To define the thermotropic behavior of galactosylceramides (GalCer) containing cis monounsaturated acyl chains, N-X:1(Δ(X-9) cis) galactosylsphingosines (GalSph) were synthesized (where X=24, 22, 20, or 18) and investigated by differential scanning calorimetry (DSC). After hydration of dried glycolipid, aqueous dispersions were prepared by repetitive heating and freeze-thaw cycles. The DSC data clearly showed that introducing a single cis double bond into the acyl chain of GalCer lowers the transition temperature of the main endothermic peak and affects the kinetics of formation of various metastable and stable gel phases. More importantly, the data emphasize the role that double bond location in concert with acyl chain length play in modulating the thermotropic behavior of GalCers. In contrast to the 18:1 GalCer and 20:1 GalCer endotherms which remain unchanged after identical repetitive heating scans and low temperature incubations, the thermotropic responses of 22:1 GalCer and 24:1 GalCer depended directly upon incubation time at lower temperatures following a heating scan. Only after extended incubation (4-5 days) did the endotherms revert to behavior observed during the initial heating scan that followed sample preparation by cyclic heating and freeze-thaw methods. The extended incubation times required for 22:1 GalCer and 24:1 GalCer to assume their more stable packing motifs appear to be consistent with nucleation events that promote transbilayer interdigitation. Yet, due to the slow kinetics of the process, the presence of cis monounsaturation in very long acyl chains that are common to GalCer may effectively inhibit transbilayer lipid interdigitation under physiological conditions. Copyright (C) 1998 Elsevier Science B.V.
Bibliographical noteFunding Information:
We thank Jan Smaby-Cook for assistance with the data analysis and presentation as well as Fred Phillips and Dr. Margot Cleary for their help with the capillary GC analyses of the glycolipid derivatives, and Carmen Hotson for secretarial services. The authors gratefully acknowledge the major support provided by USPHS Grant GM45928 and the Hormel Foundation.
- Differential scanning calorimetry
- Human immunodeficiency virus glycolipid receptor
- Hydrocarbon chain-length asymmetry
- Metastable phase formation