Beechwood carbohydrates for enzymatic synthesis of sustainable glycolipids

Abstract

Moving away from crude oil to renewable resources for the production of a wide range of compounds is a challenge for future generations. To overcome this, the use of lignocellulose as substrate can contribute to drastically reduce the consumption of crude oil. In this study, sugars from lignocellulose were used as a starting material for the enzymatic synthesis of surface-active sugar esters. The substrates were obtained by an acid-catalyzed, beechwood pretreatment process, which resulted in a fiber fraction that is subsequently hydrolyzed to obtain the monosaccharides. After purification and drying, this glucose- and xylose-rich fraction was used to create a deep eutectic solvent, which acts both as solvent and substrate for the lipase-catalyzed reaction at the same time. Finally, the successful synthesis of glycolipids from a sustainable resource was confirmed by ESI-Q-ToF mass spectrometry and multidimensional NMR experiments. Moreover, conversion yields of 4.8% were determined by LC-MS/MS.

Keywords: Deep eutectic solvents; Glycolipid synthesis; Lignocellulose; Lipase; Transesterification.

Fig. 1

The TLC shows the glycolipid standards glucose–octanoate (1) and xylose–octanoate (2), synthesized with vinyl-octanoate. S is a lab-intern rhamnolipid standard, which acts as a positive control for the TLC. P1–P4 shows extracts of the reaction in a CC:sugar mix DES with vinyl-octanoate as substrate. At a Rf between 0.1 and 0.2 there is mainly the sugars, glucose and xylose, out of the DES, which are comparable with the pure sugar standards of glucose (Gluc) and xylose (Xyl). The brownish double spots and the purple spots (P1–P4) represent different glucose–octanoates (Rf 0.4–0.5) and the slightly spot with a Rf of 0.65–0.75 xylose–octanoate. The spot on the height of the running front indicates more non-polar products, like di- or poly-acylated sugars. A negative control without enzyme and without lipase shows only glucose and xylose spots (Contr.)

Fig. 2

Transesterification between glucose and vinyl-octanoate which leads to the formation of glucose–4–O–octanoate and ethenol. Ethenol is not stable and tautomerizes to acetaldehyde. Acetaldehyde evaporates quickly, pushing the reaction forward. The reaction scheme is analogous for other monosaccharides like xylose. It might be possible that more or other C-atoms are acylated, too. Using a fatty acid like octanoic acid, one molecule of water will be formed as a side product