α-Chymotrypsin-catalyzed peptide synthesis in frozen aqueous solution using N-protected amino acid carbamoylmethyl esters as acyl donors

A kinetically controlled peptide synthesis catalyzed by α-chymotrypsin was performed in frozen aqueous solution (ice, −24 °C). The yield of the peptide was significantly improved by the use of the carbamoylmethyl (Cam) ester as the acyl donor instead of the conventional ethyl ester. The peptide yield increased up to ca. 90% when N-benzyloxycarbonyl (CBZ)-Phe-OCam and H-Phe-NH₂ were used as the acyl donor and nucleophile, respectively. Such an improvement of the peptide yield in ice was also observed in the coupling of other CBZ-amino acid Cam esters as acyl donors. Furthermore, this approach was applied to the synthesis of peptides containing d-amino acids. The peptides such as CBZ-d-Phe-Phe-NH₂, CBZ-Phe-d-Phe-NH₂ and CBZ-d-Phe-d-Phe-NH₂ were also obtained in excellent to moderate yields in ice. A high diastereoselectivity towards the l–l peptide was observed when the racemic amino acid Cam ester was used as the acyl donor in ice.