High Yield of B-Side Electron Transfer at 77 K in the Photosynthetic Reaction Center Protein from Rhodobacter sphaeroides .

The primary electron transfer (ET) processes at 295 and 77 K are compared for the Rhodobacter sphaeroides reaction center (RC) pigment-protein complex from 13 mutants including a wild-type control. The engineered RCs bear mutations in the L and M polypeptides that largely inhibit ET from the excited state P* of the primary electron donor (P, a bacteriochlorophyll dimer) to the normally photoactive A-side cofactors and enhance ET to the C 2 -symmetry related, and normally photoinactive, B-side cofactors. P* decay is multiexponential at both temperatures and modeled as arising from subpopulations that differ in contributions of two-step ET ( e.g., P* → P + B B - → P + H B - ), one-step superexchange ET ( e.g., P* → P + H B - ), and P* → ground state. [H B and B B are monomeric bacteriopheophytin and bacteriochlorophyll, respectively.] The relative abundances of the subpopulations and the inherent rate constants of the P* decay routes vary with temperature. Regardless, ET to produce P + H B - is generally faster at 77 K than at 295 K by about a factor of 2. A key finding is that the yield of P + H B - , which ranges from ∼5% to ∼90% among the mutant RCs, is essentially the same at 77 K as at 295 K in each case. Overall, the results show that ET from P* to the B-side cofactors in these mutants does not require thermal activation and involves combinations of ET mechanisms analogous to those operative on the A side in the native RC.