Static and pulsed field gradient nuclear magnetic resonance studies of water diffusion in protein matrices.

Static field gradient and pulsed field gradient NMR are used to study the temperature dependence of water diffusion in myoglobin and lysozyme matrices for low hydration levels of about 0.3 g/g. We show that in order to determine reliable self-diffusion coefficients D in a broad temperature range, it is very important to consider an exchange of magnetization between water and protein protons, often denoted as cross relaxation. Specifically, upon cooling, the observed stimulated-echo decays, which reflect water diffusion near ambient temperature, become more and more governed by cross relaxation. We demonstrate that comparison of experimental results for inhomogeneous and homogeneous magnetic fields enables successful separation of diffusion and relaxation contributions to the stimulated-echo decays. Making use of this possibility, we find that in the temperature range 230-300 K, the temperature-dependent diffusivities D exhibit a Vogel-Fulcher-Tammann behavior, where water diffusion in the studied protein matrices is substantially slower than in the bulk. By comparing present and previous data, we discuss relations between translational and rotational motions and between short-range and long-range water dynamics in protein matrices. In addition, we critically examine the significance of results from previous applications of NMR diffusometry to the temperature-dependent water diffusion in protein matrices.