Lamellar packing of a chiral N,N-dimethylphosphatidylethanolamine: electron diffraction. Evidence for a lecithin-type...

Lamellar electron diffraction intensity data from epitaxially crystallized 1,2-dipalmitoyl-sn-glycerophospho-N,N-dimethylethanolamine were used to determine the layer packing in order to compare the chiral structure to the crystal structure of a racemic homologue. After finding the chain orientation, the structure was determined by interpretation of the Patterson function, followed by independent crystallographic phase assignments with conventional direct methods (use of three phase structure invariants). The phase determination was verified by a translational search with a molecular model based on a similar lecithin structure. The final R-value is 0.29, and this is lowered to 0.18 after a correction is made for incoherent multiple electron scattering. The layer packing is found to be very much like that of a diacyl phosphatidylcholine with the N,N-dimethylethanolamine moiety parallel to the bilayer surface rather than the perpendicular arrangement of headgroups involved in an interdigitated layer, as seen for racemic homolog.