Molecular organization of hydrated dispersions of N-acylethanolamine phospholipids and mixtures with phosphatidylcholine

Abstract

The physical properties of aqueous dispersions of N-acylphosphatidylethanolamine ( N-acylPE) of natural origin with a long N-acyl chain have been studied and compared with those of phosphatidylcholine (PC). Saturated N-acylPEs form lamellar structures with a restricted head-group rotational mobility and with the same motional freedom in the hydrophobic area as PC. However, the presence of unsaturations in N-acylPE increases the mobility of the head group in the gel phase compared to that of the synthetic saturated analogue. The presence of an additional hydrocarbon chain and the suggested formation of hydrogen bonds between amide groups produces a significant increase in the thermodynamic parameters (transition temperature and enthalpy) of the main calorimetric transition. Another stabilizing factor could be related to the increase in the degree of hydration of the interfacial and head-group molecular regions of N-acylPE with respect to those of PC. Analysis of mixtures of N-acylPE and PC reveals that both phospholipids were miscible over the entire range of composition assayed. When increasing the PC content of N-acylPE bilayers, decreases in the transition temperature, transition enthalpy and cooperativity of the main transition occur. Although the mixing of the components is clearly nonideal, no lateral phase separation of the components occurs in the bilayer of the binary system. The polymorphic behaviour of the binary systems observed by 31P-NMR reveals a hydrated lamellar phase spectrum composite of the two spectral line shapes corresponding to both phospholipids with the same chemical shift anisotropy of the monocomponent systems.