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The mechanism of collapse of heterogeneous lipid monolayers
#MMPMID25185549
Baoukina S
; Rozmanov D
; Mendez-Villuendas E
; Tieleman DP
Biophys J
2014[Sep]; 107
(5
): 1136-1145
PMID25185549
show ga
Collapse of homogeneous lipid monolayers is known to proceed via
wrinkling/buckling, followed by folding into bilayers in water. For heterogeneous
monolayers with phase coexistence, the mechanism of collapse remains unclear.
Here, we investigated collapse of lipid monolayers with coexisting liquid-liquid
and liquid-solid domains using molecular dynamics simulations. The MARTINI
coarse-grained model was employed to simulate monolayers of ?80 nm in lateral
dimension for 10-25 ?s. The monolayer minimum surface tension decreased in the
presence of solid domains, especially if they percolated. Liquid-ordered domains
facilitated monolayer collapse due to the spontaneous curvature induced at a high
cholesterol concentration. Upon collapse, bilayer folds formed in the liquid
(disordered) phase; curved domains shifted the nucleation sites toward the phase
boundary. The liquid (disordered) phase was preferentially transferred into
bilayers, in agreement with the squeeze-out hypothesis. As a result, the
composition and phase distribution were altered in the monolayer in equilibrium
with bilayers compared to a flat monolayer at the same surface tension. The
composition and phase behavior of the bilayers depended on the degree of
monolayer compression. The monolayer-bilayer connection region was enriched in
unsaturated lipids. Percolation of solid domains slowed down monolayer collapse
by several orders of magnitude. These results are important for understanding the
mechanism of two-to-three-dimensional transformations in heterogeneous thin films
and the role of lateral organization in biological membranes. The study is
directly relevant for the function of lung surfactant, and can explain the role
of nanodomains in its surface activity and inhibition by an increased cholesterol
concentration.
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