Role of redox-inactive metals in controlling the redox potential of heterometallic manganese–oxido clusters

Photosystem II (PSII) contains Ca2+, which is essential to the oxygen-evolving activity of the catalytic Mn4CaO5 complex. Replacement of Ca2+ with other redox-inactive metals results in a loss/decrease of oxygen-evolving activity. To investigate the role of Ca2+ in this catalytic reaction, we investigate artificial Mn3[M]O2 clusters redox-inactive metals [M] ([M]  = Mg2+, Ca2+, Zn2+, Sr2+, and Y3+), which were synthesized by Tsui et al. (Nat Chem 5:293, 2013). The experimentally measured redox potentials (Em) of these clusters are best described by the energy of their highest occupied molecular orbitals. Quantum chemical calculations showed that the valence of metals predominantly affects Em(MnIII/IV), whereas the ionic radius of metals affects Em(MnIII/IV) only slightly. Supplementary Information The online version contains supplementary material available at 10.1007/s11120-021-00846-y.


Introduction
Plants, algae, and cyanobacteria use the water-splitting enzyme photosystem II (PSII) for oxygen evolution. The oxygen evolution proceeds at the oxygen-evolving center, the Mn 4 CaO 5 cluster. The cluster consists of a distorted cubane [Mn1, Mn2, Mn3, four oxygen atoms, and Ca 2+ ] and "dangling" Mn4 ( Fig. 1)  . The Mn 4 CaO 5 cluster has two ligand water molecules, W1 and W2, at the Mn4 site and another two ligand water molecules, W3 and W4, at the Ca 2+ site (Fig. 1). The catalytic cycle moves through a series of oxidation states, denoted as S n (n = 0, 1, 2, and 3). As electron transfer occurs, S n increases. During the catalytic cycle, four electrons from two of the substrate water molecules are removed, and O 2 evolves in the S 3 to S 0 transition (Shen 2015;Cardona and Rutherford 2019).
In the Mn 4 CaO 5 cluster, a redox-inactive Ca 2+ is essential for the oxygen evolution activity, as oxygen is not evolved when Ca 2+ is removed (Ono and Inoue 1988) or replaced with Dy 3+ , Cu 2+ , Cd 2+ (Lee et al. 2007), K + , Rb + , and Cs + (Ono et al. 2001). The Mn 4 SrO 5 cluster can evolve oxygen but the activity is lower than that of the native Mn 4 CaO 5 cluster (Yachandra and Yano 2011). Koua et al. identified that the distance between Sr 2+ and W3 (2.6 Å) was longer than that between Ca 2+ and W3 (2.4 Å) (Koua et al. 2013) and proposed that the long Sr 2+ ⋯W3 distance contributed to the decrease in the activity upon replacement of Ca 2+ with Sr 2+ .
It was speculated that Ca 2+ might be responsible for the distorted cubane structure of the Mn 4 CaO 5 cluster . However, the removal of Ca 2+ does not alter the Mn 3 CaO 4 cubane structure (Saito and Ishikita 2014;Siegbahn 2014Siegbahn , 2017, as suggested by the extended X-ray absorption fine structure (EXAFS) and the electron paramagnetic resonance (EPR) measurements (Latimer et al. 1998;Yachandra and Yano 2011;Lohmiller et al. 2012). Note that the Jahn-Teller distortion for Mn(III) ions can be affected by Ca 2+ (Yamaguchi et al. 2013). When Ca 2+ is removed, the rearrangement of water molecules in the hydrogen-bond (H-bond) network of the redox-active tyrosine (TyrZ) is observed (Saito and Ishikita 2014;Saito et al. 2020a). TyrZ is involved in electron transfer from the Mn 4 CaO 5 cluster to the reaction center chlorophyll P D1 . The rearrangement of the H-bond network increases its redox potential (E m (TyrZ)) by ~ 300 mV and inhibits the formation of the downhill electron transfer pathway from the Mn 4 CaO 5 cluster via TyrZ to P D1 (Saito et al. 2020a). Thus, Ca 2+ is essential in both maintaining the H-bond network and optimizing electron transfer. The role of Ca 2+ as the water binding site can be substituted with H 3 O + : recent theoretical studies showed that the H-bond network, including the low-barrier H-bond between TyrZ and D1-His190, remains unaltered upon the replacement of Ca 2+ with H 3 O + (Saito et al. 2020a).
Ca 2+ is a prerequisite for the low-barrier H-bond between W1 and D1-Asp61: they form a low-barrier H-bond in native PSII (Kawashima et al. 2018b;Saito et al. 2020a), whereas they cannot form in the absence of Ca 2+ (Saito et al. 2020a). That is, Ca 2+ decreases pK a (W1) electrostatically to a level of pK a (D1-Asp61) in native PSII, thus forming the lowbarrier H-bond and facilitating proton transfer from W1 to D1-Asp61.
Althogh it was proposed that Ca 2+ might electrostatically affect the properties of the cluster (e.g., pK a and E m ) (McEvoy and Brudvig 2006), the replacements of Ca 2+ with H 2 O and H 3 O + lead to different E m (Mn III/IV ) values due to different H-bond patterns in PSII (Saito et al. 2020a). Artificial Mn clusters with redox-inactive metals (Zhang et al. 2015;Mukherjee et al. 2012;Kanady et al. 2013;Lin et al. 2015) may serve as reference model systems since the corresponding H-bond network is absent. Tsui et al. synthesized   . A similar correlation between the ligand-to-metal charge transfer energy (related to E m ) and the Lewis acidity has also been reported in the Fe and Mn complexes (Bang et al. 2014;Krewald et al. 2016).
E m can be calculated as the free energy difference between the oxidized and reduced states, including the entropic effect of the solvent (Marenich et al. 2014;Pitari et al. 2015;Amin et al. 2013;Krewald et al. 2016). E m also correlates with the ionization potential as shown for various complexes, including Mn complexes (Marenich et al. 2014;Krewald et al. 2016). The ionization potential can be regarded as the free energy difference between the oxidized and the reduced states when the reorganization effect upon the redox reaction (including the electronic relaxation, the solvent reorganization, and the structural change of the molecule) is neglective. As the ionization potential (or the electronic affinity) is correlated with the energy levels of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) in density functional theory (DFT) (Kohn-Sham orbital) (Zhang and Musgrave 2007), E m should be calculated based on the HOMO or LUMO energy calculated using DFT. An electron releases from the HOMO upon oxidation, whereas an electron enters the LUMO upon reduction. Thus, the HOMO energy corresponds to the potential for one-electron oxidation, and the LUMO energy corresponds to the potential for one-electron reduction. When the redox reaction is reversible, the midpoint potential E m is located at the midpoint between the oxidation and reduction potentials, i.e., E m and the two potentials have the same tendency. Indeed, the E m of quinones can be determined based on the LUMO energy (Ishikita and Saito 2020) as accurately as the free energy difference (Kishi et al. 2017). For complexes that include transition metals, high correlations between the HOMO and/ or LUMO energy and the experimentally measured  Table S2). E m (Mn1 III/IV ) was calculated from the HOMO energies, since the value of E m for one-electron oxidation is correlated with the energy of the highest occupied molecular orbital (HOMO) (Mendez-Hernandez et al. 2013;Igarashi and Seefeldt 2003;Mandal et al. 2020). Using the optimized geometries in vacuum, the HOMO energy (E HOMO ) was calculated in dichloromethane (CH 2 Cl 2 , dielectric constant 8.93) using the polarizable continuum model (PCM). All calculations were performed with Jaguar program [Schrödinger, LLC, 2012, New York]. The initial-guess wavefunctions were obtained using the ligand field theory (Vacek et al. 1999) implemented in the Jaguar program. For the detail of ligands, see Table S1 associated with a difference between the absolute electrode potential and the Fc/Fc + electrode potential and liquid junction potential (Kishi et al. 2017). These factors depend on the size and the net charge of the QM system, the solvent, and the reference electrode (e.g., see the caption of Fig. S1). Thus, Eq. (1) (Fig. 4, red circles). .

