Extending the Angular Range of Neutron Reflectivity Measurements from Planar Lipid Bilayers: Application to a Model Biological Membrane

A novel experimental setup has been used to measure the specularly-reflected neutron intensity from a model biological membrane containing components from the outer membrane of Neurospora crassa mitochondria. The specular reflectivity from a single bilayer membrane, formed by the fusion of vesicles onto a phospholipid monolayer supported on a flat substrate, was measured in both D₂O and H₂O solvents. In D2O solvent, reflected neutron intensities down to 10^-6 were measured for wavevector transfers out to 0.25 Å^-1. A symmetric model for the neutron scattering length density profile perpendicular to the bilayer surface was constructed based on fits to the D₂O data. The overall bilayer thickness was found to be 43 ± 2 Å. The individual lipid head group and hydrocarbon tail layer thicknesses were 7.5 ± 1.4 and 28.0 ± 2.8 Å, respectively. The fitted results are consistent with the H₂O data. The integrity of the model membrane bilayer was confirmed by comparing its measured reflectivity to that obtained from a single lipid bilayer consisting of soybean phospholipids (asolectin) which was deposited on a flat substrate by the Langmuir-Blodgett technique.