The mutation, HL(M 182), in the Rhodobacter sphaeroides reaction center (RC) results in the replacement of the monomer bacteriochlorophyll (BCh1) on the inactive side (B side) of the RC with a bacteriopheophytin (BPheo; the new cofactor is referred to as phi(B)). In phi(B)-containing RCs, the first excited state of, the primary donor (P*) decays with an accelerated time constant of 2.6 +/- 0.1 ps at room temperature as compared to 3.1 +/- 0.2 ps in wild-type (WT) RCs. At low temperatures, P* decay is essentially the same in the HL(M182) mutant and WT RCs: 1.4 +/- 0.1 ps at 77 K and 1.1 +/- 0.1 ps at 9 K. The yield of the charge-separated P(+)phi(B)-state decreases from 35% at room temperature to 12% at 77 and 9 K. The decreased P(+)phi(B) - yield is apparently due to the fact that the rate of the charge separation along the A side of the RC at low temperature increases, while the rate along the B side remains essentially unchanged. From measurements of the long-lived fluorescence decay at room temperature, the standard free energy of the P(+)phi(B) - state is estimated to be about 0.16 +/- 0.04 eV below P*. Given a difference between the midpoint potentials of BCh1 and BPheo of 0.26 +/- 0.03 V, the standard free energy of the P+BB- state in WT RC is estimated to be 0.1 +/- 0.07 eV above P*.