Spin-label electron spin resonance studies on the mode of anchoring and vertical location of the N-acyl chain in N-acylphosphatidylethanolamines

Electron spin resonance (ESR) studies have been performed on N-myristoyl dimyristoylphosphatidylethanolamine (N-14-DMPE) membranes using both phosphatidylcholines spin-labeled at different positions in the sn-2 acyl chain and N-acyl phosphatidylethanolamines spin-labeled in the N-acyl chain to characterize the location and mobility of the N-acyl chain in the lipid membranes. Comparison of the positional dependences of the spectral data for the two series of spin-labeled lipids suggests that the N-acyl chain is positioned at approximately the same level as the sn-2 chain of the phosphatidylcholine spin-label. Further, similar conclusions are reached when the ESR spectra of the N-acyl PE spin-labels in dimyristoylphosphatidylcholine (DMPC) or dimyristoylphosphatidylethanolamine (DMPE) host matrixes are compared with those of phosphatidylcholine spin-labels in these two lipids. Finally, the chain ordering effect of cholesterol has also been found to be similar for the N-acyl PE spin-label and PC spin-labels, when the host matrix is either DMPC and cholesterol or N-14-DMPE and cholesterol at a 6:4 mole ratio. In both cases, the gel-to-liquid crystalline phase transition is completely abolished but cholesterol perturbs the gel-phase mobility of N-14-DMPE more readily than that of DMPC. These results demonstrate that the long N-acyl chains are anchored firmly in the hydrophobic interior of the membrane, in an orientation that is parallel to that of the O-acyl chains, and are located at nearly the same vertical position as that of the sn-2 acyl chains in the lipid bilayer. There is a high degree of dynamic compatibility between the N-acyl chains and the O-acyl chains of the lipid bilayer core, although bilayers of N-acyl phosphatidylethanolamines possess a more hydrophobic interior than phosphatidylcholine bilayers. These results provide a structural basis for rationalizing the biological properties of NAPEs.