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Deep eutectic solvents (DESs) are formed by mixing quaternary ammonium salts (e.g., choline chloride) and hydrogen-bond donors (e.g., glycerol or urea), which leads to biodegradable and readily available ionic solvents at room temperature. Analogous to other ionic liquids, DESs represent a promising reaction media if hydrophobic and hydrophilic substrates need to be combined. This paper assesses DESs as reaction media for chymotrypsin-catalyzed peptide synthesis. After careful determination of the reaction conditions (e.g., water content, enzyme loading), α-chymotrypsin displayed high activity for peptide synthesis in choline chloride/glycerol mixtures to afford productivities of ca. 20 g L-1 h-1 and with complete selectivity for the peptide, which is in contrast to the detrimental hydrolysis pathway observed in aqueous media. The nonimmobilized suspended enzyme could be reused several times by simple filtration with excellent to moderate activities. Overall, the results reported suggest that choline chloride based DESs may become promising neoteric solvents for peptide synthesis through biocatalysis. [ABSTRACT FROM AUTHOR]