Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native piginent-protem complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Len for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (p). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H-M) in Lhd-beta RCs to form D+HM- with a yield of similar to 6%. This state also forms in Mbd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D* decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit of V-L(2)/V-M(2) <= 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (k(L)/k(M)) approximate to 30 for wild-type RCs, our findings indicate that electronic factors contribute similar to 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC.