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 Mn-3[M]O-2 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.

Automated summary

The presence of calcium ions in Photosystem II is crucial for the oxygen-evolving activity of the Mn4CaO5 complex. Substituting calcium ions with other redox-inactive metals leads to a decrease in oxygen-evolving activity. Studying artificial Mn-3[M]O-2 clusters synthesized by Tsui et al. reveals that the valence of metals significantly influences the redox potentials, while the ionic radius has a minor effect.