Phase transitions
How do cholesterol and heat reshape lipid membrane domains?
Open access · cc by · source: Europe PMC
More cholesterol shrinks the thickness difference between ordered and disordered membrane domains, which should lower the boundary energy and help the ordered domains melt.
Study at a glance
- Design
- Other — Small-angle x-ray scattering of multilamellar lipid vesicles analysed with a new global two-phase model, across compositions and temperatures.
- N
- No single N; two ternary lipid mixtures, each on two tielines, with endpoint and midpoint samples, all prepared at least twice.
- Population
- Model membranes of DOPC with DPPC or DSPC plus cholesterol
- Outcome
- Bilayer thickness, area per lipid, water layer, bending fluctuation (Caillé) parameter and Ld phase fraction for each domain type
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
The two-phase fits matched the single-phase endpoints closely: disordered and ordered bilayers were about 38.1 and 48.6 Å thick in coexistence versus 37.5 and 49.7 Å at the endpoints. Ordered domains had roughly 20 Ų smaller area per lipid and weaker bending fluctuations (Caillé parameter about 0.03 versus 0.08). Raising cholesterol reduced the thickness mismatch (from 11.3 to 10 Å with DSPC and from 9.7 to 7.5 Å with DPPC) and increased how much ordered phase melted on heating. Near the transition, disordered domains unexpectedly thickened, consistent with cholesterol moving into them.
Methodology
The authors made multilamellar vesicles from three-component lipid mixtures (an unsaturated lipid, a saturated lipid and cholesterol) that split into liquid-ordered and liquid-disordered domains. They recorded x-ray scattering at a synchrotron and extended a global fitting model to treat the pattern as a sum of two phases, yielding structure and bending fluctuations for each. They validated it against single-phase endpoint samples, then varied cholesterol content and heated samples in 5 °C steps until the domains merged.
Limitations
These are simple lipid-only model membranes with micrometre domains, so they say little directly about whether nanoscale rafts exist in living cells. The method assumes domains stack in registry and ignores cross-correlations between phases, and it relies on published phase diagrams, one of whose boundaries the authors found to be wrong. Line tension was not measured but inferred from a theoretical relation to thickness mismatch, and coarse temperature steps prevent firm conclusions about critical behaviour.
How this study connects
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