Interfacial interactions of ceramide with dimyristoylphosphatidylcholine: Impact of the N-acyl chain

The mixing behavior of dimyristoylphosphatidylcholine (DMPC) with either N- palmitoyl-sphingosine (C16:0-ceramide) or N-nervonoyl-sphingosine (C24:1-ce ramide) was examined using monomolecular films. While DMPC forms highly ela stic liquid-expanded monolayers, both neat C16:0-ceramide and C24:1-ceramid e yield stable solid condensed monomolecular films with small areas and low interfacial elasticity. Compression isotherms of mixed C16:0-ceramide/DMPC films exhibit an apparent condensation upon increasing X-cer16:0 at all su rface pressures. The average area isobars, coupled with the lack of a liqui d-expanded to condensed phase transition as X-cer16:0 is increased, are ind icative of immiscibility of the lipids at all surface pressures. In contras t, isobars for C24:1-ceramide/DMPC mixtures show surface pressure-dependent apparent condensation or expansion and surface pressure-area isotherms sho w a composition and surface pressure-dependent phase transition. This sugge sts miscibility, albeit non-ideal, of C24:1-ceramide and DMPC in both liqui d and condensed surface phases. The above could be verified by fluorescence microscopy of the monolayers and measurements of surface potential, which revealed distinctly different domain morphologies and surface potential val ues for the DMPC/ C16:0- and DMPC/C24:1-ceramide monolayers. Taken together , whereas C16:0-ceramide and DMPC form immiscible pseudo-compounds, C24:1-c eramide and DMPC are partially miscible in both the liquid-expanded and con densed phases, and a composition and lateral pressure-dependent two-phase r egion is evident between the liquid-expanded and condensed regimes. Our res ults provide novel understanding of the regulation of membrane properties b y ceramides and raise the possibility that ceramides with different acyl gr oups could serve Very different functions in cells, relating to their diffe rent physicochemical properties.