High Yield of Long-Lived B-Side Charge Separation at Room Temperature in Mutant Bacterial Reaction Centers

The photosynthetic reaction center of Rhodobacter (Rb.) sphaeroides is a quasi-symmetric pigment protein complex. This near symmetry results in two potential pathways for light-induced electron transfer, labeled A and B, yet under most conditions charge separation occurs almost exclusively along the A-side cofactors in wild-type reaction centers. One exception is that upon excitation with blue light (390 nm) a transient B-side charge-separated state, B_B⁺H_B^- (the cation of the B-side monomer bacteriochlorophyll and the anion of the B-side bacteriopheophytin), is formed that decays in picoseconds at room temperature but is stable at cryogenic temperatures. To characterize the nature of this reaction further, a series of mutations involving the introduction of potentially negative amino acids in the vicinity of P and the monomer bacteriochlorophylls, B_A and B_B, were used to alter the local electrostatic environment. The most dramatic effects were observed for reaction centers with the mutations L168 His to Glu and L170 Asn to Asp. These mutant reaction centers display a stable (hundreds of picoseconds) formation of B_B⁺H _B^- not only using 390-nm excitation but also upon direct excitation of the lowest excited singlet states of the reaction center bacteriopheophytins and bacteriochlorophylls at 740 and 800 nm, respectively. Reaction centers mutated at three sites - L168 His to Glu, L170 Asn to Asp and M199 Asn to Asp - were also found to form a stable B_B ⁺H_B^- state following excitation at 390 nm but with an apparently lower yield. In the other mutant reaction centers studied, long-lived B_B⁺H_B^- formation was not observed using any excitation wavelength, though 390-nm excitation resulted in B_B⁺H_B^- formation followed by the decay of this state on the picosecond time scale, as is observed in wild type. Combining these results with past work demonstrates that there is a rich variety of photochemistry possible in the reaction center. Both the pathway and products of light-induced charge separation depend on the interplay between local electrostatic interactions and the nature of the excited states available at early times.