Extreme-Values Statistics and Dynamics of Water at Protein Interfaces

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• 作者：Jean-Pierre Korb ; Yanina Goddard ; Jason Pajski ; Galina Diakova ; Robert G. Bryant
• 刊名：Journal of Physical Chemistry B
• 出版年：2011
• 出版时间：November 10, 2011
• 年：2011
• 卷：115
• 期：44
• 页码：12845-12858
• 全文大小：1084K
• 年卷期：v.115,no.44(November 10, 2011)
• ISSN：1520-5207

文摘

Immobilized proteins present a unique interface with water. The water translational diffusive motions affect the high-frequency dynamics and the nuclear spin鈥搇attice 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. ²H₂O spin鈥搇attice 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 ²H₂O spin鈥搇attice 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鈥搇attice 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 D₂O deuterium spin鈥搇attice 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鈥搇attice 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.