The participation of tyrosine in the oxidation of water by photosystem II, a multisubunit enzyme complex involved in plant photosynthesis, exemplifies the significant role amino-acid side chains play in oxidation/reduction reactions in proteins. The influence of the surrounding protein on the properties of amino-acid radicals and the attributes necessary for electron transfer are not well understood. Here we report that modifications of reaction centres from the purple bacterium Rhodobacter sphaeroides result in the generation of a tyrosyl radical in a manner similar to that of photosystem II. Our design incorporates a highly oxidizing bacteriochlorophyll dimer possessing a midpoint potential of more than 0.8 V, which was achieved by altering its local environment. After substitution of tyrosine residues at positions corresponding to the tyrosyl radicals of photosystem II, optical and electron paramagnetic resonance spectra showed changes consistent with oxidation of the tyrosine. The properties of this reaction include initiation by light and coupling to proton transfer, most probably to residues near the tyrosines.
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