Manipulating the direction of electron transfer in the bacterial reaction center by swapping Phe for Tyr near BChl(M) (L181) and Tyr for Phe near BChl(L) (M208)

We have investigated the primary charge separation processes in Rb. capsula tus reaction centers (RCs) bearing the mutations Phe(L181) --> Tyr, Tyr(M20 8) --> Phe, and Leu(M212) --> His. In the YFH mutant, decay of the excited primary electron donor P* occurs with an 11 +/- 2 ps time constant and is t rifurcated to give (1) internal conversion to the ground state (similar to 10% yield), (2) charge separation to the L side of the RC (similar to 60% y ield). and (3) electron transfer to the M-side bacteriopheophytin BPhM (sim ilar to 30% yield). These results relate previous work in which the ionizab le residues Lys (at L178) and Asp (at M201) have been used to facilitate ch arge separation to the M side of the RC, and the widely studied L181 and M2 08 mutants. One conclusion that comes from this work is that the Tyr (M208) --> Phe and Gly(M201) --> Asp mutations near the L-side bacteriochlorophyl l (BChl(L)) raise the free energy of P(+)BChlL(-) by comparable amounts. Th e results also suggest that the free energy of P(+)BChl(M)(-) is lowered mo re substantially by a Tyr at L181 than a Lys at L178. The results on the YF H mutant further demonstrate that the free energy differences between the L - and M-side charge-separated states play a significant role in the directi onality of charge separation in the wild-type RC. and place limits on the c ontributing role of differential electronic matrix elements on the two side s of the RC.