Rafael Gustavo Oliveira

Brewster angle microscopy (BAM) is a highly surface-sensitive, noninvasive technique used to study the reflectivity and lateral domain structures of Langmuir monolayers at the air-water interface. This experimental system uniquely allows for the control of lateral compression while simultaneously measuring surface pressure and visualization of domains. The primary advantage of BAM over other methods is its ability to directly measure in situ the intrinsic properties of samples without requiring chemical modifications, external probes or further manipulations like transfer to a solid support. This chapter outlines the basic theory of BAM and introduces a new method for determining the refractive index of the monolayer, a key parameter often missing in data analysis. By combining the refractive index with reflectivity measurements, it becomes possible to calculate the thickness of films as a function of lateral packing in lipid, protein, and lipid/protein films.

There are several examples of single mutations that lead to a well-defined disease through a well-known mechanism. In other cases, a collection of mutations of the same protein produces a pathology with different degrees of severity. The accompanying work by Uusitalo et al. studies several mutants of the fatty acid binding protein P2 of the peripheral nervous system myelin. They conserve the native tertiary structure but a remarkable difference in the capacity to interact with lipids. This could be a clue to unravel the complex way in which these mutations affect myelin structure and function in a variant of Charcot–Marie–Tooth disease. Comment on: https://doi.org/10.1111/febs.16079

Purified myelin membranes (PMMs) are the starting material for biochemical analyses such as the isolation of detergent-insoluble glycosphingolipid-rich domains (DIGs), which are believed to be representatives of functional lipid rafts. The normal DIGs isolation protocol involves the extraction of lipids under moderate cooling. Here, we thus address the influence of cooling on the structure of PMMs and its sub-fractions. Thermodynamic and structural aspects of periodic, multilamellar PMMs are examined between 4°C and 45°C and in various biologically relevant aqueous solutions. The phase behavior is investigated by small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). Complementary neutron diffraction (ND) experiments with solid-supported myelin multilayers confirm that the phase behavior is unaffected by planar confinement. SAXS and ND consistently show that multilamellar PMMs in pure water become heterogeneous when cooled by more than 10–15°C below physiological temperature, as during the DIGs isolation procedure. The heterogeneous state of PMMs is stabilized in physiological solution, where phase coexistence persists up to near the physiological temperature. This result supports the general view that membranes under physiological conditions are close to critical points for phase separation. In presence of elevated Ca²⁺ concentrations (> 10 mM), phase coexistence is found even far above physiological temperatures. The relative fractions of the two phases, and thus presumably also their compositions, are found to vary with temperature. Depending on the conditions, an “expanded” phase with larger lamellar period or a “compacted” phase with smaller lamellar period coexists with the native phase. Both expanded and compacted periods are also observed in DIGs under the respective conditions. The observed subtle temperature-dependence of the phase behavior of PMMs suggests that the composition of DIGs is sensitive to the details of the isolation protocol.