Time-resolved, light-induced conformational changes in R.sphaeroides reaction centers upon QA-reduction

Abstract

By comparing peptide patterns after mild trypsination in the dark and in light, we have earlier shown that reaction center undergoes a conformational change upon illumination. We have studied the dynamics of this structural change in the presence of o-phenanthroline, which inhibits electron transfer from QA to QB. Time-resolved studies of the kinetics of charge separation and recombination were performed as a function of actinic light intensity and temperature (5 to 35° C). At each temperature the data were globally fitted to a model where charge separated reaction center can be present in two different structural states, where the light-adapted conformation does not allow the electron to recombine directly to P870+, but only via the dark-adapted conformation. Thermodynamic parameters for the model were extracted using the temperature dependence of the rate constants. The time constants for both the forward and the reverse transitions between the different charge separated conformations are in the order of minutes.