We have investigated the primary charge separation processes in Rb. capsulatus reaction centers (RCs) bearing the mutations Phe(L181) → Tyr, Tyr(M208) → 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 trifurcated to give (1) internal conversion to the ground state (∼10% yield), (2) charge separation to the L side of the RC (∼60% yield), and (3) electron transfer to the M-side bacteriopheophytin BPhM (∼30% yield). These results relate previous work in which the ionizable residues Lys (at L178) and Asp (at M201) have been used to facilitate charge separation to the M side of the RC, and the widely studied L181 and M208 mutants. One conclusion that comes from this work is that the Tyr (M208) → Pre and Gly(M201) → Asp mutations near the L-side bacteriochlorophyll (BChlL) raise the free energy of P+BChlL- by comparable amounts. The results also suggest that the free energy of P+BChlM- is lowered more substantially by a Tyr at L181 than a Lys at L178. The results on the YFH mutant further demonstrate that the free energy differences between the L- and M-side charge-separated states play a significant role in the directionality of charge separation in the wild-type RC, and place limits on the contributing role of differential electronic matrix elements on the two sides of the RC.
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