ROLE OF HEADGROUP STRUCTURE IN THE PHASE-BEHAVIOR OF N-ACYLETHANOLAMINE PHOSPHOLIPIDS - HYDROGEN-BONDING ABILITY AND HEADGROUP SIZE

The physical properties of aqueous dispersions of N-acylphosphatidylet hanolamine from natural origin with long N-acyl chain (NAPE) and headg roup modified analogues have been studied. N-Acylation of PE causes a significant increase in the gel-to-liquid crystalline lamellar phase t ransition temperature in contrast with saturated N-acyl-(dipalmitoyl) PEs, and in addition it does not restrict the headgroup rotational mob ility in gel phase. The results agree with the increase of hydration o f the phosphate group compared with that in PE and suggest the formati on of hydrogen bonds between amide groups. The modifications introduce d modulate the headgroup size and their hydrogen bonding capability. A n increasing number of methylene groups between the phosphate and amid e groups does not modify the phase behaviour observed. N-methylation o f the amide group, which prevents the possibility of intermolecular hy drogen bond formation, decreases the melting temperature and the coope rativity of the phase transition and does not change the phase behavio ur, while the hydration at the ester carbonyl groups level is decrease d. On the other hand, the addition of N-ethyl substituent to the amide group or substitution of an ester group for this group increases its tendency to form structures with inverted geometries. The behaviour of these compounds suggests that hydration forces must be more important than considerations of the lipid dynamic shape in predicting the rela tive stabilities of lamellar vs. non-lamellar phases for NAPEs with lo ng saturated N-acyl chain.