Conformational difference in the polar groups of phosphatidylcholine and phosphatidylethanolamine in aqueous phase

Proton and phosphorus nuclear magnetic resonance was used to investigate conformations of o-phosphorylcholine(OPC), o-phosphorylethanolamine(OPE) and L-alpha-glycerophosphorylethanolamine in aqueous solution, and the conformations of dipalmitoyl-3-sn-phosphatidylcholine and phosphatidylethanolamine from E. coli in methanol and chloroform solutions. It has been shown that in every case the O-C-C-N system prefers a gauche conformations, but in the choline moiety the dihedral angle around the C-C bond is distorted from the usual gauche angle, 60 degrees, to a larger one. The dihedral angle of OPC is shown to be more variable than that of OPE. This may be due to the curvature of its potential curve, i.e. asymmetrical curvature around the gauche minima. This property of the phosphatidylcholine molecule may be partly responsible for the flexibility of the phosphatidylcholine bilayer. The coupling is dominant in the P-O-C-C systems of the 5 compounds examined. The results also indicated that the two hydrocarbon chains in phosphatidylcholine or phosphatidylethanolamine are apt to take nearly parallel orientation in methanol solution. This characteristic is favourable for the formation of the bilayer structure.