Measuring Lipid Membrane Viscosity using Rotational and Translational Particle Diffusion

Abstract

The two-dimensional fluidity of lipid bilayers enables the motion of membrane-bound macromolecules and is therefore crucial to biological function. However, lipid bilayer viscosity remains poorly quantified, largely due to the difficulty of relating the diffusion coefficients of membrane-associated tracer particles to the viscosity of the underlying membrane. We address this with a new technique for measuring lipid bilayer viscosity, in which determination of both the rotational and translational diffusion coefficients of tracer particles enables quantification of viscosity as well as the effective size of the tracers. Surprisingly, we find a wide distribution of effective tracer sizes, due presumably to a wide variety of couplings to the membrane. We provide viscosity measurements for lipid bilayers with various compositions in the gel and liquid crystalline phases.