Manipulating the Energetics and Rates of Electron Transfer in Rhodobacter capsulatus Reaction Centers with Asymmetric Pigment Content

Seemingly redundant parallel pathways for electron transfer (ET), composed of identical sets of cofactors, are a cornerstone feature of photosynthetic reaction centers (RCs) involved in light-energy conversion. In native bacterial RCs, both A and B branches house one bacteriochlorophyll (BChl) and one bacteriopheophytin (BPh), but the A branch is used exclusively. Described herein are the results obtained for two Rhodobacter capsulatus RCs with an unnaturally high degree of cofactor asymmetry, two BPh on the RC's B side and two BChl on the A side. These pigment changes derive, respectively, from the His(M180)Leu mutation [a BPh (φ_B) replaces the B-side BChl (B_B)], and the Leu(M212)His mutation [a BChl (β_A) replaces the A-side BPh (H_A)]. Additionally, Tyr(M208)Phe was employed to disfavor ET to the A branch; in one mutant, Val(M131)Glu creates a hydrogen bond to H_B to enhance ET to H_B. In both φ_B mutants, the decay kinetics of the excited primary ET donor (P∗) resolve three populations with lifetimes of ∼9 ps (50-60%), ∼40 ps (10-20%), and ∼200 ps (20-30%), with P⁺φ_B^- formed predominantly from the 9 ps fraction. The 50-60% yield of P⁺φ_B^- is the highest yet observed for a φ_B-containing RC. The results provide insight into factors needed for efficient multistep ET.