The photosynthetic reaction center (RC) is an integral membrane protein that carries out the initial charge-separation reactions of photosynthesis. Upon light excitation, a pair (P) of bacteriochlorophylls (Bchls) donates an electron to a bacteriopheophytin (H-L), generating an ion-pair state (P+HL-). Previous ENDOR studies of RCs from the purple bacterium Rhodobacter sphaeroides have shown that the unpaired electron of P+ is distributed unequally between the two Bchls of P, with about 2/3 of the unpaired spin and positive charge residing on the Bchl bound to subunit L (P-L) and 1/3 on the Bchl bound to M (P-M). To investigate the protein's role in establishing the energies of the cations P-L(+) and P-M(+) through long-range electrostatic interactions, we mutated Arg L135 and Arg M164 individually to Leu or Glu and measured the effects on the Special TRIPLE and FTIR spectra of P+. These highly conserved residues occupy homologous positions on either side of P but have no hydrogen bonds or steric interactions with the pigments. Previous work has shown that replacing Arg by Leu at either site lowers the midpoint potential (E-m) of P/P+ by about 15 mV; replacing it by Glu lowers the E-m by about 35 mV. We found that the mutations also alter the spin distribution in P+. The mutation R(L135)E stabilizes P-L(+) relative to P-M(+), increasing the ratio of the spins to 3.1, whereas R(M164)E decreases the spin ratio to 1.3. Replacing R(L135) or R(M164) by Leu gave spin ratios of 2.9 or 1.6, respectively. FTIR measurements showed that the mutations also affect the vibrational spectra of P and P+ and the electronic transition between the eigenstates of P+. The R(L135)E mutation increases the splitting between the keto C=O stretching bands of P-L(+) and P-M(+), whereas the R(M164)E mutation decreases this splitting. The changes in the keto frequency are correlated with calculated changes in the projection of the local electric field along the C=O bond, and give a Stark tuning rate in the range 0.81-1.45 cm(-1)/(MV/cm). Cooling the RCs to 100 K during illumination increases the splitting between the keto bands of P-L(+) and P-M(+) in wild-type and R(L135)E RCs and shifts the electronic transition to higher energies, suggesting that relaxations of the protein further stabilize P-L(+) relative to P-M(+). Fitting the electrochemical, ENDOR, and FTIR data to a self-consistent molecular orbital model required including both vibronic and electronic coupling of P-L(+) and P-M(+) and yielded an electronic coupling constant of approximately -155 meV and a reorganization energy of approximately 220 meV.