Redox potential of quinones in photosynthetic reaction centers from Rhodobacter sphaeroides: dependence on protonation of Glu-L212 and Asp-L213.

The absolute values of the one-electron redox potentials of the two quinones (Q(A) and Q(B)) in bacterial photosynthetic reaction centers from Rhodobacter sphaeroides were calculated by evaluating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation at pH 7.0. The redox potential for Q(A) was calculated to be between -173 and -160 mV, which is close to the lowest measured values that are assumed to refer to nonequilibrated protonation patterns in the redox state Q(A)(-). The redox potential of quinone Q(B) is found to be about 160-220 mV larger for the light-exposed than for the dark-adapted structure. These values of the redox potentials are obtained if Asp-L213 is nearly protonated (probability 0.75-1.0) before and after electron transfer from Q(A) to Q(B), while Glu-L212 is partially protonated (probability 0.6) in the initial state Q(A)(-)Q(B)(0) and fully protonated in the final state Q(A)(0)Q(B)(-). Conversely, if the charge state of the quinones is varied from Q(A)(-)Q(B)(0) to Q(A)(0)Q(B)(-) corresponding to the electron transfer from Q(A) to Q(B), Asp-L213 remains protonated, while Glu-L212 changes its protonation state from 0.15 H(+) to fully protonated. In agreement with results from FTIR spectra, there is proton uptake at Glu-L212 going along with the electron transfer, whereas Asp-L213 does not change its protonation state. However, in our simulations Asp-L213 is considered to be protonated rather than ionized as deduced from FTIR spectra. The calculated redox potential of Q(A) shows little dependence on the charge state of Asp-L213, which is due to a strong coupling with the protonation state of Asp-M17 but increases by 50 mV if Glu-L212 changes from the ionized to the protonated charge state. Both are in agreement with fluorescence measurements observing the decay of SP(+)Q(A)(-) in a wide pH regime. The computed difference in redox potential of Q(B) in the light-exposed and dark-adapted structure was traced back to the hydrogen bond of Q(B) with His-L190 that is lost in the dark-adapted structure and the charge of the non-heme iron atom, which is closer to Q(B) in the light-exposed than in the dark-adapted structure.