Simulation Study of the Permeability of a Model Lipid
Membrane at the Fluid–Solid Phase Transition
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Abstract
When a range of lipid bilayers are
melted to the disordered fluid
phase from the (much less permeable) ordered gel phase, their permeability
to a variety of permeants shows a peak at the transition temperature
and drops off with increasing temperature, rather than just rising
as melting proceeds. To explore this anomalous behavior, a simulated
coarse-grained lipid membrane model that exhibits a phase transition
upon expansion or compression was studied to determine how the permeation
rate of a simple particle depends on the phase composition in the
two-phase region and on particle size. The permeation rate and each
phase’s area fraction and area density could be directly calculated,
along with the probability that the permeant would cross in either
phase or in interfacial regions. For large permeants and system sizes,
conditions could be found where permeability increases upon compression
of the bilayer. Permeation was negligible in the gel phase and, in
contrast to the predictions of the “leaky interface”
hypothesis, was not enriched in interfacial regions. The anomalous
effect could instead be attributed to an increase in the area per
lipid of fluid-phase domains. This result motivated a model for the
decrease in effective permeability barrier through fluid-phase domains
arising from a decrease in the length of the gel/fluid interface at
the midpoint of a permeation event