Structure and properties of totally synthetic galacto- and gluco-cerebrosides.

The structural and thermal properties of aqueous dispersions of the totally synthetic cerebrosides, D -erythroN-palmitoyl galactosyland glucosyl-C 18 -sphingosine (C16:0GalCer and C16:0-GluCer, respectively) have been studied using differential scanning calorimetry (DSC) and X-ray diffraction. Over the temperature range 0–100 8 C, both C16:0GalCer and C16:0-GluCer show complex thermal transitions characteristic of polymorphic behavior of exclusively bilayer phases. On heating, hydrated C16:0-GalCer undergoes an exothermic bilayer–bilayer transition at 59 8 C to produce a stable bilayer crystal form. X-ray diffraction at 70 8 C reveals a bilayer structure with an ordered hydrocarbon chain-packing arrangement. This ordered bilayer phase undergoes an endothermic chain-melting transition at 85 8 C to the bilayer liquid crystalline state. Similar behavior is exhibited by hydrated C16:0-GluCer which undergoes the exothermic transition at 49 8 C and a chain-melting transition at 87 8 C. The exothermic transitions observed on heating hydrated C16:0-GalCer and C16:0-GluCer are irreversible and dependent upon previous chain melting, prior cooling rate, and time of incubation at low temperatures. Thus, the structure and properties of totally synthetic C16:0-GalCer and C16:0-GluCer with identical sphingosine (C18:1) and fatty acid (C16:0) chains are quite similar, suggesting that the precise isomeric structure of the linked sugar plays only a minor role in regulating the properties of hydrated cerebrosides. Further, these studies indicate that the complex thermal behavior and bilayer phase formation exhibited by these single-sugar cerebrosides are intrinsic properties and not due to the heterogeneity of the sphingosine base found in natural and partially synthetic cerebrosides. —Saxena, K., R. I. Duclos, P. Zimmermann, R. R. Schmidt, and G. G. Shipley. Structure and properties of totally synthetic galactoand gluco-cerebrosides. J. Lipid Res. 1999. 40: 839–849. Supplementary key words differential scanning calorimetry • X-ray diffraction • lipid bilayers • hydrocarbon chain packing • hydrogen bonding • glycolipids Glycosphingolipids, GSL, are complex lipids with a lipophilic component (ceramide composed of a long-chain fatty acid amide-linked to a long-chain base, usually sphingosine) and a hydrophilic sugar moiety attached via an ether linkage to sphingosine. GSL are found in relatively large amounts in the myelin sheath of central and peripheral nervous system. Glycosphingolipids can also act as cell surface receptors for hormones, bacterial toxins and viruses (1–3). We have been interested in defining the structure and properties of GSL of increasing oligosaccharide complexity with a view to understanding both their structural role in membranes and their functional role as receptors; an example of the latter would include our studies of the ganglioside G M1 -cholera toxin complex (4–7). In this paper, we focus on the properties of two “simple” monoglycosyl GSL, galactoand gluco-cerebroside, GalCer and GluCer, respectively. The physical properties of both natural and partially synthetic cerebrosides have been studied by X-ray diffraction, differential scanning calorimetry (DSC), and spectroscopic methods. For example, early X-ray diffraction studies showed that bovine brain cerebrosides containing predominantly long chain fatty acids (C24:0, C24:1, and C18:0) undergo a chain-melting transition at a relatively high temperature (8). Using DSC, Curatolo (9) studied two subfractions of bovine brain cerebrosides, those containing amide-linked 2-hydroxy fatty acids and amidelinked nonhydroxylated fatty acids (HFA-CER and NFACER, respectively). NFA-CER exhibits two low temperature phases, a metastable phase converting exothermically to a stable phase at , 50 8 C ( D H 5 16–17 cal/g). In contrast, HFA-CER displays no metastability and exhibits a reversible transition at , 68 8 C ( D H 5 7.3 cal/g). Interestingly, Pascher and coworkers (10–12) have suggested that the Abbreviations: DSC, differential scanning calorimetry; C16:0-GalCer, d -erythro-N-palmitoyl-galactosyl-C 18 -sphingosine; C16:0-GluCer, d -erythro-N-palmitoyl-glucosyl-C 18 -sphingosine; GSL, glycosphingolipids; sulfatide, sulfated galactocerebroside; HIV, human immunodeficiency virus. 1 Present address: Department of Cancer Biology, Dana Farber Cancer Research Institute and Harvard Medical School, Boston, MA 02115. 2 To whom correspondence should be addressed. by gest, on Jne 3, 2013 w w w .j.org D ow nladed fom