Biological processes often involve the surfaces of proteins, where the structural and dynamic properties of the aqueous solvent are modified. Information about the dynamics of protein hydration can be obtained by measuring the magnetic relaxation dispersion (MRD) of the water 2H and 17O nuclei or by recording the nuclear Overhauser effect (NOE) between water and protein protons. Here, we use the MRD method to study the hydration of the cyclic peptide oxytocin and the globular protein BPTI in deeply supercooled solutions. The results provide a detailed characterization of water dynamics in the hydration layer at the surface of these biomolecules. More than 95% of the water molecules in contact with the biomolecular surface are found to be no more than two-fold motionally retarded as compared to bulk water. In contrast to small nonpolar molecules, the retardation factor for BPTI showed little or no temperature dependence, suggesting that the exposed nonpolar residues do not induce clathrate-like hydrophobic hydration structures. New NOE data for oxytocin and published NOE data for BPTI were analyzed, and a mutually consistent interpretation of MRD and NOE results was achieved with the aid of a new theory of intermolecular dipolar relaxation that accounts explicitly for the dynamic perturbation at the biomolecular surface. The analysis indicates that water?protein NOEs are dominated by long-range dipolar couplings to bulk water, unless the monitored protein proton is near a partly or fully buried hydration site where the water molecule has a long residence time.
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