Physical Chemistry I, Faculty of Chemistry, TU Dortmund University, Dortmund, Germany.
Biophys J. 2011 May 4;100(9):2169-77. doi: 10.1016/j.bpj.2011.03.019.
We report x-ray reflectivity and grazing incidence x-ray diffraction measurements of lipopolysaccharide (LPS) monolayers at the water-air interface. Our investigations reveal that the structure and lateral ordering of the LPS molecules is very different from phospholipid systems and can be modulated by the ionic strength of the aqueous subphase in an ion-dependent manner. Our findings also indicate differential effects of monovalent and divalent ions on the two-dimensional ordering of lipid domains. Na(+) ions interact unspecifically with LPS molecules based on their ability to efficiently screen the negative charges of the LPS molecules, whereas Ca(2+) ions interact specifically by cross-linking adjacent molecules in the monolayer. At low lateral pressures, Na(+) ions present in the subphase lead to a LPS monolayer structure ordered over large areas with high compressibility, nearly hexagonal packing of the hydrocarbon chains, and high density in the LPS headgroup region. At higher film pressures, the LPS monolayer becomes more rigid and results in a less perfect, oblique packing of the LPS hydrocarbon chains as well as a smaller lateral size of highly ordered domains on the monolayer. Furthermore, associated with the increased surface pressure, a conformational change of the sugar headgroups occurs, leading to a thickening of the entire LPS monolayer structure. The effect of Ca(2+) ions in the subphase is to increase the rigidity of the LPS monolayer, leading to an oblique packing of the hydrocarbon chains already at low film pressures, an upright orientation of the sugar moieties, and much smaller sizes of ordered domains in the plane of the monolayer. In the presence of both Na(+)- and Ca(2+) ions in the subphase, the screening effect of Na(+) is predominant at low film pressures, whereas, at higher film pressures, the structure and lateral organization of LPS molecules is governed by the influence of Ca(2+) ions. The unspecific charge-screening effect of the Na(+) ions on the conformation of the sugar moiety becomes less dominant at biologically relevant lateral pressures.
我们报告了在水-气界面处脂多糖 (LPS) 单层的 X 射线反射率和掠入射 X 射线衍射测量。我们的研究表明，LPS 分子的结构和横向有序性与磷脂系统非常不同，可以通过水亚相的离子强度以离子依赖的方式进行调制。我们的发现还表明单价和二价离子对脂质域二维有序性的影响不同。Na(+) 离子通过有效地屏蔽 LPS 分子的负电荷来与 LPS 分子非特异性相互作用，而 Ca(2+) 离子通过在单层中交联相邻分子来特异性相互作用。在低侧向压力下，亚相中存在的 Na(+) 离子导致 LPS 单层结构在大面积上有序排列，具有高压缩性、烃链近乎六边形排列和 LPS 头部区域高密度。在较高的膜压下，LPS 单层变得更加刚性，导致 LPS 烃链的排列不那么完美，呈倾斜状，并且单层上高度有序的畴的横向尺寸更小。此外，伴随着表面压力的增加，糖头部基团发生构象变化，导致整个 LPS 单层结构变厚。亚相中 Ca(2+) 离子的作用是增加 LPS 单层的刚性，导致烃链在低膜压下已经呈倾斜排列，糖部分呈垂直取向，并且单层平面上有序畴的尺寸小得多。在亚相中存在 Na(+)-和 Ca(2+) 离子的情况下，Na(+) 的屏蔽效应在低膜压下占主导地位，而在较高膜压下，LPS 分子的结构和横向组织由 Ca(2+) 离子的影响决定。Na(+) 离子对糖部分构象的非特异性电荷屏蔽效应在生物学相关的侧向压力下变得不那么占主导地位。
