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    Ionic environment, thickness and line tension as determinants of phase separation in whole Purified Myelin Membranes monolayers

    Year: 2021

    Journal: Colloid Surf. B-Biointerfaces, Volume 207, NOV

    Authors: Pusterla, Julio; Hernandez, Juan Martin; Wilke, Natalia; Schneck, Emanuel; Oliveira, Rafael G.

    Keywords: Langmuir monolayers; Myelin; Thickness mismatch; Membrane domains; Line tension

    Purified myelin membranes (PMM) were spread as monomolecular films at the air/aqueous solution interface, and visualized by Brewster Angle Microscopy (BAM) at different lateral pressures (E) on three specific aqueous solutions: absence of salts, physiological conditions and presence of calcium. Coexistence of Liquid-Expanded (LE) and Liquid Ordered (LO) phases persisted up to collapse in the presence of salts, whereas monolayers became homogeneous at E >= 35-40 mN/m when salts are absent. This PMM phase-mixing behavior in monolayers is similar to the previously reported behavior of PMM multilamellar vesicles. Reflectivities (Rp) of ppolarized light from both phases were assessed throughout the whole E -range, and film thicknesses (t) were calculated from the Rp values and measured film refractive indices (n). The LO phase was found to be more reflective and thicker than the LE phase at E <= 15 mN/m, but less reflective and thinner at higher E. We also determined the line tension (A) of PMM monolayers at the domain boundaries from the rate of domain shape relaxation, which turned out to be of the order of picoNewtons (pN) and decreased as E increased. A correlation between A and thickness differences (At) was found, suggesting that At is a molecular determinant for A in PMM monolayers. Both A and At were found to increase markedly when calcium was present in the subphase. This result corroborates the concept of divalent cations as a stabilizing factor for phase separation, in line with earlier studies on this mixture forming multilamellar membrane arrangements.
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