PSII uses light energy to split water into protons (H+) and molecular oxygen (O2). In order to drive this reaction, a huge redox potential is required and P680, the reaction center of PSII, is the most powerful biological oxidant (1.3 V) identified to date.
Unraveling the mechanism by which PSII does this is one of the holy grails in our field. If we only knew how it worked, we could make artificial systems that take advantage of the mechanism for coupling light energy to electron extraction from water (translation = ‘free’ electrical energy from water). So how does it work? We don’t know the details for sure, but here’s what we know so far…
Mechanistically, splitting water to form oxygen is an extremely difficult reaction to do. Using four successive photons, PSII must sequentially pull four electrons from two water molecules and form an O-O bond. This can be seen experimentally by measuring the amount of oxygen produced at each flash of light in a series of flashes.
The water-splitting reaction is accomplished by the Oxygen-Evolving Complex within PSII, an inorganic cluster of four manganese atoms and one calcium atom (Mn4CaO5). This works because manganese is a transitional metal that can exist in a number of different oxidation states Mn2+, Mn3+ and Mn4+. This kind of stable redox space makes it the perfect element for holding onto the four electrons required to split water and form O2. As the PSII reaction center absorbs photons, the four Mn atoms of the cluster cycle through various combinations of oxidation states as PSII turns over ultimately forming oxygen. These are termed S-states. So, mechanistically the Mn4CaO5 cluster cycles through 5 different S-states as photons are absorbed. The dark-stable state is S0. As individual photons are absorbed the Mn4CaO5 cluster proceeds through S1 to S2 to S3. Upon the absorption of the fourth photon, the system continues through a transient S4 state and the O-O bond is formed as the cluster resets back to S0.
Currently, there is much debate among PSII researchers as to what the exact combination of oxidation states is as well as what the precise structure of the Mn4CaO5 cluster is at each S-state. Researchers are using various types of spectroscopies (X-ray diffraction, X-ray absorption, X-ray emission, EPR, FTIR) and oxygen detection measurements to gain additional insights on the mechanism.