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 T_m = 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 (L_β1) to mixed interdigitated (L_β2) lamellar structures evidencing gel phase polymorphism and then iii) transition to the fluid liquid-crystal L_α phase. We demonstrated that cholesterol modulates the physical properties of milk-SM bilayers and that building up of the lamellar liquid-ordered L_o phase is completed for 33 mol% of cholesterol. The ordering effect of cholesterol on milk-SM bilayers and the temperature-independent behavior of the L_o 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).

鞘磷脂（SM）和胆固醇是参与有序域形成的生物膜的主要脂质成分。在这项研究中，我们调查了牛奶-SM双层的生物物理特性，并确定了胆固醇的作用。使用差示扫描量热法（DSC）和高通量同步辐射X射线衍射（SR-XRD）表征了牛奶-SM和牛奶-SM /胆固醇混合物的热致相行为。测定为完全水合的牛奶-SM双层的熔融相转变温度为T _m ＝ 34.3±0.1℃。牛奶-SM的热致变相行为很复杂，反映了不同分子种类的混合物。依次发生在完全水合乳-SM双层的加热结构企业重组被解释如下：i）熔融C16：0的-SM，ii）由完全相互交叉（长和饱和SM物种的转化大号_β1）为混合相互交叉（大号_β2）片状结构证明凝胶相多态性然后ⅲ）过渡到流体液晶大号_α相。我们证明了胆固醇调节了牛奶-SM双层的物理特性，并且层状液体有序L _o的建立相完成了33摩尔％的胆固醇。表征了胆固醇对牛奶-SM双层分子的有序作用以及牛奶-SM /胆固醇复合物形成的L _o相的温度独立行为。这项工作的发现将有助于更好地理解牛奶-SM所具有的生物学功能，作为其相态和与胆固醇的相互作用的函数（例如降血脂作用）。