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 13(1)-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 rho(L)/rho(M) of the unpaired electron of P.+ between P-L and P-M as determined by ENDOR/TRIPLE resonance spectroscopy. The model by Ariz 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 rho(L)/rho(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 13(1)-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 PL to Pm is between 100 and 200 meV in keeping with earlier estimates.