New insights into kinetics and mechanism of hydrolysis of sophorolipids using LC-MS and NMR spectroscopy

Abstract: Sophorolipids (SLs) generated from yeast fermentation can vary greatly not only in the amounts of residual sugars, organic acids, and oleochemicals but also in the speciation ratio of lactonic to linear forms as well as the degree of acetylation. Alkali hydrolysis to increase the amount of linear SLs and to decrease the amount of acetylation is well established. However, it would be advantageous to control the conversion of lactonic to linear as well as the degree of deacetylation since different SL species have different surface and interfacial properties. Linear SLs have a very small micelle size, higher water solubility and surface adsorption characteristics than the lactonic SLs. Thus, control over the relative amount of SLs species would be advantageous to for formulation purposes and for the needs of a specific function.
The purpose of this work was to study hydrolysis of a high ( >80%) lactonic SLs sample that resulted in an >80% linear SLs mixture while retaining a majority of the acetylated species. A quantitative LC-MS method was used to determine the types and amounts of sophorolipids at different times during the alkali hydrolysis. The quantitation of each SLs species over time allowed for the determination of the hydrolysis pathway. Standards SLs used in the LC-MS analysis were produced in-house via LC preparation separation method. Purity and structures of standards were confirmed by 1-D and 2-D NMR analysis. The NMR analysis discovered that production lots contained a unique form of lactonic monoacetylated SLP that previously has only been reported in trace amounts in the literature. Future work will include changing the hydrolysis parameters in order to tailor make SLs with more specific lactonic/linear ratios.
Sophorolipids are a group of biosurfactants that found applications in the oil & gas, mining, agriculture, household cleaning, and personal care markets and considered as greener and sustainable alternatives to synthetic surfactants.