Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstraße 6, 64289 Darmstadt, Germany.
J Chem Phys. 2011 Oct 28;135(16):164503. doi: 10.1063/1.3653943.
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.
静态磁场梯度和脉冲磁场梯度 NMR 被用于研究低水合水平（约 0.3g/g）下肌红蛋白和溶菌酶基质中水中扩散的温度依赖性。我们表明，为了在较宽的温度范围内确定可靠的自扩散系数 D，必须考虑水和蛋白质质子之间的磁化交换，通常称为交叉弛豫。具体来说，在冷却过程中，观察到的受激发射回波衰减，反映了环境温度附近的水扩散，越来越受到交叉弛豫的控制。我们证明，对比不均匀和均匀磁场的实验结果，可以成功地将扩散和弛豫贡献分离到受激发射回波衰减中。利用这种可能性，我们发现，在 230-300 K 的温度范围内，扩散系数 D 随温度的变化呈现 Vogel-Fulcher-Tammann 行为，研究中蛋白质基质中的水扩散速度明显慢于本体。通过比较目前和以前的数据，我们讨论了蛋白质基质中平移和旋转运动之间以及短程和长程水动力学之间的关系。此外，我们还批判性地检查了以前应用 NMR 扩散测量法研究蛋白质基质中温度依赖性水扩散的结果的意义。
