Control of Electron Transfer between the L- and M-Sides of Photosynthetic Reaction Centers

An aspartic acid residue has been introduced near ring V of the L-side accessory bacteriochlorophyll (BChl$_L$) of the photosynthetic reaction center in a Rhodobacter capsulatus mutant in which a His also replaces Leu 212 on the M-polypeptide. The initial stage of charge separation in the G(M201)D/L(M212)H double mutant yields ∼70 percent electron transfer to the L-side cofactors, ∼15 percent rapid deactivation to the ground state, and ∼15 percent electron transfer to the so-called inactive M-side bacteriopheophytin (BPh$_M$). It is suggested here that the Asp introduced at M201 modulates the reduction potential of BChl$_L$, thereby changing the energetics of charge separation. The results demonstrate that an individual amino acid residue can, through its influence on the free energies of the charge-separated states, effectively dictate the balance between the forward electron transfer reactions on the L-side of the RC, the charge-recombination processes, and electron transfer to the M-side chromophores.