Solar-to-Chemical Conversion

Light harvesting, excitation energy transfer, and charge separation are functions shared by all types of photosynthetic organisms. What is special about oxygenic photosynthesis is the use of water as the ultimate electron donor by PS‐II. Water oxidation takes place at an active site, the OEC, that harbors an inorganic oxo‐bridged Mn 4Ca cluster. The cluster is readily assembled from Mn 2+and Ca 2+in solution through a process known as photoassembly [66, 117]. The OEC is successively oxidized by the Y Ztyrosyl radical (D1‐Tyr161), storing up to four oxidizing equivalents before releasing dioxygen. The storage/catalytic cycle of the OEC is described by the Kok–Joliot cycle of S istates ( i = 0–4), with S 0being the lowest oxidation state of the OEC and S 4the highest state that evolves dioxygen ( Figure 3.8). Among those states S 1is the resting state of PS‐II, i.e. the state to which the enzyme reverts if left in the dark. Y Zand the Mn cluster are in close spatial and electronic contact. Along each S i→ S i+1transition, the intermediates S iY Z •formed when P680 •+oxidizes Y Zhave a finite lifetime at low temperature and can be studied by electron paramagnetic resonance (EPR) spectroscopy [86, 118–125]. Studies of these intermediates provide information about the tyrosyl radical itself, its interaction with the manganese cluster, the spin state of the cluster, and changes in hydrogen bonding and protonation occurring during the S‐state transition. Storing the four oxidizing equivalents before performing the four‐electron water oxidation provides a low‐energy pathway for oxidation of water to dioxygen and avoids formation of dangerous reactive intermediates that would result from partial oxidation of the substrate. An important feature of the OEC is that electrons and protons are removed in an alternate fashion along the S‐state cycle [126–128]. This creates a redox‐leveling effect, which means that the four successive oxidative steps can take place within a narrow range of potential.

Figure 3.8The cycle of intermediate oxidation states of the oxygen‐evolving complex. Water is assumed to bind at the S 3state of the cluster and upon reconstitution of the S 0state. The S 4state is a postulated but unobserved transient intermediate that decays spontaneously to S 0with release of dioxygen.

The geometric structure of the OEC has been the subject of speculation for a very long time [129, 130], ever since EPR studies on the S 2state in 1981 established the presence of four antiferromagnetically interacting Mn ions giving rise to a multiline g = 2 EPR signal arising from a cluster with total spin state S = 1/2 [131]. Extended X‐ray absorption fine structure (EXAFS) studies provided increasingly detailed and accurate information about the metal–metal distances within the cluster over the next decades [130, 132–139], but no unique three‐dimensional reconstruction of this information could be achieved without additional input from crystallography [140, 141]. The appearance of the first XRD structure of PS‐II in 2001 [47] and the development of crystallographic models over the following years culminated in an atomic‐resolution model of the OEC core in 2011 [55]. A particular challenge for crystallography was the control of X‐ray radiation damage that led to reduction of the Mn ions [136, 142, 143] and compromised the quality and reliability of structural information contained in the fitted structural models [144–146]. This problem was addressed to large extent [147, 148] by the use of XFEL approaches [57], although certain structural details remain debatable [56, 149–151].

Our present view of the OEC cluster in the dark‐stable S 1state is depicted in Figure 3.9. It is an asymmetric Mn 4CaO 5cluster, where three Mn ions and a Ca ion form a Mn 3CaO 4cubane unit, while a fourth Mn ion is attached externally to this cubane both by coordination to an oxo bridge of the cubane and by a fifth oxo bridge (note that the protonation state of the bridges cannot be inferred from crystallographic models). The inorganic core is mostly ligated by carboxylates provided by the D1 and CP43 proteins: D1‐Asp170, D1‐Glu189, D1‐Glu333, D1‐Asp342, CP43‐Glu354, and the C‐terminal D1‐Ala344. There is a single nitrogen‐donor ligand, D1‐His332, coordinated to Mn1. Four water‐derived ligands, i.e. H 2O or OH, are identified in the crystallographic models; two of them are attached to Mn4 (W1 and W2), and two are attached to calcium (W3 and W4).

Figure 3.9The Mn 4CaO 5cluster and its protein pocket in the dark‐stable S 1state as revealed by protein crystallography (PDB ID: 3WU2, a), and a scheme showing the commonly used labeling of the ions comprising the inorganic core.

