Novel biocatalytic esterification reactions on fatty acids: synthesis of sorbitol 1(6)-monostearate

Aspergillus terreus lipase has exhibited novel capability of catalyzing esterification reaction between fatty acids (C-4-C-18) and primary, secondary and tertiary monohydric alcohols. Although, the lipase efficiently catalyzed the esterification of saturated stearic acid (C-18:0), it failed to accept the monounsaturated oleic acid (C-18:1) as the substrate which is also a C-18 acid, but has a double bond. Thus, this enzyme has a great potential to be used as a selective catalyst for the separation of almost identical saturated and unsaturated fatty acids in their mixtures. Extending this work, highly efficient and regioselective conversion of sorbitol to its 1(6) - monostearate has been achieved by Aspergillus terreus lipase - mediated esterification in n-hexane. Ninety four per cent conversion to the ester was achieved in 12 h on optimizing four physico-chemical factors, i.e. molar ratios of substrates, temperature, solvent and water activity. Aspergillus terreus lipase immobilized on Accurel was efficient in the synthesis of sorbitol stearate and was reused three times without any significant decline in its activity, yields were further enhanced to 96 % on using the immobilized enzyme.