Optimization of Glycolipid Synthesis in Hydrophilic Deep Eutectic Solvents

Abstract

Glycolipids are considered an alternative to petrochemically based surfactants because they are non-toxic, biodegradable, and less harmful to the environment while having comparable surface-active properties. They can be produced chemically or enzymatically in organic solvents or in deep eutectic solvents (DES) from renewable resources. DES are non-flammable, non-volatile, biodegradable, and almost non-toxic. Unlike organic solvents, sugars are easily soluble in hydrophilic DES. However, DES are highly viscous systems and restricted mass transfer is likely to be a major limiting factor for their application. Limiting factors for glycolipid synthesis in DES are not generally well understood. Therefore, the influence of external mass transfer, fatty acid concentration, and distribution on initial reaction velocity in two hydrophilic DES (choline:urea and choline:glucose) was investigated. At agitation speeds of and higher than 60 rpm, the viscosity of both DES did not limit external mass transfer. Fatty acid concentration of 0.5 M resulted in highest initial reaction velocity while higher concentrations had negative effects. Fatty acid accessibility was identified as a limiting factor for glycolipid synthesis in hydrophilic DES. Mean droplet sizes of fatty acid-DES emulsions can be significantly decreased by ultrasonic pretreatment resulting in significantly increased initial reaction velocity and yield (from 0.15 ± 0.03 μmol glucose monodecanoate/g DES to 0.57 ± 0.03 μmol/g) in the choline: urea DES. The study clearly indicates that fatty acid accessibility is a limiting factor in enzymatic glycolipid synthesis in DES. Furthermore, it was shown that physical pretreatment of fatty acid-DES emulsions is mandatory to improve the availability of fatty acids.

Keywords: Candida antarctica lipase B; deep eutectic solvents; enzymatic synthesis; glycolipid; mass transfer; viscosity.

FIGURE 1

Reaction scheme of enzymatic synthesis of glucose monodecanoate.

FIGURE 2

Comparison of three extraction solvents, dimethyl carbonate (DMC), ethyl acetate (EtAc) and chloroform (CHCl₃), for glucose decanoate extraction from ChCl:Glc (ChCl:Glc:water, 5:2:5, n:n:n) and ChCl:U (ChCl:U, 1:2, n:n, 5% water). a, b, and c show statistically significant differences.

FIGURE 3

Initial reaction velocity in relation to the agitation rate. Glucose monodecanoate was determined directly by product quantification. Reaction conditions: 0.5 M vinyl decanoic acid, 50°C. a and b shows statistically significant differences. ChCl:Glc (ChCl:Glc:water, 5:2:5, n:n:n) and ChCl:U (ChCl:U, 1:2, n:n, 5% water).

FIGURE 4

Impact of different fatty acid concentrations on the initial reaction velocity in ChCl:U (ChCl:U, 1:2, n:n, 5% water) and ChCl:Glc (ChCl:Glc:water, 5:2:5, n:n:n). Glucose monodecanoate was determined directly by product quantification. Reaction conditions: 90 rpm, 50 C. a and b shows statistically significant differences between the fatty acid concentrations. * shows statistically significant differences between the two DES.

FIGURE 5

Microscopic pictures of untreated and sonicated fatty acid-DES emulsions. (A) Untreated fatty acid-ChCl:Glc emulsion, (B) sonicated fatty acid-ChCl:Glc emulsion, (C) untreated fatty acid-ChCl:U emulsion, and (D) sonicated fatty acid-ChCl:U emulsion. Fatty acid concentration in all fatty acid-DES emulsion was 0.5 mmol/mL. The images were obtained using phase contrast and a Nikon Eclipse E200 microscope.

FIGURE 6

Impact of ultrasonic pretreatment (UST) on droplet size distribution of fatty acid-DES emulsions. (A) The cumulative volumetric size distribution Q₃ and (B) the volumetric size distribution q₃.

FIGURE 7

Impact of ultrasonic pretreatment (UST) on initial reaction velocity in ChCl:Glc (ChCl:Glc:water, 5:2:5, n:n:n) and ChCl:U (ChCl:U, 1:2, n:n, 5% water). Glucose monodecanoate was determined directly by product quantification. Reaction conditions: 0.5 M vinyl decanoic acid, 50°C, 90 rpm. a and b shows statistically significant differences.