Surface viscosities of lipid bilayers determined from equilibrium molecular dynamics simulations

Membrane viscosity influences cellular function and respiration rates. Prior research has shown that bacterial cells adjust lipid composition to maintain membrane viscosity. Experimental methods have provided estimates of membrane viscosity using chromophore rotation and protein diffusion. However, these measurements vary in magnitude and interpretation. No prior work had resolved the precise molecular determinants of membrane viscosity. This gap motivated the use of equilibrium simulations to quantify how lipid chemistry affects viscosity. The need for a detailed molecular-level understanding of membrane viscosity remains unmet. Computational models offer a precise but underutilized approach to this problem. This paper contributes a systematic analysis of lipid composition effects on membrane viscosity.

The study aimed to determine how lipid chemistry influences membrane viscosity using equilibrium simulations. The researchers focused on single-component lipid bilayers to isolate molecular effects. They simulated various lipids with differing chain lengths, unsaturation, and backbone structures. The goal was to quantify the contribution of each chemical feature to membrane viscosity. They also examined the role of dispersion interactions in viscosity calculations. The study sought to compare simulated viscosities with experimental data from chromophore rotation and vesicle deformation. Understanding these relationships could clarify the molecular basis of membrane function. The work provides a framework for linking lipid chemistry to physical membrane properties.

The team used equilibrium all-atom molecular dynamics simulations of lipid bilayers. They modeled the stress-tensor autocorrelation function with a stretched exponential function. The Green-Kubo relation was applied to calculate viscosities from simulated data. The simulations covered a range of lipids with varying chain lengths and unsaturation. They also tested different backbone structures, including sphingomyelin and DPPC. The effect of dispersion interactions was systematically evaluated. Simulations were run at multiple temperatures to assess thermal dependence. The results were compared to experimental data from chromophore rotation and vesicle deformation.

Sphingomyelin membranes exhibited viscosities about 20 times higher than DPPC membranes. Chain length had a significant effect on viscosity, with longer chains increasing resistance. Unsaturation reduced viscosity compared to fully saturated lipids. Backbone structure played a key role in determining membrane viscosity. Dispersion interactions increased viscosity by up to 140% when fully included. Simulated viscosities matched experimental data from chromophore rotation and protein diffusion. However, they were significantly lower than vesicle deformation measurements. These findings suggest that lipid chemistry and intermolecular forces strongly influence membrane viscosity.

The study shows that lipid chemistry and dispersion interactions strongly influence membrane viscosity. Sphingomyelin membranes are much more viscous than DPPC membranes. Chain length, unsaturation, and backbone structure each contribute to viscosity. Dispersion interactions increase viscosity by up to 140% in simulations. Simulated viscosities align with chromophore rotation and protein diffusion data but not with vesicle deformation measurements. These results suggest that molecular-level features govern membrane viscosity. The findings support the use of equilibrium simulations to study membrane properties. The work highlights the importance of lipid composition in determining membrane behavior.