Aqueous-phase behavior and vesicle formation of natural glycolipid biosurfactant, mannosylerythritol lipid-B

Abstract

Mannosylerythritol lipids (MELs) are one of the most promising glycolipid biosurfactants produced by yeast strains of the genus Pseudozyma. In this study, the aqueous-phase behavior of a new monoacetyl MEL derivative, 1- O-β-(2′,3′-di- O-alka(e)noyl-6′- O-acetyl- d-mannopyranosyl)- d-erythritol (MEL-B), was investigated using polarized optical microscopy, small-angle X-ray scattering (SAXS), confocal laser scanning microscopy (CLSM), and differential scanning calorimetry (DSC). The present MEL-B was found to self-assemble into a lamellar (L α) phase over remarkably wide concentration and temperature ranges. According to SAXS measurement, the interlayer spacing ( d) was estimated to be almost constant (about 4.7 nm) at the low MEL-B concentration (≤60 wt.%) region where the L α phase is in equilibrium with the excess water phase (L α + W). On the other hand, at high MEL-B concentration (>60 wt.%) region, the d-spacing gradually decreased to 3.1 nm with an increase in the MEL-B concentration. The thermal stability of the liquid crystalline phase was investigated by DSC measurement. The obtained L α phase was found to be stable up to 95 °C below a MEL-B concentration of 85 wt.%; then, the melting temperature of the liquid crystalline phase dramatically decreased with an increase in MEL-B concentration (above 85 wt.%). Furthermore, we found relatively large vesicles (1–5 μm) at the low MEL-B concentration using CLSM observation. The trapped volume of the obtained MEL-B vesicle was estimated to be about 0.42 μL/μmol by glucose dialysis method. These results suggest that the natural glycolipid biosurfactant, the newly found MEL-B, would be useful in various fields of applications as an L α phase- and/or vesicle-forming lipid.