INTERACTION OF CHOLESTEROL WITH GALACTOCEREBROSIDE AND GALACTOCEREBROSIDE-PHOSPHATIDYLCHOLINE BILAYER-MEMBRANES

The interaction of the galactocerebroside, N-palmitoylgalactosylsphingosine (NPGS), with cholesterol [from pig brains] was studied by differential scanning calorimetry (DSC) and X-ray diffraction. Thermal and structural studies demonstrated complex behavior characterized by 2 endothermic transitions: transition I (TI .simeq. 50-60.degree. C) corresponding to an NPGS-cholesterol bilayer gel .fwdarw. bilayer liquid crystal transition and transition II (TII where TI < TII < TNPGS) corresponding to an NPGS bilayer crystal (stable E form) .fwdarw. bilayer liquid crystal transition. For mixtures containing from 6-80 mol% cholesterol, X-ray diffraction studies at 22.degree. C (T < TI) indicated 2 separate lamellar phases; an NPGS crystal bilayer phase and a cholesterol monohydrate phase. For cholesterol concentrations < 50 mol % at TI < T < TII, NPGS-cholesterol liquid crystal bilayer and excess NPGS crystal bilayer phases were observed. For > 50 mol % cholesterol concentrations at these temperatures, an excess cholesterol monohydrate phase coexists with the NPGS-cholesterol liquid crystal bilayers. At T > TII, complete NPGS-cholesterol miscibility was only observed for < 50 and mol % cholesterol concentrations, whereas at > 50 mol % cholesterol an excess cholesterol phase was present. The solid phase immiscibility of cerebroside and cholesterol at low temperatures was suggested to result from preferential NPGS-NPGS associations via hydrogen bonding. The unique thermal and structural behavior NPGS-cholesterol dispersions was contrasted with the behavior of cholesterol-phosphatidylcholine and cholesterol-sphingomyelin bilayers. Thermal and structural studies of NPGS in dipalmitoylphosphatidylcholine (DPPC)/cholesterol (1:1, molar ratio) bilayers were performed. For dispersions containing < 20 mol % NPGS at 22.degree. C there were no observable calorimetric transitions; X-ray diffraction studies indicated complete lipid miscibility. At > 20 mol % NPGS, a high temperature transition was observed that was shown by X-ray diffraction studies to be due to an excess NPGS crystal bilayer .fwdarw. liquid crystal bilayer transition. Complete miscibility of NPGS in DPPC/cholesterol bilayers was observed at T > TNPGS. The properties of NPGS/DPPC/cholesterol bilayers were discussed in terms of the lipid composition of the myelin sheath.