Enzymatic synthesis of a novel glycolipid biosurfactant, mannosylerythritol lipid-D and its aqueous phase behavior.

Mannosylerythritol lipids (MELs) produced by yeasts are one of the most promising glycolipid biosurfactants. In this study, we succeeded in the preparation of a novel MEL homolog having no acetyl groups, namely MEL-D. MEL-D was synthesized by lipase-catalyzed hydrolysis of acetyl groups from a known MEL, and identified as 4-O-[2',3'-di-O-alka(e)noyl-β-d-mannopyranosyl]-(2R,3S)-erythritol. The obtained MEL-D showed a higher critical aggregation concentration (CAC=1.2 × 10(-5)M) and hydrophilicity compared to known MELs, retaining an excellent surface tension lowering activity (the surface tension at the CAC was 24.5mN/m). In addition, we estimated the binary phase diagram of the MEL-D-water system based on a combination of visual inspection, polarized optical microscopy, and SAXS measurement. From these results, MEL-D was found to self-assemble into a lamellar (L(α)) structure over all ranges of concentration. Meanwhile, the one-phase L(α) region of MEL-D was extended wider than those of known MELs. MEL-D might keep more water between the polar layers in accordance with the extension of the interlayer spacing (d). These results suggest that the newly obtained MEL-D would facilitate the application of MELs in various fields as a lamellar-forming glycolipid with higher hydrate ability.