Mechanical properties of milk sphingomyelin bilayer membranes in the gel phase: Effects of naturally complex heterogeneity, saturation and acyl chain length investigated on liposomes using AFM.

Sphingomyelin (SM) molecules are major lipid components of plasma membranes that are involved in functional domains. Among natural SMs, that found in milk (milk-SM) exhibits important acyl chain heterogeneities in terms of length and saturation, which could affect the biophysical properties and biological functions of the milk fat globule membrane or of liposome carriers. In this study, the thermotropic and mechanical properties of milk-SM, synthetic C16:0-SM, C24:0-SM and the binary mixtures C16:0-SM/C24:0-SM (50:50% mol) and C24:0-SM/C24:1-SM (95:5% mol) bilayer membranes were investigated using differential scanning calorimetry and atomic force microscopy, respectively. Results showed that acyl chain length, heterogeneity and unsaturation affected i) the temperature of phase transition of SM bilayers, and ii) the mechanical properties of liposome (diameter<200nm) membranes in the gel phase, e.g. the Young modulus E and the bending rigidity k. This study increases our knowledge about the key role of naturally complex lipid compositions in tailoring the physical properties of biological membranes. It could be also used in liposomes development e.g. to select the suitable lipid composition according to usage.