Physique de la Matière Condensée, Ecole Polytechnique, CNRS, 91128 Palaiseau, France.
J Phys Chem B. 2011 Nov 10;115(44):12845-58. doi: 10.1021/jp2053426. Epub 2011 Oct 18.
Immobilized proteins present a unique interface with water. The water translational diffusive motions affect the high-frequency dynamics and the nuclear spin-lattice relaxation as with all surfaces; however, rare binding sites for water in protein systems add very low-frequency components to the dynamics spectrum. Water binding sites in protein systems are not identical, thus distributions of free energies and consequent dynamics are expected. (2)H(2)O spin-lattice relaxation rate measurements as a function of magnetic field strength characterize the local rotational fluctuations for protein-bound water molecules. The measurements are sensitive to dynamics down to the kilohertz range. To account for the data, we show that the extreme-values statistics of rare events, i.e., water dynamics in rare binding sites, implies an exponential distribution of activation energies for the strongest binding events. In turn, for an activated dynamical process, the exponential energy distribution leads to a Pareto distribution for the reorientational correlation times and a power law in the Larmor frequency for the (2)H(2)O spin-lattice relaxation rate constants at low field strengths. The most strongly held water molecules escape from rare binding sites in times on the order of microseconds, which interrupts the intramolecular correlations and causes a plateau in the spin-lattice relaxation rate at very low magnetic field strengths. We examine the magnetic relaxation dispersion (MRD) data using two simple but related models: a protein-bound environment for water characterized by a single potential well and a protein-bound environment characterized by a double potential well where the potential functions for the local motions of the bound-state water are of different depth. This analysis is applied to D(2)O deuterium spin-lattice relaxation on cross-linked albumin and lysozyme, which is dominated by the intramolecular relaxation driven by the dynamical modulation of the nuclear electric quadrupole coupling. We also separate the intramolecular from the intermolecular contribution to water proton spin-lattice relaxation by isotope dilution and show that the intramolecular proton data map onto the deuterium relaxation by a scale factor implied by the relative strength of the quadrupole and dipolar couplings. The temperature and pH dependence of the magnetic relaxation dispersion are complex and accounted for by changing only the weighting factors in a superposition of contributions from single-well and double-well contributions. These experiments show that the reorientational dynamics spectrum for water, in and on a protein, is characterized by a strongly asymmetric distribution with a long-time tail that extends at least to microseconds.
固定化蛋白质呈现出与水独特的界面。与所有表面一样，水的平移扩散运动影响高频动力学和核自旋晶格弛豫；然而，蛋白质系统中极少量的水结合位点为动力学谱添加了极低频成分。蛋白质系统中的水结合位点并不相同，因此预计会有自由能分布和相应的动力学。（2）H（2）O 自旋晶格弛豫率测量作为磁场强度的函数，可表征蛋白质结合水分子的局部旋转波动。这些测量对低至千赫兹范围的动力学很敏感。为了说明数据，我们表明，稀有事件的极值统计，即稀有结合位点中水的动力学，意味着最强结合事件的激活能呈指数分布。反过来，对于一个激活的动力学过程，指数能量分布导致重新取向相关时间的帕累托分布和低场强度下（2）H（2）O 自旋晶格弛豫率常数的拉莫尔频率的幂律。最强结合的水分子以微秒量级的时间从稀有结合位点中逃脱，这中断了分子内相关性，并导致自旋晶格弛豫率在非常低的磁场强度下出现平台。我们使用两个简单但相关的模型来检查磁共振弛豫色散（MRD）数据：一个由单个势阱表征的蛋白质结合水环境和一个由局部运动的双势阱组成的蛋白质结合水环境，其中结合态水的势函数深度不同。这种分析应用于交联白蛋白和溶菌酶的 D（2）O 氘核自旋晶格弛豫，它由核电四极耦合动态调制驱动的分子内弛豫主导。我们还通过同位素稀释将水分子的分子内与分子间贡献分离，并表明分子内质子数据通过由四极和偶极耦合相对强度暗示的比例因子映射到氘松弛上。磁共振弛豫色散的温度和 pH 依赖性很复杂，只需通过在单阱和双阱贡献的叠加中改变权重因子即可解释。这些实验表明，蛋白质中的水的重新取向动力学谱由具有长时尾巴的强烈不对称分布特征，至少延伸到微秒。
