Polymorphism of phosphatidylethanolamines from two natural sources

Abstract

Two pure phosphatidylethanolamines, isolated from porcine erythrocytes (PPE) and from larvae of Phormia regina (FPE), were analysed and found to differ only in their fatty-acyl composition. An X-ray diffraction study was made of the equilibrium structures formed by these two lipids, in water, as a function of lipid concentration and of temperature. They behave identically and the structural parameters of a widely occurring lamellar phase were determined and found to be similar to phosphatidylcholine. The lamellar phase, composed of alternating layers of lipid, of thickness d 1, and water, of thickness d w , swells only to a limited extent ( d w maximum is 13 Å) and d 1 varies from 38 Å to 43 Å as the lipid concentration varies from 70% to 90% dry weight. The equilibrium structures formed by FPE and PPE in low pH buffers are quite different, PPE being identical to that in water but FPE forming a cubic phase. The reason for the difference is unclear, but the results suggest the importance of fatty-acyl composition and of the effect of pH on the polymorphism of these systems.