Results and discussion
The removal of Y 3+ resulted in an increase of 1.5 V in E m , whereas the removal of Na + resulted in an increase of 0.5 V in E m (Fig. 5a). These results suggest that the valence of [M] is the main factor determining E m . In addition, the removal of a metal with a large radius (e.g., Sr 2+ ) resulted in a large increase in E m , whereas the removal of a metal with a small radius (e.g., Zn 2+ ) resulted in a small increase in E m (Fig. 5b). For metals with the same valence (e.g., Sr 2+ , Ca 2+ , and Zn 2+ ), the difference in E m can be explained by the difference in the ionic radius of the redox-inactive metal    (Lin et al. 2015).
In PSII, E m (Mn1 III/IV ) of the Mn 4 CaO 5 cluster changes by only ~40 mV even upon the replacement ion of Ca 2+ with H 3 O + irrespective of the loss of a +1 charge (Saito et al. 2020a). In contrast, E m (Mn III/IV ) of the Mn 3 [M]O 2 cluster changes by 450 mV upon the loss of a +1 charge (Fig. 5a). These indicate that the protein environment including the H-bond network (e.g., D1-Asp61, TyrZ, D1-His190, and water molecules) plays a key role in determining the E m (Mn 4 CaO 5 ) in PSII.
In summary, the quantum-chemically calculated HOMO energies of artificial Mn 3 [M]O 2 clusters ([M] = Na + , Sr 2+ , Ca 2+ , Zn 2+ , and Y 3+ ) correlate with the experimentally measured E m (Mn III/IV ) values (Fig. 3). The E m calculation for the metal-deleted Mn 3 O 2 clusters shows that the valence of [M] predominantly affects E m (Fig. 5a), whereas the ionic radius of [M] affects E m only slightly (Fig. 5b).

Conflict of interest
The authors declare that they have no conflict of interest.
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