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 (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.
Highlights
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 catalytic cycle moves through a series of oxidation states, denoted as Sn (n = 0, 1, 2, and 3)
Summary
Algae, and cyanobacteria use the water-splitting enzyme photosystem II (PSII) for oxygen evolution. The rearrangement of the H-bond network increases its redox potential (Em(TyrZ)) by ~ 300 mV and inhibits the formation of the downhill electron transfer pathway from the M n4CaO5 cluster via TyrZ to PD1 (Saito et al 2020a).
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