Grazing incidence X-ray off specular scattering on Langmuir monolayers from biological membranes

RAFAEL GUSTAVO OLIVEIRA; PUSTERLA, JULIO M.; GASPERINI-MALFATTI, ANTONIO

Córdoba

Simposio; 14th International Symposium on Radiation Physics; 2018

Amphiphilic molecules can be spread at the air water interface and compress under well-defined lateral compression in a Langmuir trough. The out of plane structure of these films is traditionally obtained from specular X-ray reflectivity, but this technique is seldom applied to horizontal samples as Langmuir trough at synchrotrons. The out of plane structure can alternatively be obtained from Grazing Incidence X-ray Off Specular Scattering (GIXOS), which is a technique that produces analogous information to specular X-ray reflectivity on systems with conformal roughness in just a single measurement. 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 us 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 [1]. Work is in progress with whole myelin monolayers.[1] JM. Pusterla, AA. Malfatti-Gasperini, XE. Puentes-Martinez, LP. Cavalcanti, RG. Oliveira, BBA Biomem. 1859, 2017, 924.