Comparison of confined water in Ld- and Lo-phases of cholesterol/lecithin mixtures

众所周知，胆固醇会影响脂质双分子层的结构特性，如流动性、曲率和脂质堆积，同时也会影响整体膜蛋白的位置。在多组分脂质混合物中，胆固醇可以诱导液体无序的Ld和液体有序的Lo相分离。在DOPC、dsc和胆固醇的三元混合物中，低胆固醇的Ld相和富胆固醇的Lo相在临界温度TC以下共存，而在临界温度TC以上，两相结合形成单一的、均匀的Ld相。虽然胆固醇在双分子层结构中的作用已经得到了很好的研究，但对封闭水和脂质双分子层界面之间相互作用的影响却很少被探索。回顾Heftberger等人(1)的研究数据，我们采用了最近发表的“三水层”模型（见图1）(2)。本研究的目的是确定在Lo相和Ld相中堆叠的双分子层的脂/水界面的精细结构。为了初步了解(i)头群相关、（ii）扰动和（iii）自由水区，我们确定了这些水层随温度的变化，并比较了不同脂质/胆固醇浓度下的结果。

Cholesterol is known to affect structural properties of lipid bilayers, such as fluidity, curvature, and lipid packing, while also affecting the position of integral membrane proteins. In multi-component lipid mixtures, cholesterol can induce a phase separation between liquid-disordered Ld and liquid-ordered Lo phases. In a ternary mixture of DOPC, DSPC and cholesterol, both cholesterol-poor Ld and cholesterol-rich Lo phases coexist below a critical temperature TC, while above TC, both phases coalesce to form a single, homogeneous Ld phase. While cholesterol's role on the bilayer structure has been well-studied, whereas influence on the interactions between confined-water and the lipid bilayer interface is scarcely explored. Revisiting data from a study by Heftberger et al. (1), we applied a recently published 'three-water layer' model (see Figure 1)(2). The aim of this study was to determine the fine structure at the lipid/water interface of stacked bilayers, both in the Lo and Ld phase. To obtain first insights into the (i) headgroup associated, (ii) perturbed and (iii) free water regions, we determined the changes to these water layers as a function of temperature and comparing results for different lipid/cholesterol concentrations.