Physical properties of ceramides: Effect of fatty acid hydroxylation

Abstract

The structural and thermotropic properties of α-hydroxy fatty acid (HFA) and non-hydroxy Fatty acid (NFA) ceramics (CER) have been studied using differential scanning calorimetry (DSC) and X-ray diffraction techniques. The DSC of anhydrous HFA-CER shows a single, sharp reversible transition at 95.6°C (ΔH = 15.3 kca/mol). At intermediate hydrations HFA-CER exhibited more complex behavior but at maximum hydration only a single reversible transition is observed at 80.0°C (ΔH = 8.5 kcal/mol). X-ray diffraction of hydrated (74% water) HFA-CER al 20°C shows a lamellar structure with a bilayer periodicity d = 60.7 Å; a single wide angle reflection at 4.2 Å is characteristic of hexagonal chain packing. Above tim main transition temperature at 91°C, a hexagonal (H(II)) phase is observed. In contrast, DSC of anhydrous NFA-CER demonstrates two thermal transitions at 81.3°C (ΔH = 6.8 kcal/mol) and 85.9°C (ΔH = 3.5 kcal/mol). With increasing hydration, both transitions shift towards lower temperatures; at maximum hydration, on beating, the endothermic transitions occur at 72.7°C (ΔH = 9.8 kcal/mol) and 81.1°C (ΔH = 4.0 kcal/mol). On cooling, there is hysteresis of both transitions. X-ray diffraction of NFA-CER (80% water) at 20°C shows a well- ordered lamellar structure with a bilayer periodicity d = 58.0 Å and three wide-angle reflections at 4.6 Å, 4.2 Å, and 3.8 Å. At 77°C (between the two transitions), again a lamellar structure exists with reduced bilayer periodicity d = 53.1 Å and four wide-angle reflections at 4.6 Å, 4.2 Å, and 3.8 Å are observed. Above the second transition, only a single low angle reflection at 30.O Å is observed; a diffuse reflection at 4.6 Å is indicative of a melted chain phase. Thus, HFA-CER exhibits a simple phase behavior involving the reversible conversion of a gel phase to a hexagonal phase (L(β) → H(II)). However, NFA-CER shows a more complex polymorphic phase behavior involving two gel phases.