B-side electron transfer in a Rhodobacter sphaeroides reaction center mutant in which the B-side monomer bacteriochlorophyll is replaced with bacteriopheophytin

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 φ_B). In φ_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⁺H_A^- state, as seen in WT, and a new state, P⁺φ_B^-. P⁺φ_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⁺φ_B^- state is 35 ± 5% at room temperature, while the remaining 65 ± 5% of the initial electron-transfer results in P⁺H_A^-. There does not appear to be any further electron transfer from φ_B^- to H_B. Apparently, in the H(M182)L mutant, the state P⁺φ_B^- is lower in free energy than the P⁺H_B^- state.