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Mechanical Properties of DPPC–POPE Mixed Langmuir Monolayers
journal contribution
posted on 2019-12-16, 05:30 authored by Alberto
S. Luviano, José Campos-Terán, Dominique Langevin, Rolando Castillo, Gabriel EspinosaThe mechanical properties of lipid monolayers and their
responses
to shear and compression stresses play an important role in processes
such as breathing and eye blinking. We studied the mechanical properties
of Langmuir monolayers of a model mixture, composed of an unsaturated
lipid, 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine
(POPE), and a saturated lipid, 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC). We performed isothermal compressions
and sinusoidal shear deformations of these mixed monolayers. Also,
the different phases were observed with Brewster angle microscopy.
We found that the mechanical behavior is affected by the miscibility
of both lipids. In the two-phase region, the compression elastic modulus
increases with the amount of the LC phase but does not follow the
predictions of a simple effective medium model. The discrepancies
arise from the fact that, upon compression, the domains grow at a
rate faster than the compression rate but not fast enough to reach
thermodynamic equilibrium. Before reaching the LC phase, domain percolation
is observed and compression and shear moduli become equal to those
of the pure LC phase. Most of the monolayers behave as viscoelastic
fluids at the frequencies investigated. A minimum in the compression
modulus and shear viscosity was observed for mixtures close to equimolar
composition, with the minimum being accompanied by a change in domain
shapes.