Hydroxyl hydrogen conformations in trypsin determined by the neutron diffraction solvent difference map method: relative importance of steric and electrostatic factors in defining hydrogen-bonding geometries.

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Abstract
Neutron diffraction maps have been used to assign the rotor conformations of the hydroxyl hydrogens in trypsin. Knowledge of these conformations is used to assess the relative importance of steric and electrostatic effects in conferring the H-binding geometries of these groups. A general finding was the most hydroxyl groups are rotationally ordered with their highest populated conformation near the low-energy staggered orientation. For the low-energy conformers (-60.degree., 60.degree., 180.degree.) of serine and threonine, the trans (-180.degree.) position is most highly populated followed by +60.degree.. In trypsin, only 1 of 24 serines was found in the -60.degree. conformer. Serine hydroxyls preferentially act as H-bond acceptors and rarely are observed as H-bond donors alone. Threonines were found to be more likely than serines to participate in two H bonds; tryosines were found to prefer to act as donors. In H-binding situations in which there was incompatibility between the energies defining the barrier to rotation and the local electrostatics, the electrostatic criteria dominated. Overall, the findings support a model of H bonding where there exists strong inherent complementarity between the low-energy hydroxyl orientations and the local electrostatic environment.