Tsuchikawa Hiroshi, Monji Mami, Umegawa Yuichi, Yasuda Tomokazu, Slotte J Peter, Murata Michio
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560- 0043, Japan.
Faculty of Medicine, Oita University, Oita, Oita 879-5593, Japan.
Langmuir. 2022 May 10;38(18):5515-5524. doi: 10.1021/acs.langmuir.2c00092. Epub 2022 Apr 28.
The chain melting of lipid bilayers has often been investigated in detail using calorimetric methods, such as differential scanning calorimetry (DSC), and the resultant main transition temperature is regarded as one of the most important parameters in model membrane experiments. However, it is not always clear whether the hydrocarbon chains of lipids are gradually melting along the depth of the lipid bilayer or whether they all melt concurrently in a very narrow temperature range, as implied by DSC. In this study, we focused on stearoyl-d-sphingomyelin (SSM) as an example of raft-forming lipids. We synthesized deuterium-labeled SSMs at the 4', 10', and 16' positions, and their depth-dependent melting was measured using solid-state deuterium NMR by changing the temperature by 1.0 °C, and comparing with that observed from a saturated lipid, palmitoylstearoylphosphatidylcholine (PSPC). The results showed that SSM exhibited a characteristic depth-dependent melting, which was not observed for PSPC. The strong intermolecular hydrogen bonds between the sphingomyelin amide moiety probably caused the chain melting to start from the chain terminus through the middle part and end in the upper part. This depth-dependent melting implies that the small gel-like domains of SSM remain at temperatures slightly above the main transition temperature. These sphingomyelin features may be responsible for the biological properties of SM-based lipid rafts.
脂质双层的链熔化现象常常使用量热法进行详细研究，比如差示扫描量热法（DSC），所得的主要转变温度被视为模型膜实验中最重要的参数之一。然而，脂质的烃链是沿着脂质双层的深度逐渐熔化，还是如DSC所暗示的那样在非常窄的温度范围内同时全部熔化，这一点并不总是很清楚。在本研究中，我们聚焦于硬脂酰 - d - 鞘磷脂（SSM）作为成筏脂质的一个例子。我们在4'、10'和16'位置合成了氘标记的SSM，并通过固态氘核磁共振在温度以1.0°C变化的情况下测量其深度依赖性熔化，同时与从饱和脂质棕榈酰硬脂酰磷脂酰胆碱（PSPC）观察到的情况进行比较。结果表明，SSM呈现出一种PSPC未观察到的特征性深度依赖性熔化。鞘磷脂酰胺部分之间强大的分子间氢键可能导致链熔化从链末端开始，经过中间部分，在上部结束。这种深度依赖性熔化意味着SSM的小凝胶状结构域在略高于主要转变温度的温度下仍然存在。这些鞘磷脂特征可能是基于鞘磷脂的脂筏生物学特性的原因。
