Chemistry Department, University of Virginia, Charlottesville, Virginia, USA.
Biophys J. 2010 Jan 6;98(1):138-46. doi: 10.1016/j.bpj.2009.09.054.
Rotational immobilization of proteins permits characterization of the internal peptide and water molecule dynamics by magnetic relaxation dispersion spectroscopy. Using different experimental approaches, we have extended measurements of the magnetic field dependence of the proton-spin-lattice-relaxation rate by one decade from 0.01 to 300 MHz for (1)H and showed that the underlying dynamics driving the protein (1)H spin-lattice relaxation is preserved over 4.5 decades in frequency. This extension is critical to understanding the role of (1)H(2)O in the total proton-spin-relaxation process. The fact that the protein-proton-relaxation-dispersion profile is a power law in frequency with constant coefficient and exponent over nearly 5 decades indicates that the characteristics of the native protein structural fluctuations that cause proton nuclear spin-lattice relaxation are remarkably constant over this wide frequency and length-scale interval. Comparison of protein-proton-spin-lattice-relaxation rate constants in protein gels equilibrated with (2)H(2)O rather than (1)H(2)O shows that water protons make an important contribution to the total spin-lattice relaxation in the middle of this frequency range for hydrated proteins because of water molecule dynamics in the time range of tens of ns. This water contribution is with the motion of relatively rare, long-lived, and perhaps buried water molecules constrained by the confinement. The presence of water molecule reorientational dynamics in the tens of ns range that are sufficient to affect the spin-lattice relaxation driven by (1)H dipole-dipole fluctuations should make the local dielectric properties in the protein frequency dependent in a regime relevant to catalytically important kinetic barriers to conformational rearrangements.
蛋白质的旋转固定化允许通过磁共振弛豫谱来对内部肽和水分子的动力学进行特征描述。我们使用不同的实验方法，将质子自旋晶格弛豫率的磁场依赖性测量扩展了一个数量级，从 0.01 到 300 MHz，涵盖了 (1)H，结果表明，驱动蛋白质 (1)H 自旋晶格弛豫的基本动力学在 4.5 个数量级的频率范围内是保持不变的。这一扩展对于理解 (1)H(2)O 在总质子自旋弛豫过程中的作用至关重要。事实上，蛋白质质子弛豫弥散谱在近 5 个数量级的频率范围内呈幂律关系，常数系数和指数保持不变，这表明导致质子核自旋晶格弛豫的天然蛋白质结构波动的特征在如此宽的频率和长度尺度间隔内是非常恒定的。与用 (2)H(2)O 而非 (1)H(2)O 平衡的蛋白质凝胶中的蛋白质质子自旋晶格弛豫率常数进行比较表明，在水合蛋白质的这个中间频率范围内，由于水合蛋白质中几十纳秒时间范围内水分子的动力学，水质子对总自旋晶格弛豫有重要贡献。这种水的贡献与相对罕见、长寿命且可能被束缚的水分子的运动有关，这些水分子受到限制。在几十纳秒的时间范围内，水分子的重新取向动力学足以影响由 (1)H 偶极子偶极子波动驱动的自旋晶格弛豫，这应该使蛋白质中局部介电性质在与催化相关的构象重排动力学障碍相关的频率范围内具有依赖性。
