Oxygen-17 and deuterium nuclear magnetic resonance studies of lysozyme hydration

Oxygen-17 and deuterium NMR studies of lysozyme hydration are reported for a wide range of lysozyme concentrations, and the relationship between water "activity" and water mobility in the lysozyme-water system as determined by high-field NMR is examined. In a first approximation, the effect of lysozyme activity on hydration is considered to be small because of the relatively low charge on lysozyme at pH 7 and the absence of salt in the lysozyme solutions. Correlation times are determined for tightly bound water, weakly bound water, and "multilayer" or trapped water in lysozyme at 20 degrees C. Hydration numbers are also determined for these three different water populations interacting with lysozyme. Good agreement is found between the hydration numbers determined by 17O NMR and the calculations based on the D'Arcy and Watt analysis of water sorption isotherms for proteins that considered three major water populations in hydrated lysozyme. A molecular interpretation for the three components in the D'Arcy and Watt theory of sorption isotherms is also proposed on the basis of our NMR results. Previous proton NMR spin-echo results are shown to be consistent with our findings by 17O NMR and support the view that there are at least four regions of distinct hydration behavior of lysozyme which span the whole range from solutions to solid powders.