The interfacial elastic packing interactions of different galactosylceramides (GalCers), sphingomyelins (SMs), and phosphatidylcholines (PC) were compared by determining their elastic area compressibility moduli (C(s)/-1) as a function of lateral packing pressure (π) in a Langmuir- type film balance. To assess the relative contributions of the lipid headgroups as well as those of the ceramide and diacylglycerol hydrocarbon regions, we synthesized various GalCer and SM species with identical, homogeneous acyl residues and compared their behavior to that of PCs possessing similar hydrocarbon structures. For PCs, this meant that the sn-1 acyl chain was long and saturated (e.g., palmitate) and the sn-2 chain composition was varied to match that of GalCer or SM. When at equivalent π and in either the chain-disordered (liquid-expanded) or chain-ordered (liquid-condensed) state, GalCer films were less elastic than either SM or PC films. When lipid headgroups were identical (SM and Pc), C(s)/-1 values (at equivalent π) for chain-disordered SMs, but not chain-ordered SMs, were 25- 30% higher than those of PCs. Typical values for fluid phase (liquid- expanded) GalCer at 30 mN/m and 24°C were 158 (±7) mN/m, whereas those of SM were 135 (±7) mN/m and those of PC were 123 (±2) mN/m. Pressure-induced transitions to chain-ordered states (liquid-condensed) resulted in significant increases (two- to fourfold) in the 'in-plane' compressibility for all three lipid types. Typical C(s)/-1 values for chain-ordered GalCers at 30 mN/m and 24°C were between 610 and 650 mN/m, whereas those of SM and of PC were very similar and were between 265 and 300 mN/m. Under fluid phase conditions, the π-C(s)/-1 behavior for each lipid type was insensitive to whether the acyl chain was saturated or monounsaturated. Measurement of the C(s)/-1 values also provided an effective way to evaluate the two- dimensional phase transition region of SMs, GalCers, and PCs. Modest heterogeneity in the acyl composition led to transitional broadening. Our findings provide useful information regarding the in-plane elasticity of lipids that are difficult to investigate by alternative methods, i.e., micropipette aspiration technique. The results also provide insight into the stability of sphingolipid-enriched, membrane microdomains that are thought to play a role in the sorting and trafficking of proteins containing glycosylphosphatidylinositol anchors with cells.
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We thank Howard Brockman for the use of the Langmuir film balance and for helpful comments regarding this report, Evan Evans for pointing out references regarding measurements of elastic area compressibility moduli in lipid bilayers, Margot Cleary and Fred Phillips for assistance with the capillary GC analyses of lipid acyl composition, and Kristi Hyland for synthesizing and purifying several of the sphingolipids. This investigation was supported by USPHS grant GM45928 and the Hormel Foundation.