Acyl Chain-Length Asymmetry Alters the Interfacial Elastic Interactions of Phosphatidylcholines

Abstract

Phosphatidylcholines (PCs) with stearoyl (18:0) sn-1 chains and variable-length, saturated sn-2 acyl chains were synthesized and investigated using a Langmuir-type film balance. Surface pressure was monitored as a function of lipid molecular area at various constant temperatures between 10°C and 30°C. Over this temperature range, 18:0–10:0 PC displayed only liquid-expanded behavior. In contrast, di-14:0 PC displayed liquid-expanded behavior at 24°C and 30°C, but two-dimensional phase transitions were evident at 20°C, 15°C, and 10°C. The average molecular area of 18:0–10:0 PC was larger than that of liquid-expanded di-14:0 PC at equivalent surface pressures, and the shapes of their liquid expanded isotherms were somewhat dissimilar. Analysis of the elastic moduli of area compressibility ( C s −1) as a function of molecular area revealed shallower slopes in the semilog plots of 18:0–10:0 PC compared to di-14:0 PC. At membrane-like surface pressures (e.g., 30 mN/m), 18:0–10:0 PC was 20–25% more elastic (in an in-plane sense) than di-14:0 PC. Other PCs with varying degrees of chain-length asymmetry (18:0–8:0 PC, 18:0–12:0 PC, 18:0–14:0 PC, 18:0–16:0 PC) were also investigated to determine whether the higher in-plane elasticity of fluid-phase 18:0–10:0 PC is a common feature of PCs with asymmetrical chain lengths. Two-dimensional phase transitions in 18:0–14:0 PC and 18:0–16:0 PC prevented meaningful comparison with other fluid-phase PCs at 30 mN/m. However, the C s −1 values for fluid-phase 18:0–8:0 PC and 18:0–12:0 PC were similar to that of 18:0–10:0 PC (85–90 mN/m). These values showed chain-length asymmetrical PCs to have 20–25% greater in-plane elasticity than fluid-phase PCs with mono- or diunsaturated acyl chains.