Two-particle random walk simulation of outer-sphere nuclear relaxation.

Area of Science:

• Computational Chemistry
• Magnetic Resonance Spectroscopy
• Molecular Dynamics

Background:

• Accurate computation of the outer-sphere (OS) dipolar time correlation function (DTCF) is crucial for understanding molecular dynamics and relaxation processes.
• Existing methods based on Smoluchowski diffusion equations may not fully capture the complex spatial motion of interacting molecules, especially anisotropic ones.
• The influence of solvation properties on the DTCF and its contribution to relaxivity, particularly for paramagnetic contrast agents, requires detailed investigation.

Purpose of the Study:

• To introduce a two-particle Monte Carlo method for calculating the OS-DTCF for anisotropic molecules undergoing translational and rotational diffusion.
• To assess the validity of the interspin procedure based on Smoluchowski diffusion solutions by comparing them with simulation results.
• To investigate the impact of solvation properties and molecular size on the DTCF decay rate and its implications for relaxivity.

Main Methods:

• Development and application of a two-particle Monte Carlo simulation method.
• Computation of the outer-sphere (OS) dipolar time correlation function (DTCF) for relative molecular positions.
• Analysis of the influence of molecular anisotropy, translational/rotational diffusion, and solvation layers on DTCF decay.

Main Results:

• The true spatial motion of the interspin vector can differ significantly from Smoluchowski solutions, leading to a retarded DTCF time decay.
• Molecular size and solvation layers around the target molecule M(S) play a critical role in modulating the DTCF decay rate.
• Viscous solvation layers can cause local slowdowns in relative translational diffusion, enhancing DTCF retardation and increasing the OS contribution to relaxivity.

Conclusions:

• The presented Monte Carlo method provides a more accurate approach to calculating OS-DTCF compared to Smoluchowski-based methods.
• Solvation properties, particularly viscous layers, significantly influence molecular dynamics and the resulting DTCF, impacting relaxivity.
• The findings are particularly relevant for understanding the behavior of Gd(3+)-based contrast agents in Magnetic Resonance Imaging (MRI).