Hydrophobic deep eutectic solvents as emerging green reaction media for biocatalytic processes: impacts of solvent properties and compositions

Abstract

Hydrophobic deep eutectic solvents (HDESs) are a new generation of water-immiscible solvents that have shown potential as green and sustainable reaction media for biocatalytic applications. In this study, esterification of tyrosol and hydroxytyrosol catalysed by Novozym 435 (Candida antarctica lipase B immobilised on acrylic resin) was taken as a model reaction to examine the feasibility of using HDESs as solvents for acylation reactions and the impacts of the components and physicochemical properties of the HDESs on the enzymatic performance. 21 HDESs were screened. Yields as high as 95.1% could be achieved within 2 h of reaction in an HDES, comparable to or even better than the results obtained in organic solvents. The enzymatic reactions were favoured in the HDES comprising a tertiary alcohol instead of a secondary/primary one as the hydrogen bond acceptor (HBA). Inverse correlations were found between enzyme activity and the polarity and viscosity of the HDES. Carboxylic acid-based eutectic solvents can play a dual role as both a reaction medium and a substrate reservoir for the acylation reaction. These findings provide valuable guidelines for selecting HDESs for biocatalytic applications.