Key role of lipid composition and phase on the mechanical properties of membranes probed by AFM on liposomes

Polar lipids from milk are valuable sources to design affordable and biocompatible liposomes for food or medical applications. In particular, the milk sphingomyelin (MSM) that accounts for about 30 % wt of milk polar lipids exhibits a complex composition with over 95 % mol. of saturated acyl chains (C16:0, C22:0, C23:0 and C24:0) and a melting temperature (Tm) of 35°C. The mechanical properties of the liposome’s bilayer are important to ensure e.g. physical stability. The objective of this study was to investigate the mechanical properties of liposomes as a function of their chemical composition and physical state. Liposomes (diameter < 200 nm) were produced using controlled compositions of polar lipids and immobilized onto flat silicium substrate in aqueous environment. The elastic modulus E and bending rigidity kc of their bilayer were measured using atomic force microscopy (AFM). As references, liposomes made of dipalmitoylphosphatidylcholine in the gel phase (DPPC; C16:0 ; Tm= 41°C) was found to be about 9 times more elastic at 20°C than that of dioleoylphosphatidylcholine in the fluid phase (DOPC; C18:1n-9 ; Tm=-20°C). Surprisingly, MSM liposome bilayers exhibited intermediate mechanical properties to those of DPPC and DOPC, in spite of the presence of long and saturated acyl chains. Experiments performed with liposomes of synthetic C16:0-SM, C24:0-SM and C24:1-SM showed that chain length heterogeneity or unsaturation both contribute to decrease the overall elasticity of MSM liposomes. The results demonstrate the potential of AFM indentation to assess the mechanical properties of curved and soft nano-objects such as liposomes. They also show that biologically relevant liposomes with complex lipid compositions, such as MSM, exhibit overall softer elasticity than expected, probably because of altered lateral packing of the molecules in the gel phase. This study contributes in a better understanding of the mechanical properties of membranes as a function of their composition and physical state.