Surface viscosities of several lipids determined from equilibrium molecular dynamics simulations

Lipid membrane viscosity is critical to biological function. Bacterial cells grown in different environments have been shown to alter their lipid composition in order to maintain a viscosity in a narrow range,1 and membrane viscosity has been shown to control the rate of cellular respiration.2 In order to understand the factors which determine membrane viscosity, 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 fit to a stretched exponential function. By simulating a series of lipids at different temperatures, 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 twenty times that of DPPC. Including the full range of the dispersion increases the membrane viscosity by up to 140%, alleviating some of the discrepancy between simulated and experimental viscosity measurements, although the simulated viscosities remain significantly lower than recent experimental values reported for a series of lipids.3