Switching sides—Reengineered primary charge separation in the bacterial photosynthetic reaction center

We report 90% yield of electron transfer (ET) from the singlet excited state P* of the primary electron-donor P (a bacteriochlorophyll dimer) to the B-side bacteriopheophytin (H_B) in the bacterial photosynthetic reaction center (RC). Starting from a platform Rhodobacter sphaeroides RC bearing several amino acid changes, an Arg in place of the native Leu at L185—positioned over one face of H_B and only ∼4 Å from the 4 central nitrogens of the H_B macrocycle—is the key additional mutation providing 90% yield of P⁺H_B ⁻. This all but matches the near-unity yield of A-side P⁺H_A ⁻ charge separation in the native RC. The 90% yield of ET to H_B derives from (minimally) 3 P* populations with distinct means of P* decay. In an ∼40% population, P* decays in ∼4 ps via a 2-step process involving a short-lived P⁺B_B ⁻ intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P⁺H_B ⁻ conversion takes place in ∼20 ps by a superexchange mechanism mediated by B_B. An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of A-versus B-side initial charge separation in native RCs and have implications for the mechanism(s) and timescale of initial ET that are required to achieve a near-quantitative yield of unidirectional charge separation.