Structural polymorphism of hydrated ether-linked dimyristyl maltoside and melibioside.

The structural polymorphism of two selected disaccharide glycolipids with a maltose (DMMA) and a melibiose (DMME) carbohydrate headgroup linked to dimyristyl alkyl chains were investigated by FTIR-spectroscopy, differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS) and film-balance measurements. The compounds displayed thermotropic multilamellar phases. In the gel phase, DMMA formed also a crystalline phase of orthorhombic symmetry, and DMME an interdigitated phase. The gel to liquid crystalline phase transition temperature Tc of DMMA depended on the storage and hydration conditions, a precooled sample having a Tc around 45°C, and a freshly prepared sample around 33°C. In contrast, the phase transition temperature for the gel to liquid crystalline phase of DMME was always found at 24°C. Surface pressure isotherms of the lipids on water and buffer showed that DMMA covers only a small surface area (∼35Å2) whereas DMME requires 50Å2 of space on the surface. Films of DMMA can be compressed up to a maximum compressibility Πmax of 54mNm−1 whereas the tilted DMME forms less stable films with Πmax of 34mNm−1. These different structural characteristics reflect the different conformations of the disaccharide head groups. The presence of the α1→4 linked maltose head group in DMMA and an α1→6 linked melibiose head group in DMME induces geometrical structures ranging from a slightly wedge-shaped towards a more tilted structure, and as a consequence of Israelachvilis packing model, to the formation of different phases. In addition, the structural constraints of DMME allow the formation of a phase with interdigitated hydrocarbon chains.