Orientational order and dynamics of hydration water in a single crystal of bovine pancreatic trypsin inhibitor.

The orientational order and dynamics of the water molecules in form II crystals of bovine pancreatic trypsin inhibitor (BPTI) are studied by (2)H NMR in the temperature range 6-50 degrees C. From the orientation dependence of the single crystal quadrupole splitting and linewidth, the principal components of the motionally averaged quadrupole interaction tensor and the irreducible linewidth components for the orthorhombic crystal are determined. With the aid of water orientations derived from neutron and x-ray diffraction, it is shown that the NMR data can be accounted for by a small number of highly ordered crystal waters, some of which have residence times in the microsecond range. Most of these specific hydration sites must be located at intermolecular contacts. The surface hydration layer that is also present in dilute solution is likely to be only weakly ordered and would then not contribute significantly to the splitting and linewidth from the protein crystal. To probe water dynamics on shorter time scales, the (2)H longitudinal relaxation dispersion is measured for a polycrystalline BPTI sample. The observed dispersion is dominated by rapidly exchanging deuterons in protein side chains, undergoing restricted rotational motions on a time scale of 10 ns.