CRYSTALLOGRAPHIC AND SPECTROSCOPIC STUDIES OF 1,2-SN-DIPALMITOYLGLYCEROL WITH A COMMENT ON LIPID PHASES AND MEMBRANES

The crystal structure of 1,2-sn-dipalmitoylglycerol (DPG) has been stu died with diffraction data collected at 87 K. The data were of excelle nt quality, and a very precise description of this structure was obtai ned. The crystallographic refinement converged at R = 0.0256. The mole cular conformation of DPG at low temperature confirms the results from a less precise room temperature study (Dorset and Pangborn, 1988); it is similar to that of the dilauroyl analog (Pascher, Sundell, and Hau ser, 198 1) but differs from some studied phospholipids. The refined b onding parameters of the hydrocarbon chains are remarkably homogeneous . A semiquantitative correction for charge asphericity and rigid body motion gave mean values 1.530 angstrom/112.9-degrees for the C-C bond lengths and C-C-C angles along the chains. This agrees favorably with the results from gas electron diffraction studies of various alkanes. The molecular packing involves one H bond and several 0 ... H contacts , of which some are shorter than the normal van der Waals type, indica ting a certain amount of electrostatic interaction. Raman and infrared spectroscopic data for DPG and other glycerides have been reexamined in the light of the new crystallographic results. The carbonyl stretch ing mode assignments comply with rules relating frequency and chain fo lding (Mushayakarara and Levin, 1982); a precise correlation with chai n conformation is not apparent. Interpretation of the rocking-twisting and Raman longitudinal modes was only in part successful. This spectr al region is rich in detail and very sensitive even to small differenc es in structure. Many more studies combining spectral and structural o bservations are needed to establish more precise relations. The orient ation of the methyl groups, and thus of the chains themselves, was pre dicted previously from Raman spectroscopy. These assignments are confi rmed by the crystallographic study. Asymmetric C-H stretching modes, s hifted up about 17 cm-1 relative to the values commonly accepted for h ydrocarbons, were ascribed to the methyl group on the sn-2 chain. This shift can now be associated with an observed shift of methyl carbon C (36) 0.52 angstrom out of the main chain plane.