Lipase-Catalyzed Synthesis of Sugar Esters in Honey and Agave Syrup

Abstract

Honey and agave syrup are high quality natural products and consist of more than 80% sugars. They are used as sweeteners, and are ingredients of cosmetics or medical ointments. Furthermore, both have low water content, are often liquid at room temperature and resemble some known sugar-based deep eutectic solvents (DES). Since it has been shown that it is possible to synthesize sugar esters in these DESs, in the current work honey or, as vegan alternative, agave syrup are used simultaneously as solvent and substrate for the enzymatic sugar ester production. For this purpose, important characteristics of the herein used honey and agave syrup were determined and compared with other available types. Subsequently, an enzymatic transesterification of four fatty acid vinyl esters was accomplished in ordinary honey and agave syrup. Notwithstanding of the high water content for transesterification reactions of the solvent, the successful sugar ester formation was proved by thin-layer chromatography (TLC) and compared to a sugar ester which was synthesized in a conventional DES. For a clear verification of the sugar esters, mass determinations by ESI-Q-ToF experiments and a NMR analysis were done. These environmentally friendly produced sugar esters have the potential to be used in cosmetics or pharmaceuticals, or to enhance their effectiveness.

Keywords: agave syrup; deep eutectic solvent; glycolipid synthesis; honey; lipase; sugar ester; transesterification.

Figure 1

Visualization of synthesized glycolipids in honey and agave syrup after dying with an anisaldeyhde solution. Ten microliters of the extracts and four microliters of the standards were spotted on the TLC plate. VP, vinyl palmitate; VL, vinyl laurate; VD, vinyl decanoate; VO, vinyl octanoate; H, synthesis in honey; A, synthesis in agave syrup; Std, lab intern rhamnolipid standard; Glu, glucose based DES with VO; Fru, fructose based DES with VO; G, glucose, solved in an ethanol-water mixture; F, fructose in an ethanol-water mixture.

Figure 2

Lipase-catalyzed esterification between glucose and vinyl octanoate leads to the formation of glucose-octanoate and ethanol. The vinyl alcohol tautomerizes to acetaldehyde and evaporates, which prevents the back reaction. The same scheme applies to fructose and other fatty acid vinyl esters.