The mutation, H(M182)L, in the Rhodobacter sphaeroides reaction center (RC) results in the replacement of the monomer bacteriochlorophyll on the inactive side (B-side) of the RC with a bacteriopheophytin (the new cofactor is referred to as phi(B)) In phi(B) containing RCs, P* stimulated emission decays with an accelerated time constant of 2.6 +/- 0.1 ps at room temperature compared to 3.1 +/- 0.2 ps in WT RCs. Analysis of the time-resolved spectra implies that two states are being formed during the initial reaction in the mutant: the usual P+HA- state, as seen in WT, and a new state, P(+)phi(B)(-) .P(+)phi(B)(-) is formed during the decay of P* and recombines to the ground state with a lifetime of 200 +/- 20 ps. The yield of the P(+)phi(B)- State is 35 +/- 5% at room temperature, while the remaining 65 +/- 5% of the initial electron-transfer results in P+HA-. There does not appear to be any further electron transfer from phi(B)(-) to Hg Apparently, in the H(M182)L mutant, the state P(+)phi(B)(-) is lower in free energy than the P+HB- state.