Aliphatic chain packing in three crystalline polymorphs of a saturated racemic phosphatidylethanolamine. A quantitative electron diffraction study.

Taking advantage of the adequacy of the kinematical diffraction assumption for ab initio crystal structure analysis of long chain lipids with single crystal electron diffraction intensity data, the polymethylene chain packing in three microcrystalline polymorphs of 1,2-dipalmitoyl-rac-glycero-3-phosphoethanolamine was determined. The aliphatic chains of major crystal form pack in the new parallel/perpendicular orthorhombic methylene subcell already seen for three other compounds. The unit cell dimension s of this form are: a = 7.76 A, b = 10.03 A, c = 56.0 A. The two minor polymorphs pack in commonly-found subcells, i.e. perpendicular orthorhomibic (a(s) = 5.10 A, b(s) = 7.61 A) and hexagonal (a(s) = 4.68 A, gamma = 120 degrees). Similarities between the two orthorhombic subcells are discussed.