The phylloquinones of photosystem I (PS I), A1A and A 1B, exist in near-equivalent protein environments but possess distinct thermodynamic and kinetic properties. Although the determinants responsible for the different properties of the phylloquinones are not completely understood, the strength and geometry of hydrogen bond interactions are significant factors in tuning and control of function. This study focuses on characterizing the hydrogen-bonding interactions of the phylloquinone acceptor, A1A, by 1H and 14N HYSCORE spectroscopy. Photoaccumulation of PS I complexes at pH 8.0 results in the trapping of the phyllosemiquinone anion, A1A-, on the A-branch of cofactors. The experiments described here indicate that A1A - forms a single H-bond. Using a simple point dipole approximation, we estimate its length to be 1.6 ± 0.1 Å. The value of the 1H isotropic hyperfine coupling constant suggests that the H-bond has significant out-of-plane character. The 14N HYSCORE spectroscopy experiments support the assignment of a H-bond wherein, the 14N quadrupolar coupling constant is consistent with a backbone amide nitrogen as the hydrogen bond donor.
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