Reconstitution of the lipid matrix of the outer membrane of Gram-negative bacteria as asymmetric planar bilayer

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Abstract
This paper is a report on the reconstitution of the lipid matrix of the outer membrane of Gram-negative bacteria as an asymmetric planar bilayer. This is the first time that a planar membrane is described, which consists on one side of a phospholipid (PL) mixture and on the other side of lipopolysaccharide (LPS). Therefore, strong emphasis is placed on a physical characterization of this membrane via its electrical properties. The membranes were prepared from spread monolayers or from vesicle-derived monolayers. Contrary to observations for symmetric phospholipid membranes, specific capacitances of (0.67±0.02) μF·cm−2, breakdown voltages between 200 and 400 mV and specific conductances between 10−8 and 2×10−7S·cm−2 were obtained independent of the preparation method. The LPS-containing membranes were stable up to 3 hr if they were formed and kept at temperatures above the hydrocarbon chain melting temperature of the LPS. For the specific capacitance, a dependence on the aperture radius was observed. This is explained by assuming a toroidal transition zone at the rim of the aperture. First results on the action of the pore-forming α-toxin fromStaphylococcus aureus on bilayers of different composition demonstrate particular characteristics of this asymmetric bilayer system. The pore-formation rate is highest in symmetric phospholipid bilayers, considerably lower in asymmetric PL/LPS systems and fully inhibited in LPS/LPS systems.