A biomimetic model system for the water oxidizing triad in photosystem II

In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox 'triad' consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium(II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn(III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn(III)/(IV) state. The redox potentials in our system are comparable to those in Photosystem II. Thus, our synthetic redox 'triad' mimics important elements in the electron donor 'triad' in Photosystem II, significantly advancing the development of systems for artificial photosynthesis based on ruthenium-manganese complexes.