Resolving the Differences Between the 1.9 Å and 1.95 Å Crystal Structures of Photosystem II: A Single Proton Relocation Defines Two Tautomeric Forms of the Water‐Oxidizing Complex

Abstract

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.

Resolving Ambiguous Protonation and Oxidation States in the Oxygen Evolving Complex of Photosystem II

“…For example, the energy separation between 2Ha (W2: H 2 O, O5: O 2– ) and 2Hb (W2: OH – , O5: OH – ) isomers with His337 (HIP) Δ H 2Ha–2Hb decreases from 22.0 to −2.7 kcal/mol (Figures –). This supports the choice of protonated O5 under the low paradigm , and also rationalizes the choice of O5 (O 2– ) in S 1 state under the high paradigm. ,− …”

“…The existence of the mixture of S states may be caused by the extensive (1 week) dark adaption used in an attempt to depopulate higher S states that are unusually stable in PSII crystals, which can lead to a significant fraction of the PSII cores poised in the S 0 state, rather than only S 1 . Under the low oxidation state paradigm, models typically have more protons, since O5 could be protonated as OH – or H 2 O, , which generates the observed longer Mn–O5 distance. However, the low paradigm models have been criticized for producing too long Mn–Mn distances .…”

“…In their examination of S 1 low-paradigm models, they were unable to produce short Mn–Mn distances to match the intermetallic EXAFS-derived distances. Later, Pace and Stranger suggested a S 1 low-paradigm model with O5 as OH – and oxidation state III–IV–III–II, that was not examined by Neese and which was able to produce the short Mn–Mn distances consistent with the XFEL structure . A DFT analysis by Shoji also concluded that O5 was most likely protonated in S 1 state .…”

Reconciling Structural and Spectroscopic Fingerprints of the Oxygen-Evolving Complex of Photosystem II: A Computational Study of the S2 State

“…A considerable amount of structural data concerning the OEC has been collected from crystallography (X-ray diffraction (XRD)), − X-ray spectroscopy, − electron paramagnetic resonance (EPR), − electron nuclear double resonance (ENDOR), , and computational modeling. − The XRD structure of the resting S 1 state shows that a core comprised of an oxo-bridged heterocubane, CaMn 3 O 4 , that is oxo-bridged to a “dangler” Mn4 connecting the cuboidal part through one or two oxo bridges (O4 and O5) depending upon conditions (Figure B). The structure also reveals four direct water ligands (W1 to W4), two of which are bound to Mn4 and the other two bonded to Ca.…”

“…These alternative oxidation state assignments initially evolved from EPR data of the S 2 state, which exhibits a complex multiline signal (MLS) centered at approximately g = 2.0, representing a spin S = 1/2 ground state, ,,, which was shown to be compatible with magnetic couplings to n Mn­(III) and m Mn­(IV) ions, where n + m = 4 and n / m is either 1/3 or 3 (refs and ). Various experiments, including X-ray emission spectroscopy, chemical reduction, photoassembly, , substrate water exchange kinetics, , EPR analysis, , and computational studies, ,,, have been conducted to investigate these two possibilities. Detailed arguments for and against these two alternatives can be found elsewhere. , …”