Can Lipids be used as Mobility Standards in Artificial Bilayers?

Abstract

The literature data and results presented in this article show that in lipidic bilayers lipids diffuse with various diffusion coefficients ranging from D = 3.7 ± 0.4 to 13.9 ± 0.6 μm2s-1, Interestingly, a transmembrane peptide, having nearly the same radius as lipids and whose hydrophobic thickness matches that of the bilayer, exhibits a D value of 9.6 ± 0.4 μm2s-1. Since the lipids and the transmembrane peptide possess a similar diameter and lipids do not span the whole membrane, the diffusion coefficient of peptide is expected to be smaller than that of lipids, which is not the case in our experiments. Our systematic study and reinterpretation of literature data suggests that the slower diffusion of lipids, as compared to that of the transmembrane peptide, is caused by the formation of dynamic lipid nanopatches that diffuse like a single object with an increased radius. These nanopatches form more spontaneously when lipids are saturated. Molecular dynamics simulations confirm our experimental results and show that the nanopatches formation also occurs in bilayers made of single lipid in absence of dye.Consequently, one should be cautious when comparing the mobility of transmembrane proteins and lipids. Transmembrane peptides should be used as a reference instead of lipids.