| Enabling the abundant, cheap and carbon-free energy sources to be used as efficiently as coal and oil is crucial to meet the global energy demand in a sustainable manner. Using the abundant energy from sunlight and storing it as electrical and chemical energy by extracting electrons from water and converting its H+ to H2, in other words, splitting H2O with solar energy, is considered to be the greenest and the most sustainable alternative. Extracting electrons from H2O (i.e. H2O oxidation) with sunlight demands a perfect synchronization of multi-step processes involving light absorption, energy, electron and proton transfer, catalysis, and redox leveling of the catalytically active site. In nature, photosystem II (PSII) is the only enzyme to catalyze light-driven H2O splitting. Its complexity and efficiency make this 20-subunit, 350-kDa protein one of the most entrancing natural machineries. A firm understanding of this macromolecular machinery is a prerequisite to achieve our goal of a sustainable energy-powered world.;The key property of PSII function is the formation of an O–O bond from two molecules of H2O. Due to its highly transient nature, the mechanism of the O–O bond-forming step is unknown. The indirect, yet crucial effects of proton- and electron-transfer processes on this transient O–O bond-forming intermediary state also remain unclear. The work presented in this thesis aims to establish a new approach to solve these decades-long puzzles and to lead to a promising application in the solar energy field.;We established a control over the kinetics of PSII catalysis and slowed down the catalytic cycle to capture highly reactive intermediary stages of PSII catalysis. We discovered that small molecules, such as zwitterionic betaine, affect the catalytic rate without perturbing the core catalytic cycle. The site of betaine binding is most likely the proton transfer channels built within PSII. The newly identified zwitterionic small molecules can be used as tools to control and investigate the water-oxidizing reaction kinetics of PSII.;We introduced a novel route for electron transfer in PSII. We identified a site on the PSII surface that is suitable for anchoring cationic small molecules. Cobalt(III) coordination complexes with suitable reduction potentials were shown to bind to and harvest electrons extracted from substrate water molecules, most likely by associating to this site. This newly identified site could now be used to short-circuit electrons from their natural route to our delegated alternative route. This is the first example of an artificial redirection of the pathway of electrons extracted from water in higher plant PSII, and constitutes a viable approach towards utilizing nature's tools to harness solar energy. |
