Composition of Supported Model Membranes Determined by Neutron Reflection

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Abstract
We have investigated the formation of supported bilayers by coadsorption of dipalmitoyl phosphatidylcholine (DPPC) with the nonionic surfactant β-d-dodecyl maltoside. The adsorption of mixed phospholipid−surfactant micelles on hydrophilic silica surfaces at 25 °C was followed as a function of bulk concentration by neutron reflection. Using chain-deuterated d25-β-d-dodecyl maltoside and d62-DPPC, we demonstrate that it is possible to determine the composition of the bilayers at each stage of a sequential dilution process, which enriches the adsorbed layer in phospholipid and leads to complete elimination of the surfactant. The final supported bilayers have thicknesses of 51 ± 3 Å and are stable to heating to 37 °C once all surfactant has been removed, and the structures agree well with other published data on DPPC supported bilayers. The coadsorption of cholesterol in a DPPC−surfactant mixture was also achieved, and the location and volume fraction of cholesterol in the DPPC bilayer was determined. Cholesterol is located in a 18 ± 1 Å thick layer below the lipid headgroup region and leads to an increased bilayer thickness of 58 ± 2 Å at 26 mol % of cholesterol.