Quantifying Membrane Viscosity by Monitoring the Rotational and Translational Diffusion of Tracer Particles

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 difficult to quantify, largely due to the diffusion coefficients of membrane-associated tracer particles being non-trivially related the viscosity of the underlying membrane. We address this with a new technique in which determination of both the rotational and translational diffusion coefficients of membrane-linked particles enables quantification of viscosity, measurement of the effective radii of the tracers, and assessment of theoretical models of membrane hydrodynamics. Surprisingly, we find a wide distribution of effective tracer sizes, due presumably to a wide variety of couplings to the membrane. The measured relationship between translational and rotational diffusion for two different lipids with phosphatidylcholine headgroups provides support for the classic hydrodynamic models, and is also well fit by a recent extension of this model that accounts for local membrane deformation. We further compare the effective viscosities measured for different membrane geometries, such as planar membranes and giant vesicles.