(2)H NMR study of the water dynamics in hydrated myoglobin.

We use 1D and 2D (2)H NMR to study the temperature-dependent mechanism for the rotational motion of myoglobin hydration water. The results show that isotropic and anisotropic water reorientation is observed at high and low temperatures, respectively, with a continuous crossover in the temperature range of 200-230 K. The anisotropic low-temperature motion has a large angular amplitude. It exhibits a broad distribution of geometries and pronounced dynamical heterogeneities, which are long-lived at least at T approximately 176 K. Exploiting the possibility to vary the angular resolution of (2)H NMR experiments, we find that the large solid angle accessible to low-temperature water reorientation is explored via large-angle rather than small-angle elementary steps; i.e., the rotational motion is not diffusive. Quantitative analysis of the NMR data using random-walk simulations implies that the number of sites involved in the observed water reorientation decreases from an infinite number during essentially isotropic motion above 230 K to a few, possibly two, below 165 K. Although the changes in the mechanism for water rotational motion may be accompanied by a mild change in the temperature dependence of the rotational correlation times, the (2)H NMR data provide strong evidence against the existence of a sharp fragile-to-strong transition at about 225 K. The present results are discussed in the context of previous experimental findings for hydrated proteins.