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

Abstract

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 (E_m) 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 E_m(Mn^III/IV), whereas the ionic radius of metals affects E_m(Mn^III/IV) only slightly.

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 Mn₄CaO₅ cluster. The cluster consists of a distorted cubane [Mn1, Mn2, Mn3, four oxygen atoms, and Ca²⁺] and “dangling” Mn4 (Fig. 1) (Umena et al. 2011). The Mn₄CaO₅ cluster has two ligand water molecules, W1 and W2, at the Mn4 site and another two ligand water molecules, W3 and W4, at the Ca²⁺ 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 O₂ evolves in the S₃ to S₀ transition (Shen 2015; Cardona and Rutherford 2019).

Fig. 1

Structure of the Mn₄CaO₅ cluster of PSII. Dotted lines indicate ligations to Ca²⁺

In the Mn₄CaO₅ cluster, a redox-inactive Ca²⁺ is essential for the oxygen evolution activity, as oxygen is not evolved when Ca²⁺ is removed (Ono and Inoue 1988) or replaced with Dy³⁺, Cu²⁺, Cd²⁺ (Lee et al. 2007), K⁺, Rb⁺, and Cs⁺ (Ono et al. 2001). The Mn₄SrO₅ cluster can evolve oxygen but the activity is lower than that of the native Mn₄CaO₅ cluster (Yachandra and Yano 2011). Koua et al. identified that the distance between Sr²⁺ and W3 (2.6 Å) was longer than that between Ca²⁺ and W3 (2.4 Å) (Koua et al. 2013) and proposed that the long Sr²⁺⋯W3 distance contributed to the decrease in the activity upon replacement of Ca²⁺ with Sr²⁺.

It was speculated that Ca²⁺ might be responsible for the distorted cubane structure of the Mn₄CaO₅ cluster (Kawakami et al. 2011). However, the removal of Ca²⁺ does not alter the Mn₃CaO₄ cubane structure (Saito and Ishikita 2014; Siegbahn 2014, 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 Ca²⁺ (Yamaguchi et al. 2013). When Ca²⁺ 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 Mn₄CaO₅ 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 Mn₄CaO₅ cluster via TyrZ to P_D1 (Saito et al. 2020a). Thus, Ca²⁺ is essential in both maintaining the H-bond network and optimizing electron transfer. The role of Ca²⁺ as the water binding site can be substituted with H₃O⁺: 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 Ca²⁺ with H₃O⁺ (Saito et al. 2020a).

Ca²⁺ 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 Ca²⁺ (Saito et al. 2020a). That is, Ca²⁺ decreases pK_a(W1) electrostatically to a level of pK_a(D1-Asp61) in native PSII, thus forming the low-barrier H-bond and facilitating proton transfer from W1 to D1-Asp61.

So far, the role of the Ca²⁺ can be summarized as follows: (i) maintaining the TyrZ H-bond network (Saito et al. 2011, 2020a; Kawashima et al. 2018a), including the low-barrier H-bond between TyrZ and D1-His190 (Kawashima et al. 2018b; Saito et al. 2020a); (ii) optimizing E_m(TyrZ) in the electron transfer cascade (Saito et al. 2020a); and (iii) electrostatically decreasing pK_a(W1) and facilitating proton transfer via the low-barrier H-bond with D1-Asp61 (Saito et al. 2020a).

Althogh it was proposed that Ca²⁺ might electrostatically affect the properties of the cluster (e.g., pK_a and E_m) (McEvoy and Brudvig 2006), the replacements of Ca²⁺ with H₂O and H₃O⁺ 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; Tsui et al. 2013; Kanady et al. 2013; Tsui and Agapie 2013; Lin et al. 2015) may serve as reference model systems since the corresponding H-bond network is absent. Tsui et al. synthesized artificial Mn₃[M]O₂ and clusters with redox-inactive metals [M] ([M] = Mg⁺, Ca²⁺, Zn²⁺, Sr²⁺, and Y³⁺) (Fig. 2), showing that E_m(Mn^III/IV) depends on the Lewis acidity of [M] (i.e., the pK_a of aqua complexes of [M]) (Tsui et al. 2013). 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).

Fig. 2

Structure of the Mn₃[M]O₂ cluster for [M] = Ca²⁺, Sr²⁺, and Y³⁺ (Tsui et al. 2013) a Chemical structure. b Three-dimensional structure. For the detail of ligands, see Table S1

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 E_m value were observed [e.g., organic compounds (Mendez-Hernandez et al. 2013), β-diketones complexes, including Mn and Fe (Conradie 2015), and FeCo proteins (Dance 2006)]. For the natural Mn₄CaO₅ cluster in the PSII protein environment, E_m can be determined based on the HOMO energy (Mandal et al. 2020; Saito et al. 2020a, b). Here we calculate the E_m values of the artificial clusters (Tsui et al. 2013; Tsui and Agapie 2013) based on the HOMO energy and explain how the redox-inactive metal affects E_m(Mn^III/IV).

