The influence of the local environment on the formation of a tyrosyl radical was investigated in modified photosynthetic reaction centers from Rhodobacter sphaeroides. The reaction centers contain a tyrosine residue placed ∼10 Å from a highly oxidizing bacteriochlorophyll dimer. Measurements by both optical and electron paramagnetic resonance spectroscopy revealed spectral features that are assigned as arising primarily from an oxidized bacteriochlorophyll dimer at low pH values and from a tyrosyl radical at high pH values, with a well-defined transition that occurred with a pKa of 6.9. A model based on the wild-type structure indicated that the Tyr at M164 is likely to form a hydrogen bond with His M193 and to interact weakly with Glu M173. Substitution of Tyr or Glu for His at M193 increased the pKa for the transition from 6.9 to 8.9, while substitution of Gln for His M193 resulted in a higher pKa value. Substitution of Glu M173 with Gln resulted in loss of the partial formation of the tyrosyl that occurs in the other mutants at low pH values. The results are interpreted in terms of the ability of the residues to act as proton acceptors for the oxidized tyrosine, with the pKa values reflecting those of either the putative proton acceptor or the tyrosine, in accord with general models of amino acid radicals.
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