Water molecule dynamics in hydrated lysozyme. A deuteron magnetic resonance study.

Proton nuclear magnetic resonance relaxation investigations of water dynamics in hydrated protein powders have the serious drawback that protein-water intermolecular dipolar interactions make the unambiguous interpretation of the results difficult. To circumvent this difficulty, deuteron spin-lattice and spin-spin relaxation times in lysozyme powder hydrated with deuterium oxide were measured as a function of temperature and at two frequencies. Although the deuteron relaxation results are compatible with a water molecule dynamics model based on either a bimodal distribution of correlation times or anisotropic motion, a comparison of the present results with proton data suggests than an anisotropic motion model is more likely to provide a reasonable description of the water molecule motion. An analysis based on an anisotropic motion model that uses two correlation times to characterize the motion shows that most of the water molecules rotate about their twofold axis of symmetry at a rate that is only approximately 100 times smaller than the rate of isotropic diffusion in the bulk liquid. The reorientation of the twofold axis of symmetry itself is characterized by a correlation time of approximately 10(-7) s.