Pigment-protein interactions in bacterial reaction centers and their influence on oxidation potential and spin density distribution of the primary donor

To study the specific influence of the protein environment in bacterial photosynthetic reaction centers on the oxidation potential and the spin density distribution of the primary electron donor (P), a dimer of the two bacteriochlorophyll (BChl) a molecules P_L and P_M, site-directed mutants at positions L131 and M160 near the 13¹-keto groups of the BChls were constructed, in which the native Leu residue was exchanged to either Ser, Asn, Asp, Gln, Glu, or His. In addition, each mutation at one position was combined with the change of Leu to His at the respective other position. For each series of mutants the P/P^.+ oxidation potential V^ox was determined by electrochemical methods and related to the spin density distribution ρ_L/ρ_M of the unpaired electron of P^.+ between P_L and P_M as determined by ENDOR/TRIPLE resonance spectroscopy. The model by Artz et al. (Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 13582-13587) was revised, extended, and applied to the four series of mutants. Despite its simple nature, the model is able to reproduce the observed related changes of V^ox and ρ_L/ρ_M as a result of hydrogen bonding to the 13¹-keto group of one dimer half and allows for reasonable estimates of orbital energy shifts due to pigment-protein interactions when long-range effects and electron-phonon coupling are considered. On the basis of this model, a hydrogen bond from His to the 13¹-keto group stabilizes the HOMO of BChl a by approximately 100 meV, which is in reasonable agreement with other experimental data. The electronic coupling between the dimer halves is on the order of 120 to 160 meV, and the reorganization energy associated with a complete charge transfer from P_L to P_M is between 100 and 200 meV in keeping with earlier estimates.