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

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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.