Surface behavior of myelin monolayers

Abstract

Myelin can be spread as a stable monomolecular layer, with reproducible properties, at the air–water interface. The major lipids and proteins of myelin are represented in this monolayer in molar ratios similar to those in the original membrane. A well-defined collapse point of the myelin monolayer occurs at ca. 46 mN/m. At a surface pressure of ca. 20 mN/m, the surface pressure–molecular area isotherm of the myelin monolayer shows a change in its compressibility, exhibited as a diffuse but reproducible inflection with a clearly marked change of the surface compressional modulus; the surface potential–area curve shows a change of slope at the same surface pressure. The myelin monolayer shows considerable hysteresis during the first compression–decompression cycle; no detectable protein unfolding under expansion; and decreased hysteresis after the first cycle. The average molecular areas, the inflection at 20 mN/m, the variation of the surface potential per unit of molecular surface density, and the hysteresis properties of the myelin monolayer indicate that this membrane undergoes changes of intermolecular organization mostly ascribed to the protein fraction, above a lateral surface pressure of ca. 20 mN/m. The behavior is consistent with a surface pressure-dependent relocation of protein components in the film. This has marked effects on the stability, molecular packing, and dipolar organization of the myelin interface.