Refractive index and thickness determination in Langmuir monolayers of myelin lipids

Abstract

Langmuir monolayers at the air/water interface are widely used as biomembrane models and for amphiphilic molecules studies in general. Under controlled intermolecular organization (lateral molecular area), surface pressure, surface potential, reflectivity (R) and other magnitudes can be precisely determined on these planar monomolecular films. However, some physical parameters such as the refractive index of the monolayer (n) still remain elusive. The refractive index is very relevant because (in combination with R) it allows for the determination of the thickness of the film. The uncertainties of n determine important errors that propagate non-linearly into the calculation of monolayers thickness. Here we present an analytical method for the determination of n in monolayers based on refractive index matching. By using a Brewster angle microscopy (BAM) setup and monolayers spread over subphases with variable refractive index (n), a minimum in R is search as a function of n. In these conditions, n equals n. The results shown correspond to monolayers of myelin lipids. The n values remain constant at 1.46 upon compression and equals the obtained value for myelin lipid bilayers in suspension. The values for n and R allow for the determination of thickness. We establish comparisons between these thicknesses for the monolayer and those obtained from two X-ray scattering techniques: 1) GIXOS for monolayers at the air/water interface and 2) SAXS for bilayers in bulk suspension. This allows us to conclude that the thickness that we measure by BAM includes the apolar and polar headgroup regions of the monolayer.

Unmodified Clay Nanosheets at the Air–Water Interface

“…This is shown in the inset of Figure 5 , was obtained from data fitting using a two-slab model for DPPC headgroups and tails. It matches the value obtained from the approximation d = 1.5π/ q min , 74 , 85 where q min is the value where the GIXOS curve has the first minimum. The other values were obtained in a similar way; however, this simple model is not able to fit the increased intensity around 0.5 nm –1 .…”

Unusual Behavior of the Bipolar Molecule 25-Hydroxycholesterol at the Air/Water Interface—Langmuir Monolayer Approach Complemented with Theoretical Calculations

“…Since the refractive index of a film composed of 25-OH is unknown, we assumed its value equal to the refractive index of cholesterol (1.47). 22 As can be noticed, the monolayer thickness during the first rise of surface pressure reaches a value of ca. 2.35 nm, which corresponds to a single layer of 25-OH molecules anchored to the surface of the water with one hydroxyl group.…”

Surface Plasmon Resonance Sensing on Naturally Derived Membranes: A Remyelination-Promoting Human Antibody Binds Myelin with Extraordinary Affinity

“…In all experiments, the SPR shift was ∼20 nm after a 30 min myelin immobilization step. When considering the refractive index of myelin lipid membrane to be 1.46 and the calculated (using a rigorous coupled wave analysis-based Matlab script) surface sensitivity at the silica-buffer interface about 29 nm/RIU per 1 nm of added continuous thin film, we can estimate the surface coverage of myelin to be about 5.3 × 10 6 nm 3 /μm 2 . This is rather a lower estimate as myelin comprises not only lipid (about 70–85% of the dry content and protein (between 15% and 30%), but also about 40% of water .…”