Grazing incidence X-ray off specular scattering on biomembrane Langmuir monolayers

Full text: Grazing incidence X-ray off specular scattering (GIXOS) is a technique that produces analogous information to specular X-ray reflectivity on systems with conformal roughness in just a single measurement. This technique is mainly applied to Langmuir monolayers at the air/water interface which serves as biomembrane model systems. We performed GIXOS at XRD-2 (LNLS) on Langmuir monolayers of myelin lipids. These measurements play a central role for comparison with myelin lipid bilayers (measured by SAXS) on one side, and myelin lipid monolayers measured in a new setup based on light reflectivity using a Brewster Angle Microscope (BAM) . Our BAM setup, with variable refractive index on the subphase, allows the matching of the refractive indexes from the monolayer and the subphase, and thus the determination of the monolayer refractive index. The last plus the reflectivity renders the thickness of the monolayer. Nevertheless, due to the fact that the monolayer is not optically homogeneous in the z direction, it was not clear what includes the measured thickness. In this sense, comparisons between SAXS, GIXOS and BAM on the same material in monolayers and bilayers allowed to conclude that the BAM setup measures all the thickness of the monolayer. Additionally, we found that the phosphor to phosphor thickness of the bilayer (4.60 nm) is bigger than twice the monolayer (2.13 nm each) even in the thicker state. We postulate that this is due to roughness at the air/water interface that under monolayer apposition, in order to build up a bilayer generates a steric repulsion. This force separates each monolayer from the contact plane by half roughness, which is 0.15 nm. This separation reconstitutes the correct P-P thickness as well as the proper electron density profiles of the methylene minimum as seen by SAXS in bilayers. (author)