Glycerol Conformation and the Crystal Structure of Lipids II. A Further Study of Tripalmitin and Conformationally Fixed Analogs by Small-Angle X-Ray Diffraction and Reflection Electron Diffraction

Abstract

Abstract The utility of analogs to glycerol-containing lipids based on the configurational isomers of cyclo-pentane-1,2,3-triol for ab initio crystal structure analysis via electron diffraction is assessed further. Such analogs of tripalmitin are examined with the natural triglyceride via low angle X-ray diffraction. The 1,3/2 (all-trans) and 1,2/3 (cis-trans) analogs give long spacing dimensions some 23 Å greater than found for the β-2 form of the natural compound, consistent with the long spacing observed for a β-3 form. The 1,2,3/0 (all-cis) analog gives a long spacing near that of the a-form of the triglyceride. Reflection electron diffraction measurements on the 1,3/2 and 1,2/3 analogs reveal a chain tilt near 60° for both and untilted chains for 1,2,3/0. A more accurate tilt determination from X-ray long spacings of the homologous series of 1,3/2 pseudotrilaurin to 1,3/2 pseudo-tripalmitin confirm the 67° tilt expected for the β-3 form. Therefore, given the same T,, methylene subcell, the molecular packing is very close to natural triglycerides. The subtle influences of the cyclopentane ring are overcome for 1,3/2 analogs based on stearic and arachidic acids. This emphasizes the utility of these structural analogs for ab initio crystal structure determinations of glycerol containing lipids.