Dependence of tyrosine oxidation in highly oxidizing bacterial reaction centers on pH and free-energy difference

The pH and temperature dependences of tyrosine oxidation were measured in reaction centers from mutants of Rhodobacter sphaeroides containing a tyrosine residue near a highly oxidizing bacteriochlorophyll dimer. Under continuous illumination, a rapid increase in the absorption change at 420 nm was observed because of the formation of a charge-separated state involving the oxidized dimer and reduced primary quinone, followed by a slow absorption decrease attributed to tyrosine oxidation. Both the amplitude and rate of the slow absorption change showed a pH dependency, indicating that, at low pH, the rate of tyrosine oxidation is limited by the transfer of the phenolic proton to a nearby base. Below 17 °C, the rate of the slow absorption change had a strong exponential dependence on the temperature, indicating a high activation energy. At higher pH and temperature, the overall rate of tyrosyl formation appears to be limited by a proposed conformational change in the reaction center that is also observed in reaction centers that do not undergo tyrosine oxidation. The yield of tyrosyl formation measured using electron paramagnetic resonance spectroscopy decreased significantly at 4 °C compared to 20 °C and was lower at both temperatures in mutants expected to have a slightly smaller driving force for tyrosyl formation.