PHYSICAL-PROPERTIES OF CERAMIDES - EFFECT OF FATTY-ACID HYDROXYLATION

The structural and thermotropic properties of alpha-hydroxy fatty acid (HFA) and non-hydroxy fatty acid (NFA) ceramides (CER) have been stud ied using differential scanning calorimetry (DSC) and X-ray diffractio n techniques. The DSC of anhydrous HFA-CER shows a single, sharp rever sible transition at 95.6 degrees C (Delta H = 15.3 kcal/mol). At inter mediate hydrations HFA-CER exhibited more complex behavior but at maxi mum hydration only a single reversible transition is observed at 80.0 degrees C (Delta H = 8.5 kcal/mol). X-ray diffraction of hydrated (74% water) HFA-CER at 20 degrees C shows a lamellar structure with a bila yer periodicity d = 60.7 Angstrom; a single wide angle reflection at 4 .2 Angstrom is characteristic of hexagonal chain packing. Above the ma in transition temperature at 91 degrees C, a hexagonal (H-II) phase is observed. In contrast, DSC of anhydrous NFA-CER demonstrates two ther mal transitions at 81.3 degrees C (Delta H = 6.8 kcal/mol) and 85.9 de grees C (Delta H = 3.5 kcal/mol). With increasing hydration, both tran sitions shift towards lower temperatures; at maximum hydration, on hea ting, the endothermic transitions occur at 72.7 degrees C (Delta H = 9 .8 kcal/mol) and 81.1 degrees C (Delta H = 4.0 kcal/mol). On cooling, there is hysteresis of both transitions. X-ray diffraction of NFA-CER (80% water) at 20 degrees C shows a well-ordered lamellar structure wi th a bilayer periodicity d = 58.6 Angstrom and three wide-angle reflec tions at 4.6 Angstrom, 4.2 Angstrom, and 3.8 Angstrom. At 77 degrees C (between the two transitions), again a lamellar structure exists with reduced bilayer periodicity d = 53.1 Angstrom and four wide-angle ref lections at 4.6 Angstrom, 4.2 Angstrom, and 3.8 Angstrom are observed. Above the second transition, only a single low angle reflection at 30 .0 Angstrom is observed; a diffuse reflection at 4.6 Angstrom is indic ative of a melted chain phase. Thus, HFA-CER exhibits a simple phase b ehavior involving the reversible conversion of a gel phase to a hexago nal phase (L(beta)-->H-II). However, NFA-CER shows a more complex poly morphic phase behavior involving two gel phases.