Electrogenic reactions on the donor side of Mn-depleted photosystem II core particles in the presence of $MnCl_2$ and synthetic trinuclear Mn-complexes

An electrometric technique was used to investigate the generation of a photovoltage ($\Delta\psi$) by Mn-depleted spinach photosystem II (PS II) core particles incorporated into liposomes. In the presence of $MnCl_2$, the fast kinetically unresolvable phase of $\Delta\psi$ 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 (${\tau}\;{\sim}\;20\;{\mu}s$, ~5% of the phase attributed to ${Y_Z}^{OX}{Q_A}^-$). 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 ${\tau}\;{\sim}\;50\;{\mu}s$ (~4% of the ${Y_Z}^{OX}Q_A$) and ${\tau}\;{\sim}\;160\;ms$ (~25%). The fast electrogenic component could be ascribed to reduction of ${Y_Z}^{OX}$ 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 theMn-containing complex-1 attached to the protein-water boundary to the oxidized Mn at the protein-embedded Mn-binding site.