On the Molecular Interactions in Lipid Bilayer-Water Assemblies of Different Curvatures

This work concerns the importance of intermolecular interactions present in aqueous lipid assembly systems depending on the type of aggregates they form. We have studied aqueous mixtures of diglycerol monooleate, Capmul glycerol monoleate (GMO-50) and polyoxyethylene (20) sorbitan monooleate (Polysorbate 80, P80) using small-angle X-ray scattering (SAXS) measurements to reveal the structure of liquid crystalline phases. On the basis of the SAXS data, a phase diagram was constructed. We discuss the effect of curvature changes of the lipid-aqueous interface obtained by changing the water content and the temperature. The results are related to the intermolecular interactions, as revealed by Raman spectroscopy, with a focus on the bilayer type of system of different curvature and bilayer flexibility, namely, the lamellar phase, bicontinuous cubic phase, and sponge phase. All phases show large similarities in their chain conformation and head group interactions as revealed by the Raman spectra, arising from the fact that all three structures are formed by lipid bilayers. However, subtle differences in the molecular organization of the sponge phase were revealed by employing Raman difference spectroscopy and by analysis of key spectroscopic indicators, which show a less dense hydrocarbon chain packing compared to the inverse bicontinuous cubic or lamellar phase.