Madhavi W A Monika, Weerasinghe Samantha, Momot Konstantin I
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT) , GPO Box 2434, Brisbane, Qld 4001, Australia.
J Phys Chem B. 2017 Dec 7;121(48):10893-10905. doi: 10.1021/acs.jpcb.7b07551. Epub 2017 Nov 29.
The rotational motion of water molecules plays the dominant role in determining NMR spin-relaxation properties of liquid water and many biological tissues. The traditional theory of NMR spin relaxation predominantly uses the assumption that the reorientational dynamics of water molecules is described by a continuous-time rotational-diffusion random walk with a single rotational-diffusion coefficient. However, recent experimental and theoretical studies have demonstrated that water reorientation occurs by large, discrete angular jumps superimposed on a continuous-time rotational-diffusion process. We have investigated the rotational-diffusion propagator of the proton-proton (H-H) vector of water molecules in liquid water at 298 K using molecular dynamics (MD) simulations. Analysis of the MD-simulated reorientational trajectories reveals that reorientation of the intramolecular H-H vector occurs through a combination of the two mechanisms: rotational diffusion proper and discrete large-angle jumps. We demonstrate that, empirically, the rotational-diffusion propagator of the intramolecular H-H vector in liquid water can be described in terms of multiple rotational-diffusion coefficients. A model with two rotational-diffusion coefficients was found to provide a reasonable (albeit imperfect) fit of the MD-simulated propagator on the time scales relevant to NMR spin relaxation near room or physiological temperature (picoseconds to nanoseconds). We report the apparent values of the two rotational-diffusion coefficients determined from the propagator analysis at 298 K and discuss their physical meaning.
水分子的旋转运动在决定液态水和许多生物组织的核磁共振自旋弛豫特性方面起着主导作用。传统的核磁共振自旋弛豫理论主要采用这样的假设，即水分子的重新取向动力学由具有单一旋转扩散系数的连续时间旋转扩散随机游走描述。然而，最近的实验和理论研究表明，水的重新取向是通过叠加在连续时间旋转扩散过程上的大的、离散的角跳跃发生的。我们使用分子动力学（MD）模拟研究了298K下液态水中水分子质子 - 质子（H - H）矢量的旋转扩散传播子。对MD模拟的重新取向轨迹的分析表明，分子内H - H矢量的重新取向通过两种机制的组合发生：适当的旋转扩散和离散的大角度跳跃。我们证明，根据经验，液态水中分子内H - H矢量的旋转扩散传播子可以用多个旋转扩散系数来描述。发现在与室温或生理温度附近的核磁共振自旋弛豫相关的时间尺度（皮秒到纳秒）上，具有两个旋转扩散系数的模型能够对MD模拟的传播子提供合理（尽管不完美）的拟合。我们报告了在298K下从传播子分析确定的两个旋转扩散系数的表观值，并讨论了它们的物理意义。
