Molecular organization in mixed SOPC and SDPC model membranes: Water permeability studies of polyunsaturated lipid bilayers.

Polyunsaturated phospholipids in plasma membranes modulate order and dynamics of the lipid bilayer and influence integral membrane protein function. In this study, we investigate the effects of polyunsaturated fatty acid (PUFA) chains in phosphatidylcholine (PC) on osmotic water permeability and packing characteristics, using a droplet interface bilayer (DIB) as a model cell membrane. The lipid bilayer system is composed of mixtures of PC lipids with varying ratio of 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18: 0-18:1 PC) to 1-stearoyl-2-docosahexaenoylphosphatidylcholine (SDPC, 18:0-22:6 PC). Water permeability coefficients (P) were derived from the kinetics of osmotic filtration across the DIB. Confocal Raman microspectroscopy probed the structural and packing properties of the mixtures. The water transport parameters and Raman spectral analyses together indicate that P and aggregate ordering of a lipid bilayer are significantly influenced by the progressive addition of PUFA chains to a bilayer. The permeability rates across bilayers of the mixtures are a linear function of SDPC mol%. The mean disorder for the bilayer hydrocarbon region, as determined by Raman disorder parameter from the saturated chain, scales linearly with P. The linear trend for P versus degree of unsaturation in double-chain PC lipid bilayers is about half the value of that obtained for a single-chain lipid system, with implications for an evolutionary preference for phospholipids during the postulated biotic transition from single-chain lipid protocellular to modern cell membranes. The amassed results highlight the role of PUFA in modifying membrane order as manifest in bulk membrane physical properties, with characteristic differences depending on single-chain or double-chain systems.