Subpicosecond time-resolved absorption measurements at 77 K on two reaction center (RC) mutants of Rhodobacter capsulatus are reported. In the D-LL mutant the D helix of the M subunit has been substituted with the D helix from the L subunit, and in the D-LL-FYLFM mutant, three additional mutations are incorporated that facilitate electron transfer to the M side of the RC. In both cases the helix swap has been shown to yield isolated RCs that are devoid of the native bacteriopheophytin electron carrier H-L (Chuang, J. I.; Boxer, S. G.; Holten, D.; Kirmaier, C. Biochemistry 2006, 45, 3845-3851). For D-LL, depending whether the detergent Deriphat 160-C or N-lauryl-N,N-dimethylamine-N-oxide (LDAO) is used to suspend the RCs, the excited state of the primary electron donor (P*) decays to the ground state with an average lifetime at 77 K of 330 or 170 ps, respectively; however, in both cases the time constant obtained from single-exponential fits varies markedly as a function of the probe wavelength. These findings on the D-LL RC are most easily explained in terms of a heterogeneous population of RCs. Similarly, the complex results for D-LL-FYLFM in Deriphat-glycerol glass at 77 K are most simply explained using a model that involves (minimally) two distinct populations of RCs with very different photochemistry. Within this framework, in 50% of the D-LL-FYLFM RCs in Deriphat-glycerol glass at 77 K, P* deactivates to the ground state with a time constant of similar to 400 ps; similar to the deactivation of P* in the D-LL mutant at 77 K. In the other 50% of D-LL-FYLFM RCs, P* has a 35 ps lifetime and decays via electron transfer to the M branch, giving P+HM- in high yield (>= 80%). This result indicates that P* --> P+HM- is roughly a factor of 2 faster at 77 K than at 295 K. In alternative homogeneous models the rate of this M-side electron-transfer process is the same or up to 2-fold slower at low temperature. A 2-fold increase in rate with a reduction in temperature is the same behavior found for the overall L-side process P* --> P+HL- in wild-type RCs. Our results suggest that, as for electron transfer on the L side, the M-side electron-transfer reaction P* --> P+HM- is an activationless process.