Embargoed until 2pm U.s. Eastern Time on the Thursday before This Date:

could stabilize the oxidized form of the cytochrome subunit (12). From the temperature dependence of the P:QA – → P:QA charge recombination reaction, it was proposed that structural changes occurring in response to electron transfer decrease the free energy gap between P and QA – of RCsph by about 12 kJ mol (23). Similar measurements of the pH dependence of the RCvir charge recombination reaction showed the P:QA – state to be stabilized by a chemical group with an approximate pKa of 9 (24). Prolonged exposure to bright light is also known to reversibly stabilize the P:QA – state of RCsph, slowing the charge recombination reaction by up to three orders of magnitude (3, 25, 26). Light-induced deprotonation and conformational switching of TyrL162 could contribute to all three stabilization effects, because the creation of a phenolate anion in immediate proximity to the special pair effectively neutralizes the energetic penalty associated with the buried positive charge on P. Electron paramagnetic resonance spectroscopy studies of RCsph mutants that increase the midpoint potential of the special pair have shown that TyrL162 can form a (deprotonated) tyrosyl radical in response to charge separation (27), confirming that TyrL162 can be deprotonated by photooxidation of the special pair. Photooxidation of photosystem II also creates a tyrosyl radical (TyrZ ) (28, 29), which oxidizes the manganese cluster and leads to the synthesis of molecular oxygen from water. Crystal structures of photosystem II (30) reveal that TyrZ lies between the oxygen-evolving center and the special pair in a position functionally analogous to that occupied by TyrL162 of RCvir (Fig. 4). Our findings suggest that the deprotonation and coupled conformational switching of TyrL162 may have aided the spontaneous formation of tyrosine radicals in an ancient reaction center, thereby creating the chemical potential to extract electrons from clusters of manganese atoms (31) and ultimately oxidize water to oxygen.