FTIR Spectroscopy Studies of the Conformational Order and Phase Behavior of Ceramides

Abstract

Ceramides, the major lipid component of the stratum corneum (SC), provide many of its unique physical properties. Surprisingly, only a few biophysical studies of hydrated ceramides have been reported. The current Fourier transform infrared (FTIR) spectroscopy investigation provides the first detailed study of intermolecular and intramolecular chain and headgroup interactions in hydrated non-hydroxy fatty acid (NFA) and hydroxy fatty acid (HFA) ceramides. Information about NFA and HFA ceramide chain subcell structure and conformational order is derived from the temperature dependence of the methylene stretching, scissoring, and rocking mode frequencies. At low temperatures, NFA ceramide is highly ordered and packed in an orthorhombic subcell structure that undergoes a solid−solid phase transition to a conformationally ordered hexagonal phase at 60 °C. A second transition to a conformationally disordered non-bilayer-like phase occurs at 80 °C. The lipid chains of HFA ceramide undergo a single transition from a conformationally ordered, orthorhombic subcell, phase to a conformationally disordered phase at 76 °C. In NFA ceramide the amide I and II modes of the headgroup are each split into two bands, indicating strong intermolecular headgroup coupling between two NFA ceramide headgroups in a factor group perpendicular to the bilayer plane. In contrast, splitting is not observed for either amide mode in HFA ceramide. However, the presence of strong H bonding indicates an interaction between molecules in the bilayer plane. The contrasting behavior of the headgroups in NFA and HFA ceramide suggests that these molecules make distinct contributions to the structural integrity of the stratum corneum. The implications of these findings to the recently proposed domain mosaic model of the stratum corneum lipid barrier are discussed.

The physical chemistry of the stratum corneum lipids

“…The ceramide molecules have different headgroup architectures, all of them considerably smaller and less polar than those of the lipids in a typical phospholipid membrane. The different classes of ceramides molecules can participate in different inter-and intramolecular interactions with the other lipid molecules and thus make distinct contributions to the organization and the structural integrity of the lipid matrix [31][32][33][34][35]. Both the ceramides and the fatty acids of the SC have unusually long hydrocarbon chains with a broad distribution of lengths [29,30,[35][36][37].…”

Progress in the analysis of Stratum corneum ceramides

“…Phase transitions can be recognised as abrupt changes in band positions or band intensities. In that way, the phase behavior of natural ceramides extracted from biological sources has been investigated [102][103][104][105]. For synthetic ceramides of phytosphingosine type, the polymorphism and the hydrogen-bonding network of the headgroups have been elucidated [106][107][108][109][110][111].…”

Properties of Ceramides and Their Impact on the Stratum Corneum Structure:

“…In monolayers the chain packing is hexagonal, in contrast to the orthorhombic packing in multilayers. A splitting of the head group amide IR band, observed in multilamellar samples [51] , is not found. This absence of amide mode splitting supports the assumption that in CER[NS] bulk samples there is a transversal hydrogen bonding between opposite head groups in adjacent bilayers; this arrangement is impossible for the monolayer.…”