Rand R P
Department of Biological Sciences, Brock University, St Catharines, Ontario, Canada.
Int Rev Cytol. 2002;215:33-48. doi: 10.1016/s0074-7696(02)15004-2.
Lipids at the bilayer-water interface are highly disordered and mobile, and large areas of the bilayer undergo thermal undulations. Water penetrates significantly down to the hydrocarbon chain level. This water, and water out to about 10 A from the surface, is perturbed by the lipid surface in a way that produces a strong hydration repulsion and precludes molecular contact between bilayers. Its removal costs work, but most of this water is neither a permeable barrier nor unavailable to solvate other solutes. All hydrophilic surfaces show this "hydration force." Most lipids have an excess higher free energy when packed within a bilayer membrane since in isolation they pack into high curvature assemblies with polar groups on the concave side. Osmotic stress measurements of those curved assemblies yield a measure of monolayer elastic parameters and the excess higher free energy, which likely controls embedded proteins. Osmotic stress experiments can determine whether water is energetically significant, or not, in almost any system. The osmotic effect of solutes, independent of specific binding, is to compete with lipids and proteins for water. Solute affinity for water can modify lipid packing and protein conformation, coupling lipid and protein structure and function to osmolality at the molecular level.
双层-水界面处的脂质高度无序且可移动，双层的大片区域会发生热波动。水会显著渗透到烃链层。从表面向外约10埃范围内的这些水以及表面的水，会受到脂质表面的扰动，从而产生强烈的水合斥力，并阻止双层之间的分子接触。去除这些水需要做功，但这些水大部分既不是渗透屏障，也并非无法用于溶剂化其他溶质。所有亲水性表面都表现出这种“水合力”。大多数脂质在双层膜内堆积时具有较高的自由能，因为在孤立状态下它们会堆积成高曲率聚集体，极性基团位于凹面。对这些弯曲聚集体进行渗透压测量可得出单层弹性参数和较高的自由能，这可能会控制嵌入的蛋白质。渗透压实验几乎可以确定在任何系统中，水在能量方面是否具有重要意义。溶质的渗透效应（与特异性结合无关）是与脂质和蛋白质争夺水。溶质对水的亲和力可以改变脂质堆积和蛋白质构象，在分子水平上将脂质和蛋白质的结构与功能与渗透压联系起来。
