Different effects of long- and short-chain ceramides on the gel-fluid and lamellar-hexagonal transitions of phospholipids. A calorimetric, NMR and X-ray diffraction study

¹ Unidad de Biofísica
² Universidad de Murcia
³ Servicio general de Resonancia Magnetica Nuclear

^* To whom correspondence should be addressed. E-mail: gbpaliza{at}lg.ehu.es.

Submitted on December 14, 2004
Revised on January 5, 2005
Accepted on 25 January 2005

	Abstract

The effects on dielaidoylphosphatidylethanolamine (DEPE)bilayers of ceramides containing different N-acyl chains have been studied by differential scanning calorimetry(DSC), small angle X-ray diffraction and 31P-NMR spectroscopy. N palmitoyl (Cer16), N-hexanoyl (Cer6) and N-acetyl (Cer2) sphingosines have been used. Both the gel-fluid and the lamellar-inverted hexagonal transitions of DEPE have been examined in the presence of the various ceramides in the 0-25 mol % concentration range. Pure hydrated ceramides exhibit cooperative endothermic order-disorder transitions at 93 °C (Cer16), 60 °C (Cer6) and 54 °C(Cer2). In DEPE bilayers Cer16 does not mix with the phospholipid in the gel phase, giving rise to high-melting ceramide-rich domains. Cer16 favours the lamellar-hexagonal transition of DEPE, decreasing the transition temperature. Cer2 on the other hand is soluble in the gel phase of DEPE, decreasing the gel-fluid and increasing the lamellar-hexagonal transition temperatures, thus effectively stabilizing the lamellar fluid phase. In addition, Cer2 was peculiar in that no equilibrium could be reached for the Cer2-DEPE mixture above 60 °C, the lamellar-hexagonal transition shifting with time to temperatures beyond the instrumental range. The properties of Cer6 are intermediate between those of the other two, this ceramide decreasing both the gel-fluid and lamellar-hexagonal transition temperatures. Temperature-composition diagrams have been constructed for the mixtures of DEPE with each of the three ceramides. The different behaviour of the long- and short-chain ceramides can be rationalized in terms of their different molecular geometries, Cer16 favouring negative curvature in the monolayers, thus inverted phases, and the opposite being true of the micelle-forming Cer2. These differences may be at the origin of the different physiological effects that are sometimes observed for the long- and short-chain ceramides.

Key Words: 31P-NMR, ceramides, differential scanning calorimetry, lipid phase transitions, low-angle X-ray diffraction, non-lamellar lipid phases

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