Effect of Hydrogen Bond Strength on the Redox Properties of Phylloquinones: A Two-Dimensional Hyperfine Sublevel Correlation Spectroscopy Study of Photosystem I

The phylloquinones of photosystem I (PS I), A(1A) 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 phyllo-quinones 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 ofthe phylloquinone acceptor, A(1A), by H-1 and N-14 HYSCORE spectroscopy. Photoaccumulation of PS I complexes at pH 8.0 results in the trapping of the phyllosemiquinone anion, A(1A)(-), on the A-branch of cofactors. The experiments described here indicate that A(1A)(-) forms a single H-bond. Using a simple point dipole approximation, we estimate its length to be 1.6 +/- 0.1 angstrom. The,value of the H-1 isotropic hyperfine coupling constant suggests that the H-bond has significant out-of-plane character. The N-14 HYSCORE spectroscopy experiments support the assignment of a H-bond wherein, the N-14 quadrupolar coupling constant is consistent with a backbone amide nitrogen as the hydrogen bond donor.