Composition of Supported Model Membranes Determined by Neutron Reflection

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.

Antimicrobial peptide dendrimer interacts with phosphocholine membranes in a fluidity dependent manner: A neutron reflection study combined with molecular dynamics simulations

“…All NR profiles were analyzed using the Motofit package [30], which uses the Abeles optical matrix method to calculate the reflectivity of thin layers. In order to keep a physically realistic model, we constrained the bilayer to maintain a constant area per lipid headgroup and tail in both leaflets [26]. The neutron scattering length density of the molecules, SLD m , was calculated from the constituent atoms and the molecular volumes, V m : …”

“…d = thickness, φ = solvent volume fraction, SLD = fitted scattering length density of the dendrimer layer in the specified contrast without including the contribution from the solvent. The fitting errors were determined by the quality of the fits [26]. to 100 ± 1 v/v%.…”

“…It thus allows probing the internal structure of the lipid bilayer including layer thickness, interfacial roughness, chemical composition, and the volume fraction of lipids and other membrane components. These parameters can be determined from the neutron scattering length density (SLD) profile, which gives information on the nuclear isotopic composition of the lipid layer [26,27]. The bilayer can be non-invasively modified by substituting hydrogen by its isotope deuterium to highlight different parts of the system.…”

“…A mol = molecular area per lipid component including (wet) or excluding solvent (dry). The fitting errors of the thicknesses and solvent fractions were determined by the quality of the fits[26]. The error in the area per molecule was calculated A mol , where φ is the lipid volume fraction (1 − solvent [%]).…”

New sources and instrumentation for neutrons in biology

“…When biological interactions occur at planar surfaces, at interfaces, or in layered phases, neutron reflectometry (NR) can provide information on assembly, surface association and material penetration under a wide variety of experimental conditions [30][31][32][33][34][35][36][37]. When combined with synthetic and in vivo production of specifically D-labelled biomacro-molecules or polymers, H/D-labelled components of complex systems and hybrid materials can then be selectively highlighted, located and analyzed in situ.…”