Computational details

The crystal structures of synthetic Mn₃[M]O₂ clusters ([M] = Na⁺, Sr²⁺, Ca²⁺, Zn²⁺, and Y³⁺) (Tsui et al. 2013) and Mn₃[M]O₄ clusters ([M] = Sr²⁺, Ca²⁺, Zn²⁺, Mn³⁺, Sc³⁺ and Y³⁺) (Tsui and Agapie 2013) were used as the basis for geometry optimization using unrestricted DFT (UDFT), with the B3LYP functional and LACVP* basis set (for optimized structures, see Supporting Information). For efficiency, the cluster was considered to comprise ferromagnetically coupled Mn atoms, (Tsui et al. 2013) where the total spin, S, was 12/2 for the Mn₃[M]O₂ cluster and 10/2 for the Mn₃[M]O₄ cluster. We note that, in the calculation of the native Mn₄CaO₅ of PSII, the difference in S (e.g., S = 1/2 in S₂ (Zimmermann and Rutherford 1986), high, low, ferromagnetic, and antiferromagnetic) did not affect the (i) resulting geometry (Ames et al. 2011; Isobe et al. 2012), (ii) potential energy profile of proton transfer (Kawashima et al. 2018b), (iii) redox potential of each Mn site (Mandal et al. 2020), or (iv) pK_a values of ligand water molecules W1–W4 (Saito et al. 2020c). The resulting oxidation states for three Mn atoms were Mn(III)₃ and Mn(IV)Mn(III)₂ for the Mn₃[M]O₂ and the Mn₃[M]O₄ clusters, respectively (for atomic spin density, see 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 (CH₂Cl₂, 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.

Results and discussion

The calculated E_HOMO values for the artificial Mn₃[M]O₂ clusters ([M] = Na⁺, Sr²⁺, Ca²⁺, Zn²⁺, and Y³⁺) show a correlation with the experimentally measured E_m(Mn^III/IV) values (Fig. 3) in CH₂Cl₂ and are best fitted to Eq. (1).

$$E_{{\rm{m}}} \;{\text{(V}}\;{\text{vs}}{.}\;{\text{Fc/Fc}}^{ + } {) = } -{0}{{.302}}\;E_{{{\rm{HOMO}}}} \;({\text{eV}}) -1.710,$$

(1)

where Fc/Fc+ denotes ferrocene electrode. A similar correlation is also observed in the Mn₃[M]O₄ cubane clusters ([M] = Sr²⁺, Ca²⁺, Zn²⁺, Mn³⁺, Sc³⁺, and Y³⁺) (Tsui and Agapie 2013) (Fig. S1). The coefficient of −0.302 in Eq. (1) is the conversion factor from MO energy to E_m, which may be associated with the solvation effect (Schmidt am Busch and Knapp 2005), whereas the offset of −1.710 V is 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) is applicable only to similar molecular groups (e.g., artificial Mn₃[M]O₂ clusters).

Fig. 3

Experimentally measured E_m(Mn^III/IV) values in CH₂Cl₂ (Tsui and Agapie 2013) and calculated HOMO energy levels (E_HOMO) of the Mn₃[M]O₂ clusters. The coefficient of determination (R²) is 0.96

Using Eq. (1), E_HOMO can be converted to E_m. The calculated E_m values correlate with the experimentally measured E_m values (Fig. 4, blue diamonds). In the Mn₃[M]O₂ clusters synthesized by Tsui, each [M] has different ligand groups (Table S1). Accordingly, the absolute E_m values are affected by [M] and the ligand groups. To evaluate the direct influence of electrostatic and the van der Waals interactions with [M] on the E_m of the Mn₃[M]O₂ cluster, the metal [M] was removed from the geometry-optimized Mn₃[M]O₂ cluster. The calculated E_m values for the metal-removed clusters do not correlate with the experimentally measured E_m values (Fig. 4, red circles)._.

Fig. 4

Calculated E_m(Mn^III/IV) values of the Mn₃[M]O₂ clusters (blue diamonds) and calculated E_m(Mn^III/IV) values of the metal-removed Mn₃O₂ clusters (red circles) plotted with experimentally measured E_m(Mn^III/IV) values in CH₂Cl₂ (Tsui and Agapie 2013)

The removal of Y³⁺ 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., Sr²⁺) resulted in a large increase in E_m, whereas the removal of a metal with a small radius (e.g., Zn²⁺) resulted in a small increase in E_m (Fig. 5b). For metals with the same valence (e.g., Sr²⁺, Ca²⁺, and Zn²⁺), the difference in E_m can be explained by the difference in the ionic radius of the redox-inactive metal [M]. As the radius of [M] increases, the distance between [M]²⁺ and Mn increases, leading to weak electrostatic interactions between Mn and [M]²⁺. Thus, of the [M]²⁺ metals, [M] with large radii have a smaller influence on E_m. The effect that the ionic radius has on the difference in E_m can be explained in terms of the Lewis acidity of [M] because the Lewis acidity decreases with an increase in the ionic radius (Lin et al. 2015).

Fig. 5

Shift in E_m upon the removal of [M] ([M] = Na⁺ (red), Sr²⁺, Ca²⁺, Zn²⁺ (green), and Y³⁺ (blue)) a with respect to the valence (R² = 0.93) and b with respect to the ionic radius (Shannon 1976) of [M] ([M] = Sr²⁺, Ca²⁺, and Zn²⁺) (R² = 0.99)

In PSII, E_m(Mn1^III/IV) of the Mn₄CaO₅ cluster changes by only ~40 mV even upon the replacement ion of Ca²⁺ with H₃O⁺ irrespective of the loss of a +1 charge (Saito et al. 2020a). In contrast, E_m(Mn^III/IV) of the Mn₃[M]O₂ 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(Mn₄CaO₅) in PSII.

In summary, the quantum-chemically calculated HOMO energies of artificial Mn₃[M]O₂ clusters ([M] = Na⁺, Sr²⁺, Ca²⁺, Zn²⁺, and Y³⁺) correlate with the experimentally measured E_m(Mn^III/IV) values (Fig. 3). The E_m calculation for the metal-deleted Mn₃O₂ 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).