn2-Mn3 distances of 2.65 and 2.70 in the 2.9 resolution XRD structure, but are significantly shorter than the corresponding Mn-Mn distances of 2.80 and 2.90 seen in the 1.9 resolution XRD structure.The discrepancies between the XRD and EXAFS data have led some to suggest [3,4,7] that the crystal structures generally, and the 1.9 resolution structure in particular, have undergone photoreduction of the Mn atoms during data collection, increasing the Mn II content and distorting the cluster from the functional S 1 state to as low as S À3 . This is despite the fact that in the later studies great pains were taken to minimize such X-ray exposure. The claims of ph...

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