Comparative studies were made of the polymerization of L-aspartic and L-glutamic acid dialkyl esters using polyethylene gl¥ col-modified papain as a catalyst in phosphate buffer (pH 7.5) and in benzene. Changes in the substrate specificity of papain and in the composition of oligomerized products were observed. In the buffer, the diethyl and di-n-propyl esters of L-glutamic acid were sufficiently converted to high molecular weight oligomers with the accumulation of dimer and trimer, but L-aspartic acid esters were very poor substrates. In benzene, L-aspartic acid esters became more reactive than L-glutamic acid esters. In particular, from L-aspartic acid dimethyl ester the product, which was mainly composed of heptamer to decamer, was obtained in a 90% yield. The reaction in benzene required desalted substrates and a small amount of water to proceed extensively.

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