Structure of Cerebrosides I. Phrenosine at 23 °C and 66 °C

Abstract

Small angle and wide angle powder diffraction diagrams (SAXS and WAXS) have been ob­tained from samples of dry phrenosine cerebrosides prepared from beef brain. Two kinds of lamellae bundles exist within the powder at 23 °C. The first (structure I) with 15% of the molecules having a long period P of 65 Å and a paracrystalline g-value (Hosemann and Bagchi [1962]) of 4.2% consists of orthorhombic subcells similar to CnH2n + 2 (n ≧ 26) buth with larger lateral dimensions. It has an angle of 60° between the chain direction and the lamellae surface which is parallel to the (201) netplanes of the paraffin-like subcells. The second (structure II) constitutes 22% of these lamellae and 63% with a period of 49 Å and triclinic subcells similar to C20H42 . The angle between the chain direction and lamellae surface is here 45°. The SAXS reflections of structure II are analysed with a bimodal statistic which proves that the 65 Åand 49 Å lamellae are mixed statistically. At 66 °C only a structure III exists with a 52 Å long periodicity. Here all orthorhombic lamellae are built in but now their (301) netplanes are parallel to the lamellae surface. They obviously stabilize the structure instead of the (201)-netplanes which stabilize at 23 °C. A Fourier-analysis of the first 4 SAXS reflections of the relatively undistorted structures II and III projected to the normal of the lamellae surface shows a series of 21 Å wide step functions with a periodicity of 51 or 52 Å. This proves directly a head to head and teil to tail structure. The 21 Å wide heads are attributed to two galactose rings with their amide linkages and oxygens of the fatty acids and sphingosines. Knowing their size (9 Å) one unequivocally can phase the other 6 SAXS reflections which give rise to a gap of some Å’s thick between them. The gap is deeper for structure II because here lamellae of two different thicknesses are mixed. Two further small gaps in the center of the bilayers indicate the ends of the double chain molecules. From the line widths of WAXS it is evident that small paracrystallites (mPC’s) exist with 40 to 100 Å lateral dimensions. From SAXS line widths one concludes that bundles of 4 to 7 bilayers build up macroparacrystallites (MPC’s), where each bilayer consists of 10 to 25 mPC’s. Obviously the galactose rings on the surface of the bilayers produce much larger paracrystalline distortions than the methyl-end-groups in paraffins but much smaller ones than the backfolds in mats of polyethylene single crystals. Consequently 4 to 7 lamellae can be found in each MPC. The galactose rings are the cause of larger lateral distances between the paraffin-like chains and generate distortions cor­responding to a g-value of ~2.6% in the mPC’s. This is explained by the high degree of con­formational variety that the chains adopt which is larger than in synthetic polyethylene. This would further explain why all our attempts to obtain better crystallinity failed.