Photolysis of Water

Photolysis - the light-driven splitting of water - is carried out by a dedicated protein complex on Photosystem II called the Oxygen Evolving Complex (OEC), also known as the water-splitting complex, and its activity depends on two cofactors, manganese ions (Mn2+) and chloride ions (Cl-). When PS II is activated by light, it drives the splitting of a water molecule into hydroxide (OH-) and hydrogen (H+) ions; the hydroxide ions then combine with each other to regenerate water while simultaneously releasing molecular oxygen and electrons. Stepwise, this can be written as $4H_2O \rightarrow 4H^+ + 4OH^-$, then $4OH^- \rightarrow 2H_2O + O_2 + 4e^-$, which together simplify to the net reaction $2H_2O \rightarrow 4H^+ + O_2 + 4e^-$ - so splitting two water molecules ultimately yields four protons, one oxygen molecule, and four electrons available for the electron transport chain. The strong oxidant responsible for actually pulling electrons out of water in the first place has not been definitively identified and is provisionally labelled Z (or Yz) in the literature. The most widely accepted mechanistic model for how this happens is the water-oxidising clock, or S-state mechanism, proposed by Kok and co-workers in 1970: the Oxygen Evolving Complex cycles through a series of five states, labelled S0, S1, S2, S3 and S4, and at each step from one state to the next it absorbs one photon (hv) and accumulates one additional positive charge. By the time the complex reache …