The second coordination sphere of the OEC contains the redox‐active tyrosine and its hydrogen‐bonded histidine partner D1‐His190. The tyrosine hydrogen‐bonds directly with one of the Ca‐bound waters and hence is in close interaction with the cluster. Additional residues such as D1‐His337, CP43‐Arg357, D1‐Asp61, and D2‐Lys317, as well as a functionally required chloride ion, interact with the inorganic core and its ligands mostly via hydrogen bonds. These residues play important roles in regulating properties of the cluster and its ligands [152–157], such as the magnetic interaction between specific pairs of Mn ions and local p K avalues of various groups, and may influence or directly participate in proton translocation. An important additional aspect of the local environment of the OEC is the system of water channels and hydrogen‐bonding networks that surround it. These channels and networks are crucial for connecting the active site of water oxidation to the solvent‐exposed surface of the protein and play critical roles in substrate delivery, proton transfer, and product release [158–173].

There is a strong but complex connection between the geometric and electronic structure of the OEC. This connection is key for deciphering the structure of the other S‐states and, eventually, for understanding the mechanism of biological water oxidation [7]. In the following, some of the currently most well‐supported ideas about the geometric and electronic structure of the other S‐states will be presented, with the caveat that there exist significant open questions and ambiguities about many of the specifics [7, 8].

A central question concerns the oxidation states of the Mn ions, their distribution within the cluster, and how they change along the S i–S i+1transitions. Important information on the electronic structure of the cluster can be obtained from magnetic resonance methods, as well as from XAS and XES [128, 174–183]. Structural interpretations of such data can in turn be achieved by spectroscopy‐oriented quantum chemical methods [146, 150, 179, 184–193] that have been extensively benchmarked for high‐valent manganese systems [150, 179, 186, 194–198] and additionally incorporate geometric information from EXAFS and crystallographic models. The dominant view is that the Mn oxidation states evolve from Mn(III) 3Mn(IV) in the S 0state to Mn(III) 2Mn(IV) 2in S 1, Mn(III)Mn(IV) 3in S 2, and Mn(IV) 4in S 3. This assignment is called the “high oxidation state scheme” [199] as opposed to the low oxidation state hypothesis [200–205] that assigns two more electrons to the Mn ions, with oxidation states ranging from Mn(III) 3Mn(II) in S 0to Mn(III) 2Mn(IV) 2in S 3. EPR spectroscopy has helped to identify spin states of all observable intermediates ( S = 1/2 for S 0[206–213], S = 0 for S 1with a low‐lying S = 1 state [212–217], two forms of S 2with S = 1/2 and S ≥ 5/2 [218–225], and S = 3 for S 3[226–228]) but cannot uniquely assign absolute oxidation states. 55Mn electron nuclear double resonance (ENDOR) studies of the S 2state first demonstrated the “3+1” Y‐shaped configuration of the cluster [176, 229], while 55Mn hyperfine coupling parameters supported the Mn(III)Mn(IV) 3oxidation state assignment for S 2[176, 230, 231] and confirmed the absence of Mn(II) in the S 0state [230, 232]. Electron–electron double resonance (ELDOR) detected nuclear magnetic resonance experiments (EDNMR) of the S 3state [228] demonstrated that all Mn ions are similar and isotropic, consistent with the Mn(IV) 4oxidation state assignment. X‐ray absorption and emission spectroscopies broadly agree with the results of magnetic resonance spectroscopies regarding oxidation states and localization of oxidation events. X‐ray absorption near‐edge spectroscopy (XANES) shows a shift of the Mn K‐edge to higher energies with each S‐state transition, consistent with successive Mn‐based oxidation [181, 233, 234] and a change in coordination in the S 2→ S 3transition [235]. XES studies observe changes in the Kβ′, Kβ 1,3, and Kα lines. Recent time‐resolved studies of Kβ 1,3emission spectra of the OEC at room temperature that included comparisons with reference compounds support the high oxidation state assignment described above as well as Mn(III)–Mn(IV) oxidation in the S 2→ S 3transition [182], while room‐temperature Kα XES studies that similarly compared data on the OEC with those on synthetic compounds in different oxidation states confirmed that the OEC reaches the Mn(IV) 4oxidation level in the S 3state [183]. The same assignment of oxidation states is supported by independent studies of photoactivation of PS‐II microcrystals, which measure the number of flash‐driven electron removals required for assembly of an active manganese cofactor from Mn(II) and the Mn‐free enzyme [236].