Effect of 1-palmitoyl lysophosphatidylcholine on phase properties of 1,2-dipalmitoyl phosphatidylethanolamine: a thermodynamic and NMR study

Abstract

The effect of 1-palmitoyl lysophosphatidylcholine (PLPC) on the phase behaviour of 1,2-dipalmitoyl phosphatidylethanolamine (DPPE) in excess water (34 wt%) has been examined by differential scanning calorimetry, scanning dilatometry and isothermal compressibility measurements. Mole percentages of PLPC in DPPE between 14 and 62% have been studied over the temperature range 30–75°C. The temperature dependence of orientational ordering at selected sites in 2H-labelled PLPC and 2H 2O has been determined from measurement of time-averaged chemical shift anisotropies and quadrupole splittings in the 31P- and 2H-NMR spectra. These data have been used to further characterize phase behaviour. At less than equimolar contents of PLPC, when a single phase transition with a reduced transition temperature is observed, spectral and calorimetric data indicate complete miscibility of the two lipid components. An equimolar mixture of PLPC and DPPE shows a sharp first order transition at 47.3°C and a second order transition at 62.5°C. NMR data are consistent with the existence of a defective bilayer at intermediary temperatures. In this range it is proposed that PLPC molecules prefer regions with high curvature in the vicinity of the defects, while DPPE molecules are mostly confined to flatter regions of the bilayer. A possible molecular model is described. At temperatures above 62.5°C, PLPC and DPPE are completely miscible and exist as lamellae. At higher PLPC content (> 50 mol%), thermodynamic and spectral data are indicative of phase separation of the two components over the temperature range examined.