Reflectance and Topography of Glycosphingolipid Monolayers at the Air-Water Interface

In this paper we report the dynamic response of the lateral topography and reflectance of monolayers of glycosphingolipids under changes of surface pressure (π) and composition. Epifluorescence and Brewster angle microscopy (BAM) observations have been carried out on pure monolayers formed with ceramide (Cer) and a series of glycosphingolipids (galactocerebroside, GM1 ganglioside, and asialo-GM1), and on mixed films formed with Cer and GM1. The curves of reflectance versus π for films of pure components are very dependent on the type and length of the oligosaccharide chain in the polar headgroup and are in agreement with the phase state of the π-area isotherms and with the overall molecular reorientation under compression shown by the variation of the perpendicular resultant dipole moments. In turn, the surface appearance of films formed with mixtures of GM1 and Cer is homogeneous as assessed by BAM, but epifluorescence microscopy reveals the coexistence of expanded and condensed phase domains that are under the limit of resolution of our BAM. The addition of Cer, which has a small headgroup, to GM1, which has the largest headgroup among the lipids studied, produced an average decrease of the dependence of reflectance on π (dI/dπ). Also, we analyzed the changes of reflectance and of dI/dπ with respect to the proportion of fluorescent and nonfluorescent phases. On the basis of these data it appears that the incorporation of GM1 in the Cer-enriched phase induces a drastic increase of reflectance without marked changes of dI/dπ with respect to phases of pure Cer. Cer incorporated into GM1-enriched phases of the mixed monolayers does not affect markedly the optical properties (reflectance and dI/dπ values) compared to films of pure GM1.