Sphingomyelin (SM) and cholesterol are major lipid components of biological membranes involved in the formation of ordered domains. In this study, we investigated the biophysical properties of milk-SM bilayers and determined the effect of cholesterol. The thermotropic phase behaviours of milk-SM and milk-SM/cholesterol mixtures were characterized using differential scanning calorimetry (DSC) and high flux synchrotron radiation X-ray diffraction (SR-XRD). The melting phase transition temperature determined for fully hydrated milk-SM bilayers was Tm = 34.3 ± 0.1 °C. The thermotropic phase behavior of milk-SM is complex, reflecting the mixture of different molecular species. Structural reorganisations successively occurring on heating of fully hydrated milk-SM bilayers were interpreted as follows: i) melting of C16:0-SM, ii) conversion of long and saturated SM species from fully interdigitated (Lβ1) to mixed interdigitated (Lβ2) lamellar structures evidencing gel phase polymorphism and then iii) transition to the fluid liquid-crystal Lα phase. We demonstrated that cholesterol modulates the physical properties of milk-SM bilayers and that building up of the lamellar liquid-ordered Lo phase is completed for 33 mol% of cholesterol. The ordering effect of cholesterol on milk-SM bilayers and the temperature-independent behavior of the Lo phase formed by milk-SM/cholesterol complexes were characterized. The findings of this work will contribute in a better understanding of the biological functions exerted by the milk-SM as a function of its phase state and interactions with cholesterol (e.g. hypocholesterolemic effect).
Keywords: Bilayer structure; Lipid raft; Membrane domain; Milk phospholipid; Sphingolipid; X-ray diffraction.
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