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

This is the authors' abstract. We don't add key points for this paper.

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.