In Situ Sensing of Salinity in Oriented Lipid Multilayers by Surface X-ray Scattering

Abstract

The influence of LiCl solutions on liposomal and surface-supported phosphatidylcholine/water systems (dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), respectively) has been studied by small-angle X-ray techniques. In liposomal dispersions of DPPC, an osmotically stressed liquid-crystalline phase, denoted as Lalpha osm, forms readily after rapid mixing with salt solutions. The transition from Lalpha -->Lalpha osm proceeds in two steps. The first step takes place within seconds and is due to water diffusion from the liposome into the bulk solution. The second, slower process (minutes) can be attributed to the relaxation of initially deformed intermediate liposomes into spherical ones. In experiments with aligned lipid bilayers supported on silicon wafers, it was possible to reproducibly exchange different concentrations of LiCl solutions on a single sample and to determine the lattice changes by time-resolved X-ray scattering at grazing incidence. Independently of the deposition technique (spray- or spin-coating, respectively), none of the investigated POPC samples displayed an osmotically stressed liquid-crystalline phase. While liposomes can be considered nearly defect-free, supported bilayer stacks show a high abundance of defects, such as oily streaks typical of the Lalpha phase. Thus, the alkali ions are free to diffuse into the interbilayer water regions and to cause a slight increase of the bilayer separation (about 1 Angstroms). It is concluded that low to medium concentrations of Li+ ions partially screen the attractive van der Waals force between adjacent membrane layers. However, upon annealing the defect regions or regions of high curvature in the oriented lipid matrix, e.g. by low amounts of oleyl alcohol (OA), the system is able to sense osmotic stress upon addition of a salt solution.

Rigidification of Neutral Lipid Bilayers in the Presence of Salts

“…In this report, we focus on the effects of sodium and calcium chloride on phosphatidylcholines (PCs) in the fluid L a phase. Monovalent salts are known to swell neutral lipid bilayers with increasing concentration (14)(15)(16)19,24,25). The prevailing understanding of this behavior is that both cations and anions bind about equally to the lipid headgroups, with reported binding constants of 0.15 M À1 for Na 1 and 0.20 M À1 for Cl À in EggPC bilayers (7).…”

Calcium Triggered Lα-H2 Phase Transition Monitored by Combined Rapid Mixing and Time-Resolved Synchrotron SAXS

“…For instance, Weiss et al [8] reported on the spontaneous formation of unilamellar vesicles (ULVs) by rapidly mixing two oppositely charged micellar solutions. In other investigations, the internal transfer of materials from nanostructured emulsions [15], the impact of rapid mixing of salt solutions with phospholipid vesicles [16], [17], and the salt-induced formation of calcium carbonate [18] have been studied by time-resolved SAXS. Coupling rapid mixing to other techniques helps also to unravel other interesting processes [19], [20].…”

Effects of phosphonium-based ionic liquids on phospholipid membranes studied by small-angle X-ray scattering

“…The magnitude of the change in lamellar distance and the effects of increasing concentration are likely due to the different kinetics of liposomal dispersions compared to those of supported multilayers. For example, Amenitsch et al (Amenitsch et al, 2004) measured the effect of lithium chloride concentration on DPPC and POPC MLVs and on supported multilayers and observed that while a significant 280 reduction in bilayer thickness occurred in liposomes exposed to 0.1-0.5 M lithium chloride, lamella in supported stacks actually expanded when exposed to the salt.…”