Solid-state NMR and calorimetry of structural waters in helical peptides

The peptide hydrates Gly-Gly-Val x 2H(2)O (GGV) and Gly-Ala-Leu x 3H(2)O (GAL) are known to adopt alpha-helical configurations containing waters of hydration in which each water is H-bonded to three or four peptide groups. Herein we report a thermodynamic and solid-state NMR ((2)H and (17)O) study of these peptides. From TGA and DSC, the average enthalpy per H-bond is 15 kJ/mol. The dynamics and average orientation of the hydrate are studied by powder and single-crystal (2)H NMR. Whereas waters that are shown by the X-ray structure to be coordinated by four hydrogen bonds do not yield observable (2)H NMR signals at room temperature, two of the three triply coordinated waters yield residual (2)H quadrupole coupling tensors characteristic of rapid 180 degrees flip motions and the orientation of the residual tensor is that expected from the X-ray structure-derived H-bonding pattern. At -65 degrees C, the flip motions of triply coordinated water in GGV slow into the (2)H NMR intermediate exchange regime whereas the tetrahedrally coordinated water approaches the slow-exchange limit and yields an observable NMR signal. Extensive isotope exchange between water vapor and crystalline GGV establishes the presence of additional hydrate dynamics and solid-state proton transfer along a chain of water-bridged protonated alpha-amino groups.