Surface viscosities of lipid bilayers determined from equilibrium molecular dynamics simulations

Lipid membrane viscosity is critical to biological function. Bacterial cells grown in different environments alter their lipid composition in order to maintain a specific viscosity, and membrane viscosity has been linked to the rate of cellular respi-ration. To understand the factors that determine the viscosity of a membrane, we ran equilibrium all-atom simulations of single component lipid bilayers and calculated their viscosities. The viscosity was calculated via a Green-Kubo relation, with the stress -tensor autocorrelation function modeled by a stretched exponential function. By simulating a series of lipids at different temper-atures, we establish the dependence of viscosity on several aspects of lipid chemistry, including hydrocarbon chain length, unsaturation, and backbone structure. Sphingomyelin is found to have a remarkably high viscosity, roughly 20 times that of DPPC. Furthermore, we find that inclusion of the entire range of the dispersion interaction increases viscosity by up to 140%. The simulated viscosities are similar to experimental values obtained from the rotational dynamics of small chromophores and from the diffusion of integral membrane proteins but significantly lower than recent measurements based on the deformation of giant vesicles.SIGNIFICANCE Viscosity is a critical property of cell membranes that is actively regulated and known to control the rate of reactions that require the diffusion and encounter of proteins and small molecules. However, experimental measurements span more than an order of magnitude in the obtained viscosity depending on the technique and analysis. Extensive simulations of membrane viscosity are presented in order to make progress toward a unified understanding of membrane viscosity.