Innovative sophorolipid analogues with tailor-made physico-chemical and biological properties

Sophorolipids are an interesting class of glycolipid biosurfactants. They are produced by different yeast species, mainly Starmerella bombicola, through fermentation. The yeast uses glucose as hydrophilic carbon source and an oil or fatty acid as hydrophobic carbon source. Being amphiphilic compounds, sophorolipids have been commercialized by different companies in their detergent compositions. Nevertheless, their application potential in the detergent sector is limited because they have a competitive disadvantage compared to synthetic surfactants in terms of production cost. Being renewable products with a complex structure and interesting biological properties, sophorolipids can be regarded as excellent building blocks for chemical modification. In the first part, a synthetic pathway was developed to transform the major microbial product, i.e. diacetylated sophorolipid lactone, into a C9 sophorolipid aldehyde intermediate. This pathway constitutes of three consecutive steps, i.e. methanolysis, protection of the sugar head and ozonolysis. The fermentation conditions and concomitant purity of the starting product proved to have an influence on the chemical derivatization. Also the ozonolysis reaction proved to be a key factor in this synthetic pathway. Efforts were made to render this reaction as sustainable as possible while preserving a high product yield and purity. The synthetic pathway was extended to the production of a sophorolipid alcohol intermediate by adjusting the reductive work-up of the ozonolysis reaction and to the production of a C12 sophorolipid aldehyde via the incorporation of petroselinic acid in the sophorolipid structure. In the second part, the sophorolipid aldehyde intermediate was subsequently used as chemical building block for the synthesis of a variety of innovative sophorolipid analogues. The new derivatives comprise sophorolipid amines, sophorolipid quaternary ammonium salts, sophorolipid amine oxides and bolaamphiphilic sophorolipids. Multiple properties were evaluated for these new classes of sophorolipid derivatives. For example, some derivatives showed high antimicrobial activity against Gram-positive bacterial strains such as Staphylococcus aureus and Bacillus subtilis. Also, promising results were obtained for the application of sophorolipid derivatives as vectors for gene delivery. The deprotected quaternary ammonium sophorolipids with an octadecyl chain on the nitrogen atom proved to be the most interesting compounds. The presence of the carbohydrate head proved to have a positive influence on the antimicrobial activity and on the cell viability in the transfection assay.