Rotational-Diffusion Propagator of the Intramolecular Proton-Proton Vector in Liquid Water: A Molecular Dynamics...

Area of Science:

• Physical Chemistry
• Biophysics
• Computational Chemistry

Background:

• Nuclear Magnetic Resonance (NMR) spin relaxation in liquid water and biological tissues is primarily governed by water molecule rotational motion.
• Traditional NMR relaxation theories often assume a single rotational-diffusion coefficient for water molecule reorientation.
• Emerging evidence suggests water reorientation involves a combination of continuous diffusion and discrete large-angle jumps.

Purpose of the Study:

• To investigate the rotational-diffusion propagator of the proton-proton (H-H) vector in liquid water using molecular dynamics (MD) simulations.
• To determine if water reorientation can be accurately described by multiple rotational-diffusion coefficients.

Main Methods:

• Utilized molecular dynamics (MD) simulations to generate reorientational trajectories of water molecules in liquid water at 298 K.
• Analyzed the rotational-diffusion propagator of the intramolecular H-H vector from the simulated trajectories.
• Fitted the propagator using a model incorporating multiple rotational-diffusion coefficients.

Main Results:

• MD simulations confirmed that water molecule reorientation occurs via both rotational diffusion and discrete large-angle jumps.
• A model with two rotational-diffusion coefficients provided a reasonable empirical fit to the simulated propagator.
• The fit was particularly relevant for time scales (picoseconds to nanoseconds) crucial for NMR spin relaxation near room temperature.

Conclusions:

• The rotational dynamics of water molecules in liquid water are more complex than a single rotational-diffusion coefficient model suggests.
• A multi-coefficient model, specifically with two coefficients, offers an improved empirical description of water reorientation relevant to NMR.
• The study provides apparent values for these coefficients at 298 K, contributing to a deeper understanding of water's role in NMR relaxation.