Abstract
Selective interactions of ions with charge-neutral saccharides can have far-reaching consequences in biological and wet-technological contexts but have so far been observed only indirectly. Here, we directly quantify by total-reflection X-ray fluorescence the preferential accumulation of ions near uncharged saccharide surfaces in the form of glycolipid Langmuir monolayers at air/water interfaces exhibiting different levels of structural ordering. Selective interactions with ions from the aqueous subphase are observed for monolayers featuring crystalline ordering of the saccharide headgroups, as determined by grazing-incidence X-ray diffraction. The attracted ion species depend on the structural motifs displayed by the ordered saccharide layer. Our results may constitute a basis to understand the salt-specific swelling of wood materials and various phenomena in membrane biophysics.
Highlights
Glycolipids are essential constituents of biological membranes
We directly quantify by totalreflection X-ray fluorescence the preferential accumulation of ions near uncharged saccharide surfaces in the form of glycolipid Langmuir monolayers at air/water interfaces exhibiting different levels of structural ordering
Selective interactions with ions from the aqueous subphase are observed for monolayers featuring crystalline ordering of the saccharide headgroups, as determined by grazing-incidence X-ray diffraction
Summary
Selectivity to only one ion species in a salt solution inevitably leads to a charge separation that, in turn, results in electrostatic repulsion between two such surfaces.[6,49] This notion provides a route to a better understanding of the ion-specificity in the swelling of wood.[6] In a biological context, the effective surface charge induced by preferential interactions of ions with headgroup-ordered glycolipid microdomains (“lipid rafts”)[50] is suited to attract proteins and to accelerate their binding Such preferential ion interactions lead to additional coupling between the lateral and perpendicular equations of state of multilamellar membrane systems.[51] Our results motivate further systematic investigations with the aim to identify correlations between the structural features of the crystalline saccharide surfaces and the preferentially adsorbing ion species.
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