Study of the structure of N-acyldipalmitoylphosphatidylethanolamines in aqueous dispersion by infrared and Raman spectroscopies.

The effect of the headgroup chain length on the structure and on the thermotropic behavior of N-acyldipalmitoylphosphatidylethanolamines (N-acyl-DPPEs) has been studied by infrared and Raman spectroscopies. The results show that the N-acyl-DPPEs can be divided in two classes depending on the N-acyl chain length. When the N-acyl chain contains 10 carbon atoms or more, it penetrates into the bilayer while it remains at the level of the glycerol backbone for shorter N-acyl chains. For both classes of N-acyl-DPPEs, the rotation of the lipid chains in the liquid-crystalline phase is hindered by the presence of the N-acyl group. In addition, the disruption of the hydrogen bonds between the amino and phosphate groups by N-acylation of the amino group results in an increase of the hydration of the phosphate group compared to that in DPPE. The hydration occurred at both the phosphate and amide group levels; the phosphate group is more hydrated for phospholipids with long N-acyl chains while in the case of short-chain derivatives both the phosphate and amide groups are hydrated. This higher degree of hydration coupled with the immobilization of the lipid molecule may contribute to the bilayer stabilizer role of N-acyl-PEs since hydration is an important factor in bilayer stability.