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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2020-01, Vol.117 (2), p.865-871
• Main Authors: Laible, Philip D., Hanson, Deborah K., Buhrmaster, James C., Tira, Gregory A., Faries, Kaitlyn M., Holten, Dewey, Kirmaier, Christine
• Format: Article
• Language: English
• Subjects: Amino acid sequence; Amino Acid Substitution; Amino acids; Bacteriochlorophyll; bacteriochlorophyll dimer; Bacteriochlorophylls - metabolism; BASIC BIOLOGICAL SCIENCES; Chlorophyll; Decay; Dimers; Electron transfer; Electron Transport; Internal conversion; Molecular Dynamics Simulation; mutant reaction center; Mutation; Pheophytins - chemistry; Pheophytins - metabolism; Photosynthesis; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - metabolism; Physical Sciences; protein distributions; protein dynamics; Protein Engineering; Reengineering; Rhodobacter sphaeroides - metabolism; Separation; SOLAR ENERGY; ultrafast spectroscopy; ultrafast transient absorption spectroscopy
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1916119117

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 (HB) 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 HB and only ∼4 Å from the 4 central nitrogens of the HB 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 HB 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⁺BB⁻ intermediate, analogous to initial charge separation on the A side of wild-type RCs. In an ∼50% population, P* → P⁺HB⁻ conversion takes place in ∼20 ps by a superexchange mechanism mediated by BB. An ∼10% population of P* decays in ∼150 ps largely by internal conversion. These results address the long-standing dichotomy of Aversus 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.