Subdiffusion and lateral diffusion coefficient of lipid atoms and molecules in phospholipid bilayers

TL;DR

This study combines long molecular dynamics simulations with theoretical modeling to analyze lipid atom and molecule dynamics in phospholipid bilayers, revealing distinct diffusive regimes and proposing a memory function approach for comprehensive analysis.

Contribution

It introduces a memory function model that accurately describes lipid dynamics across all diffusion regimes in bilayers, validated against extensive MD simulations.

Findings

Identification of three distinct dynamical regimes: ballistic, subdiffusive, and Fickian diffusion.

Development of a memory function approach matching MD simulation results.

Insights into subdiffusive behavior's impact on neutron scattering measurements.

Abstract

We use a long, all-atom molecular dynamics (MD) simulation combined with theoretical modeling to investigate the dynamics of selected lipid atoms and lipid molecules in a hydrated diyristoyl-phosphatidylcholine (DMPC) lipid bilayer. From the analysis of a 0.1 $\mu$s MD trajectory we find that the time evolution of the mean square displacement, [\delta{r}(t)]^2, of lipid atoms and molecules exhibits three well separated dynamical regions: (i) ballistic, with [\delta{r}(t)]^2 ~ t^2 for t < 10 fs; (ii) subdiffusive, with [\delta{r}(t)]^2 ~ t^{\beta} with \beta<1, for 10 ps < t < 10 ns; and (iii) Fickian diffusion, with [\delta{r}(t)]^2 ~ t for t > 30 ns. We propose a memory function approach for calculating [\delta{r}(t)]^2 over the entire time range extending from the ballistic to the Fickian diffusion regimes. The results are in very good agreement with the ones from the MD simulations. We also examine the implications of the presence of the subdiffusive dynamics of lipids on the self-intermediate scattering function and the incoherent dynamics structure factor measured in neutron scattering experiments.