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) in the bacterial photosynthetic reaction center (RC). Starting from a platform RC bearing several amino acid changes, an Arg in place of the native Leu at L185-positioned over one face of H and only ∼4 Å from the 4 central nitrogens of the H macrocycle-is the key additional mutation providing 90% yield of PH This all but matches the near-unity yield of A-side PH charge separation in the native RC. The 90% yield of ET to H 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 PB intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → PH conversion takes place in ∼20 ps by a superexchange mechanism mediated by 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.