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The coupling of proton chemistry with redox reactions is important in many enzymes and is central to energy transduction in biology. However, the mechanistic details are poorly understood. Here, we have studied tyrosine oxidation, a reaction in which the removal of one electron from the amino acid is linked to the release of its phenolic proton. Using the unique photochemical properties of photosystem II, it was possible to oxidize the tyrosine at 1.8 K, a temperature at which proton and protein motions are limited. The state formed was detected by high magnetic field EPR as a high-energy radical intermediate trapped in an unprecedentedly electropositive environment. Warming of the protein allows this state to convert to a relaxed, stable form of the radical. The relaxation event occurs at 77 K and seems to involve proton migration and only a very limited movement of the protein. These reactions represent a stabilization process that prevents the back-reaction and determines the reactivity of the radical.

Resolving intermediates in biological proton-coupled electron transfer: A tyrosyl radical prior to proton movement