Despite the biological significance of sphingomyelins (SMs), there is far less structural information available for SMs compared to glycerophospholipids. Considerable confusion exists in the literature regarding even the phase behavior of SM bilayers. This work studies both palmitoyl (PSM) and egg sphingomyelin (ESM) in the temperature regime from 3 °C to 55 °C using X-ray diffraction and X-ray diffuse scattering on hydrated, oriented thick bilayer stacks. We observe clear evidence for a ripple phase for ESM in a large temperature range from 3 °C to the main phase transition temperature (T_M) of ∼38 °C. This unusual stability of the ripple phase was not observed for PSM, which was in a gel phase at 3 °C, with a gel-to-ripple transition at ∼24 °C and a ripple-to-fluid transition at ∼41 °C. We also report structural results for all phases. In the gel phase at 3 °C, PSM has chains tilted by ∼30° with an area/lipid ∼45 Ų as determined by wide angle X-ray scattering. The ripple phases for both PSM and ESM have temperature dependent ripple wavelengths that are ∼145 Å near 30 °C. In the fluid phase, our electron density profiles combined with volume measurements allow calculation of area/lipid to be ∼64 Ų for both PSM and ESM, which is larger than that from most of the previous molecular dynamics simulations and experimental studies. Our study demonstrates that oriented lipid films are particularly well-suited to characterize ripple phases since the scattering pattern is much better resolved than in unoriented samples.

尽管鞘磷脂（SMs）具有生物学意义，但与甘油磷脂相比，有关SM的结构信息要少得多。关于SM双层的相行为，文献中存在相当大的困惑。这项工作使用X射线衍射和X射线扩散在水合的定向厚双层堆栈上进行的研究，研究了从3°C到55°C温度范围内的棕榈酰（PSM）和卵鞘磷脂（ESM）。我们观察到清晰的证据表明，在从3°C到主相变温度（T _M）〜38°C。对于PSM，未观察到波纹相的这种异常稳定性，PSM在3°C时处于凝胶相，在〜24°C时发生了由凝胶到波纹的转变，在〜41°C时发生了从波纹到流体的转变。 。我们还将报告所有阶段的结构结果。在3℃的凝胶相，PSM具有由约30°倾斜与区域链/脂质〜45埃²通过广角X射线散射测定。PSM和ESM的纹波相位均具有温度相关的纹波波长，在30°C时约为145Å。在流体相中，我们的电子密度分布与体积的测量相结合允许区域/脂质的计算是〜64埃²对于PSM和ESM而言，它比以前的大多数分子动力学模拟和实验研究中的值都要大。我们的研究表明，定向的脂膜特别适合表征波纹相，因为与未定向的样品相比，散射图的分辨率要好得多。