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 inact ive 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 dec ays 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 spectr a implies that two states are being formed during the initial reaction in t he 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 t he 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(M18 2)L mutant, the state P(+)phi(B)(-) is lower in free energy than the P+HB- state.