FTIR SPECTROSCOPY STUDIES OF THE CONFORMATIONAL ORDER AND PHASE-BEHAVIOR OF CERAMIDES

Ceramides, the major lipid component of the stratum corneum (SC), prov ide many of its unique physical properties. Surprisingly, only a few b iophysical studies of hydrated ceramides have been reported. The curre nt Fourier transform infrared (FTIR) spectroscopy investigation provid es the first detailed study of intermolecular and intramolecular chain and headgroup interactions in hydrated non-hydroxy fatty acid (NFA) a nd hydroxy fatty acid (HFA) ceramides. Information about NFA and HFA c eramide chain subcell structure and conformational order is derived fr om the temperature dependence of the methylene stretching, scissoring, and rocking mode frequencies. At low temperatures, NFA ceramide is hi ghly ordered and packed in an orthorhombic subcell structure that unde rgoes a solid-solid phase transition to a conformationally ordered hex agonal phase at 60 degrees C. A second transition to a conformationall y disordered non-bilayer-like phase occurs at 80 degrees C. The lipid chains of HFA ceramide undergo a single transition from a conformation ally ordered, orthorhombic subcell, phase to a conformationally disord ered phase at 76 degrees C. In NFA ceramide the amide I and II modes o f the headgroup are each split into two bands, indicating strong inter molecular headgroup coupling between two NFA ceramide headgroups in a factor group perpendicular to the bilayer plane. In contrast, splittin g is not observed for either amide mode in HFA ceramide. However, the presence of strong H bonding indicates an interaction between molecule s in the bilayer plane. The contrasting behavior of the headgroups in NFA and HFA ceramide suggests that these molecules make distinct contr ibutions to the structural integrity of the stratum corneum. The impli cations of these findings to the recently proposed domain mosaic model of the stratum corneum lipid barrier are discussed.