Esterification of N‐benzyloxycarbonyldipeptides in ethanol–water with immobilized papain

The esterification of some N-benzyloxycarbonyl (Z)-dipeptides in ethanol-containing water was investigated using papain as a catalyst. The esterification took place in ethanol containing a small amount of water (2% v/v, pH 9) with free papain at room temperature. The yield (after 24 h) of the ethyl ester was in the range of 25% to 50%. Any peptide bond cleavage of the substrates was not observed during esterification, indicating that the unfavorable amidase activity of papain was well depressed under these conditions. However, dipeptides having a D-amino amino acid (Z-valyl-D-alanine) or a bulky amino acid (Z-valylvaline) at the C-terminal position could not be esterified. It was found that the immobilization of papain on Amberlite XAD-8 increased the yield of the ester significantly as compared with free papain. In the esterification of Z-valylalanine using immobilized papain, the optimum water content, pH of an added buffer, and temperature were found to be 2% (v/v), 9, and 40 degrees C, respectively. The water content affected the yield of the product ester significantly.