Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

Subpicosecond time-resolved absorption measurements at 77 K on two reaction center (RC) mutants of Rhodobacter capsulatus are reported. In the DLL mutant the D helix of the M subunit has been substituted with the D helix from the L subunit, and in the DLL-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 HL (Chuang, J. I.; Boxer, S. G.; Holten, D.; Kirmaier, C. Biochemistry 2006, 45, 3845-3851). For DLL, 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 DLL RC are most easily explained in terms of a heterogeneous population of RCs. Similarly, the complex results for DLL-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 DLL-FYLFM RCs in Deriphat-glycerol glass at 77 K, P * deactivates to the ground state with a time constant of 400 ps, similar to the deactivation of P * in the DLL mutant at 77 K. In the other 50 % of DLL-FYLFM RCs, P * has