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Publication

Switching Sides - Re-Engineered Primary Charge Separation in the Bacterial Photosynthetic Reaction Center

Authors

Laible, Philip; Hanson, Deborah; Tira, Gregory; Faries, Kaitlyn; Holten, Dewey; Kirmaier, Christine

Abstract

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 similar to 4 angstrom from the 4 central nitrogens of the H-B macrocycle-is the key additional mutation providing 90% yield of P+HB-. This all but matches the near-unity yield of A-side P+HA- 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 similar to 40% population, P* decays in similar to 4 ps via a 2-step process involving a short-lived P+BB- intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an similar to 50% population, P* -> P+HB- conversion takes place in similar to 20 ps by a superexchange mechanism mediated by B-B. An similar to 10% population of P* decays in similar to 50 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.