Electrogenic reactions on the donor side of Mn-depleted photosystem II core particles in the presence of MnCl₂ and synthetic trinuclear Mn-complexes

An electrometric technique was used to investigate the generation of a photovoltage (Δψ) by Mn-depleted spinach photosystem II (PS II) core particles incorporated into liposomes. In the presence of MnCl₂, the fast kinetically unresolvable phase of Δψ generation, related to electron transfer between the redox-active tyrosine Y_Z and the primary plastoquinone acceptor Q_A was followed by an additional electrogenic phase (τ∼ 20 μs, ∼5% of the phase attributed to Y_Z^oxQ_A^-). The latter phase was ascribed to the transfer of an electron from the Mn, bound to the Mn-binding site of the PS II reaction center to the Y_Z^ox. An additional electrogenicity observed upon addition of synthetic trinuclear Mn complex-1 has a τ∼ 50 μs (∼4% of the Y_Z ^oxQ_A) and τ∼ 160 ms (∼25%). The fast electrogenic component could be ascribed to reduction of Y_Z ^ox by Mn, delivered to the Mn-binding site in Mn-depleted samples after the release of the tripod ligands from the complex-1 while the slow electrogenic phase to the electron transfer from the Mn-containing complex-1 attached to the protein-water boundary to the oxidized Mn at the protein-embedded Mn-binding site.