Electron diffraction study of the hexagonal polymorphic form of some choline-containing phospholipids

Abstract

Since the kinematical diffraction assumption has been found to be applicable to the analysis of electron diffraction patterns from thin organic crystals, single crystal electron diffraction patterns were obtained from thin anhydrous microcrystals of a lecithin, a lysolecithin and a beef brain sphingomyelin. Crystal structure analysis using the hk0 intensity data affirms the correctness of a free methylene rotor model for this projection of the aliphatic chain packing. Because the aliphatic chains are affixed to the polar moiety of a phospholipid molecule, it is proposed that there is helical twisting along the polymethylene chains, rather than the rigid body chain rotation proposed for the hexagonal packing of long chain paraffins. Lack of upper layer diffraction from the lecithin crystals which would be indicative of repeats along the chains leads to a speculation of translational disorders along the chain axes.