Studies of the photosynthetic reaction center, Photosystem I (PSI), have shown that its polypeptide core contains highly coupled chlorophyll molecules that serve as the primary electron donor and acceptor. Notably, a recent study found that the primary acceptor, A0, is a dimer of chlorophyll a molecules, Chl2 and Chl3, where the electron spin density on the reduced acceptor, A0- , is distributed on both molecules. Previous biochemical studies have shown that the replacement of the soft base sulfur axial ligand of Chl3A from a methionine residue to a hard base nitrogen ligand of a histidine in the M688HPsaA variant of PSI severely impacts forward electron transfer from the A0A cofactor. In this study, we determine the electronic structure of the A0- state of M688HPsaA PSI using a combination of experimental hyperfine sublevel correlation (HYSCORE) spectroscopy and computational analysis including molecular dynamics and density functional theory (DFT). Understanding the electronic structure of the dimeric A0 acceptor in the wild type and M688HPsaA variant of PSI has widespread implications ranging from the evolution of naturally occurring reaction centers to the development of a new generation of highly efficient artificial photosynthetic systems.

Spectroscopic and computational analysis of the dimeric chlorophyll acceptor in the M688HPsaA genetic variant of Photosystem I

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