Permeability of bilayers composed of mixtures of saturated phospholipids: evidence for compositional segregation of lipids at phase boundaries.
Lipid bilayers composed of a binary mixture of saturated phospholipids display broad permeability maxima in the temperature region of phase separation. We have proposed that this enhanced diffusion occurs through defects which develop at boundaries between liquid crystal and gel state domains and that one of the lipid components of the mixture might specifically congregate at these defects. To explore this postulate we have performed the following type of experiment. Liposomes were formed from different mole ratios of the lipid pairs 1,2-dimyristoyl-sn-glycero-3-phosphocholine (dimyristoyl GPC)--1,2-distearoyl-sn-glycero-3-phosphocholine (distearoyl GPC) and dimyristoyl GPC--1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (dimyristoyl GPE) and 22Na effluxes measured over an extensive temperature span (15 to 60 degrees C). For a given mixture we plotted 22Na movement through the boundary regions which develop during phase separation against the fraction of dimyristoyl GPC in the coexisting solid and liquid phases at that temperature. As dimyristoyl GPC is added to either distearoyl GPC or dimyristoyl GPE, boundary efflux remains low until the mole fraction of dimyristoyl GPC in the coexisting solid and liquid phases reaches 0.1 and 0.4 (for dimyristoyl GPC--distearoyl GPC) and 0.3 and 0.7 (for dimyristoyl GPC--dimyristoyl GPE), respectively. As the fraction of dimyristoyl GPC in the coexisting phases exceeds these values, boundary permeability rises and with further increments in this phospholipid, the mixtures begin to assume permeability characteristics similar to that of dimyristoyl GPC alone. These observations suggest that at low mole fractions of dimyristoyl GPC, the boundary regions are populated by the distearoyl GPC or dimyristoyl GPE molecules and hence efflux rates are small.(ABSTRACT TRUNCATED AT 250 WORDS)
D004134:Dimyristoylphosphatidylcholine; D008051:Lipid Bilayers; D010539:Permeability; D010714:Phosphatidylethanolamines; D010715:Phosphatidylglycerols; D012965:Sodium Chloride; D013329:Structure-Activity Relationship; D013816:Thermodynamics
D008051:Lipid Bilayers; D010714:Phosphatidylethanolamines; D010715:Phosphatidylglycerols; D012965:Sodium Chloride; C002773:dimyristoylphosphatidylglycerol; D004134:Dimyristoylphosphatidylcholine; C018525:1,2-dimyristoylphosphatidylethanolamine