The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.
The
Gram-negative bacterial outer membrane (GNB-OM) is asymmetric
in its lipid composition with a phospholipid-rich inner leaflet and
an outer leaflet predominantly composed of lipopolysaccharides (LPS).
LPS are polyanionic molecules, with numerous phosphate groups present
in the lipid A and core oligosaccharide regions. The repulsive forces
due to accumulation of the negative charges are screened and bridged
by the divalent cations (Mg2+ and Ca2+) that
are known to be crucial for the integrity of the bacterial OM. Indeed,
chelation of divalent cations is a well-established method to permeabilize
Gram-negative bacteria such as Escherichia coli. Here, we use X-ray and neutron reflectivity (XRR and NR, respectively)
techniques to examine the role of calcium ions in the stability of
a model GNB-OM. Using XRR we show that Ca2+ binds to the
core region of the rough mutant LPS (RaLPS) films, producing more
ordered structures in comparison to divalent cation free monolayers.
Using recently developed solid-supported models of the GNB-OM, we
study the effect of calcium removal on the asymmetry of DPPC:RaLPS
bilayers. We show that without the charge screening effect of divalent
cations, the LPS is forced to overcome the thermodynamically unfavorable
energy barrier and flip across the hydrophobic bilayer to minimize
the repulsive electrostatic forces, resulting in about 20% mixing
of LPS and DPPC between the inner and outer bilayer leaflets. These
results reveal for the first time the molecular details behind the
well-known mechanism of outer membrane stabilization by divalent cations.
This confirms the relevance of the asymmetric models for future studies
of outer membrane stability and antibiotic penetration.
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