Aqueous-phase behavior of natural glycolipid biosurfactant mannosylerythritol lipid A: sponge, cubic, and lamellar phases

The aqueous-phase behavior of mannosylerythritol lipid A (MEL-A), which is a glycolipid biosurfactant produced from vegetable oils by yeast strains of the genus Pseudozyma, was investigated using polarized optical microscopy, small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). MEL-A was found to self-assemble into a variety of distinctive lyotropic liquid crystals including sponge (L3), bicontinuous cubic (V2), and lamella (Lalpha) phases. On the basis of SAXS measurements, we determined the structure of the liquid crystals. The estimated lattice constant for Lalpha was 3.58 nm. DSC measurement revealed that the phase transition enthalpies from the liquid crystal to the fluid isotropic phase were in the range of 0.22-0.44 kJ/mol. Although the present MEL-A phase diagram closely resembled that obtained from relatively hydrophobic poly(oxyethylene) or fluorinated surfactants, the MEL-A L3 region was spread considerably over a wide temperature range (20-65 degrees C) compared to L3 of those surfactants: this is probably due to the unique structure which is molecularly engineered by microorganisms. In this paper, we clarify the aqueous phase diagram of the natural glycolipid biosurfactant MEL-A, and we suggest that the obtained lyotropic crystals are potentially useful as novel nanostructured biomaterials.     

Photooxidative mineralization of microorganisms-produced glycolipid biosurfactants by a titania-mediated advanced oxidation process

The photooxidative mineralizations of two microorganisms-produced glycolipid biosurfactants 4-O-(4′,6′-di-O-acetyl-2′,3′-di-O-alkanoyl-β-d-mannopyranosyl)-d-erythritol (MEL-A) and 1-O-(6′-O-acetyl-2′,3′-di-O-alkanoyl-β-d-mannopyranosyl)-d-erythritol (MEL-B) were examined by monitoring the temporal changes in UV absorption, the time profiles of CO₂ evolution and the changes in interfacial tension occurring by an advanced oxidation process in the presence of a metal-oxide (TiO₂). Features of their mineralization are compared to the photomineralization of the anionic dodecylbenzene sulfonate (DBS) surfactant carried out under otherwise identical conditions. The adsorption of surfactants on the TiO₂ surface and the positions of attack of the photogenerated OH radicals on the surfactants’ structure were assessed by molecular orbital (MO) calculations of partial charges and frontier orbital electron densities, respectively. The photodegradation of DBS was faster than the MELs as also evidenced by surface tension measurements, whereas the photomineralization of the anionic DBS surfactant was definitely slower than that of the MEL biosurfactants due to the hydrophobic alkyl chain in the DBS structure. Possible initial mechanistic stages of the photooxidation of MEL-A and MEL-B are proposed based on experimental data and comparison with MO calculations.     

Phase behavior of ternary mannosylerythritol lipid/water/oil systems

Due to the great importance of new therapeutic routes for morphine in pain treatment, several investigations are under development. In this way, the design of a liquid system for the oral administration of morphine would be of great help, especially in patients with difficulties in swallowing (children and elderly people). The systems studied in this work are kollidon^® SR microparticles, a biodegradable polymer classically used as excipient in the design of solid dosage forms, as vehicles for morphine. A detailed investigation of the capabilities of the polymer particles to load this drug at their surface is described. Electrophoretic mobility and optical absorbance determinations were used with this aim. The main factors determining the drug incorporation, after incubation of the microparticles in the morphine solutions, were the adsorption time, the type of electrolyte and its concentration, and the drug concentration. The optimum loading conditions were used to perform morphine release evaluations, finding that the release profiles were biphasic since the drug adsorbed was slowly released during 24 h after an initial burst release phase.