Shift of the special pair redox potential: Electrostatic energy computations of mutants of the reaction center from Rhodobacter sphaeroides

Shifts of the special pair redox potential of the photosynthetic reaction center of Rhodobacter sphaeroides are considered for several point mutations [Lin, X., Murchison, H. A., Nagarijan, V., Parson, W. W., Alien, J. P., & Williams, J. C. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 10265-10269] in the neighborhood of the special pair. The shifts are calculated from electrostatic energies by solving Poisson's equation for energy-minimized structures of the reaction center. Different conditions for the evaluation of the electrostatic energy are probed. To test the influence of the hydrogen bonding at the acetyl groups of the special pair, the orientation and torsion potential of the acetyl groups are varied. The calculated shifts of the midpoint potential of double and triple mutants can approximately be obtained from the corresponding shifts of the single point mutations. The calculated shifts agree with the measured values for all single and double mutants considered. However, a clear decision between different acetyl group conformations was only possible for the mutants HF(L168) and HF(L168) + LH(L131) where the calculated shifts of the redox potential agree with experiments only if the acetyl oxygen atom at D-M points toward the Mg2+ ion of D-L. This is corroborated by computations of the interaction energy of the acetyl group at D-M, which adopts a lower value in the wild-type reaction center if its oxygen atom is bonded to the Mg2+ ion of D-L.