The electrical charge of biological membranes and thus the resulting alignment of water molecules in response to this charge are important factors affecting membrane rigidity, transport, and reactivity. We tune the surface charge density by varying lipid composition and investigate the charge-induced alignment of water molecules using surface-specific vibrational spectroscopy and molecular dynamics simulations. At low charge densities, the alignment of water increases proportionally to the charge. However, already at moderate, physiologically relevant charge densities, water alignment starts to saturate despite the increase in the nominal surface charge. The saturation occurs in both the Stern layer, directly at the surface, and in the diffuse layer, yet for distinctly different reasons. Our results show that the soft nature of the lipid interface allows for a marked reduction of the surface potential at high surface charge density via both interfacial molecular rearrangement and permeation of monovalent ions into the interface.

中文翻译：

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电荷在模型膜表面的水诱导排列的饱和度。

生物膜的电荷以及由此导致的水分子响应于该电荷的排列是影响膜刚性，运输和反应性的重要因素。我们通过改变脂质成分来调节表面电荷密度，并使用表面特异性振动光谱和分子动力学模拟研究水分子的电荷诱导排列。在低电荷密度下，水的取向与电荷成比例地增加。然而，尽管标称表面电荷增加了，但在中等的生理相关电荷密度下，水的排列开始饱和。饱和在斯特恩层，直接在表面和扩散层中都发生，但由于明显不同的原因。