Phase transitions
How do fleeting lipid clusters differ from true membrane domains?
Open access · cc by · source: Europe PMC
Above the demixing temperature lipid bilayers contain short-lived, rough-edged clusters that do not line up across the two leaflets, whereas true phase domains are persistent, smoother and largely aligned.
Study at a glance
- Design
- Computational / modelling — Martini coarse-grained molecular dynamics of binary DPPC/DUPC and ternary DPPC/DUPC/cholesterol bilayers across temperature, with partial substitution by a hybrid lipid.
- N
- No participant N; simulated bilayers of 4608 or 18,432 lipids, run for 10-30 μs at temperatures from 270 to 340 K.
- Population
- Simulated model lipid bilayers (gel/liquid and liquid-ordered/liquid-disordered mixtures)
- Outcome
- Correlation length and time of fluctuations, cluster size, boundary length, interleaflet overlap, order parameters and area per lipid
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Key findings
On cooling, bilayers went from a mixed state with nanometre-sized, nanosecond-lived clusters to phase-separated domains persisting over 10 μs; fluctuations grew in size and lifetime near the transition. Fluctuations had much longer, rougher boundaries than domains and almost no overlap between leaflets, while domains overlapped substantially. The hybrid lipid lowered the transition temperature, shrank domains and fluctuation correlation lengths, and roughened boundaries, but it did not concentrate at domain edges as a popular theory predicts; instead it made the disordered phase more ordered, shrinking the difference between phases.
Methodology
The authors simulated two model membranes with a coarse-grained force field: a saturated/unsaturated lipid mix that forms gel and fluid phases, and a mix with cholesterol that forms liquid-ordered and liquid-disordered phases. They ran each over a range of temperatures above and below the point where the lipids separate, and in some runs replaced part of the unsaturated lipid with a hybrid lipid that has one saturated and one unsaturated tail. They clustered lipids by local composition and measured the size, lifetime, edge roughness and cross-leaflet overlap of the clusters.
Limitations
The coarse-grained Martini model underestimates how bilayer properties change with temperature, so transition temperatures cannot be compared quantitatively with experiments. Box sizes of tens of nanometres cap the size of both fluctuations and domains, so the simulations cannot say whether domains would eventually become macroscopic, and correlation times carried large statistical uncertainty. The membranes are simple three-component mixtures without proteins or asymmetry, so conclusions about rafts in real cells are suggestive only.
How this study connects
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