Can We Trust Hydrodynamic Models to Determine the Bilayer Viscosity Experienced by Transmembrane Proteins?

Abstract

Although the measurement of molecular mobility in bilayers is relatively straightforward, the influence of the nature of the bilayer on the diffusing molecule i.e. the viscosity of the bilayer is delicate and not well understood.It is unclear if the viscosity sampled by the diffusing micron size gel structure is a measure of the nanoscopic viscosity of the bilayers experienced by much smaller proteins. Indeed for transmembrane proteins, theoretical considerations make questionable the use of hydrodynamic models to fit experimental data since the diffusing particle is about the same size as the particles composing the fluid in which it is diffusing.We will present partial answers to questions that are still open in this context:- Do we observe viscosity changes from the value of macro-viscosity to the nano-scale and if it is so for what length-scale we observe a crossover from nano to macro?- What is the relation between nanoviscosity determined from the diffusion of nanoprobes such as proteins and macro-viscosity obtained from the diffusion of gel domains?- And finally how nanoviscosity, extracted from Fluorescence Recovery After Photobleaching (FRAP) and hydrodynamic models, compares to the nanoviscosity measured with molecular rotors by Fluorescence Lifetime Imaging Microscopy (FLIM)?