How to best estimate the viscosity of lipid bilayers

International audienceThe viscosity of lipid bilayers is a property relevant to biological function, as it affects the diffusion of membrane macromolecules. To determine its value, and hence portray the membrane, various literature-reported techniques lead to significantly different results. Herein we compare the results issuing from two widely used techniques to determine the viscosity of membranes: the Fluorescence Lifetime Imaging Microscopy (FLIM), and Fluorescence Recovery After Photobleaching (FRAP). FLIM relates the time of rotation of a molecular rotor inserted into the membrane to the macroscopic viscosity of a fluid. Whereas FRAP measures molecular diffusion coefficients. This approach is based on a hydrodynamic model connecting the mobility of a membrane inclusion to the viscosity of the membrane.We show that:- The first method is very sensitive to local changes in viscosity; however, most often it would only provide the viscosity of the hydrophobic part of the membrane.- The membrane viscosity is adequately estimated when the hydrodynamic model approach is applied to the mobility of micrometric size membrane inclusion but not for nanometric size inclusions such as lipids or proteins. In this case, the calculated value extracted from the same hydrodynamic model characterizes the interaction of the given nano-inclusion with the bilayer instead of the bilayer viscosity.This article emphasizes the pitfalls to be avoided and the rules to be observed in order to obtain a value of the bilayer viscosity that characterizes the bilayer instead of interactions between the bilayer and the embedded probe