Early indications for manganese oxidation state changes during photosynthetic oxygen production: A personal account

One of the major questions yet to be answered in photosynthesis research today is what is the chemical mechanism for the oxidation of water into molecular oxygen. It is well established that an inorganic cluster of four manganese ions and at least one calcium ion form the catalytic core. As the oxidation potential generated by the Photosystem II reaction center is accumulated over the four sequential steps needed to produce O ₂, changes in the oxidation state of the catalytic manganese occur, though the formal oxidation states that are involved are still a matter of considerable debate. Much of what is currently known has come from direct measurements of the catalytic manganese using electron paramagnetic resonance (EPR) and X-ray spectroscopy. However, in the early attempts to attack this problem, the catalytic manganese was monitored indirectly by its paramagnetic effect on the nuclear magnetic resonance (NMR) relaxation rates of solvent water protons. In this contribution, a description of the proton relaxation rate phenomenon and its use to indicate manganese oxidation state changes during O ₂ production is presented.