Is an average viscosity tenable in lipid bilayers and membranes? A comparison of semi-empirical equivalent viscosities given by unbound probes: a nitroxide and a fluorophore.

Relative variations of fluidity in bilayers and membranes are currently evaluated by numerous physical methods, but comparison between different systems remain difficult because the effects of order (anisotropy) and fluidity are involved in the diffusion coefficients for correlation times, or frictional coefficients) given by experiment. The present report represents an attempt to generalize the use of isotropic liquids as viscosity standards for disordered lipidic systems. It advances a simple check to verify the quasi-isotropic behaviour of probe environments and avoids the introduction of estimated values of the molecular dimensions in Perrin-Einstein relations. The equivalent viscosities obtained with 1,6-diphenyl hexatriene and with 2-pentyl-2'-butyl-4,4'-dimethyl oxazolidinoxyl are strikingly similar in egg lecithin vesicles above 0 degrees C, while in dipalmitoylphosphatidylcholine dispersions above their transition temperature, a discrepancy of about 30% seems to remain, even at high temperatures.