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

This work concerns the importance of the type of intermolecular interactions present in aqueous lipid-assembly systems depending on the type of aggregates they form. We have studied aqueous mixtures of diglycerolmonooleate (DGMO), Capmul glycerolmonooleate (GMO-50) and polyoxyethylene (20) sorbitanmonooleate (Polysorbate 80, P80) using small angle X-ray scattering measurements to reveal the structure of liquid crystalline phases. Based on 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 focus on the bilayer type of system of different curvature and bilayer flexibility, namely lamellar, 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 molecular organization of the sponge phase were revealed by employing Raman difference spectroscopy and analysis of key spectroscopic indicators, which show less dense hydrocarbon chain packing compared to the inverse bicontinuous cubic or lamellar phase.