A combined picture from theory and experiments on water oxidation, oxygen reduction and proton pumping

In order to illustrate how theory and experiments can be combined, examples are taken from two projects that have been going on for a decade. The goal is to obtain the full mechanistic picture of water oxidation in photosystem II and proton pumping in cytochrome c oxidase. It is argued that for obtaining a complete description of these processes, both experiments and theoretical calculations are needed. It is obvious that there are aspects, which are out of reach for computations, but there are also key aspects that can not be obtained by experiments. This concerns very short-lived species but also, in the case of photosynthesis in particular, structural information that is presently out of reach. The main contributions from theory in the present cases, is for photosynthesis a mechanism for O–O bond formation including new and improved structures for all S-states, and for proton pumping a plausible and simple mechanism for proton gating. The examples also illustrate that sometimes rather qualitative experimental information can be of highest importance.