The following crude peptide derivatives obtained by thermolysin catalysis in the presence of calcium acetate, were dissolved in methanol and in methanol‐containing calcium acetate to determine the possible occurrence of transesterification: Z‐Asn‐Leu‐Gly‐OEt, Boc‐Asn‐Leu‐Gly‐OEt, Moz‐Asn‐Leu‐Gly‐OBzl, Moz‐Asn‐Leu‐Gly‐OtBu, Moz‐Gln‐Leu‐Gly‐OEt, Moz‐Asn‐lle‐Gly‐OEt, and Moz‐Asn‐Leu‐Ala‐OEt. Only Z‐Asn‐Leu‐Gly‐OEt and Moz‐Gln‐Leu‐Gly‐OEt were transesterified in methanol, indicating the existence of a peptide derivative‐Ca2+ catalytic complex that may favor the reaction. In the presence of calcium acetate, all protected peptide esters except the t‐butyl esters were transesterified. The transesterification of several other di‐ and tripeptide derivatives of different structures in methanol‐containing calcium acetate was detected by HPLC and confirmed by the isolation and characterization of some of the protected peptide methyl esters obtained. Boc‐Leu‐Gly‐ and Moz‐Asn‐Leu‐Gly‐Merrifield resin were also transesterified in these solutions.

N-acetyl-tyrosine (Ac-Tyr-OH), N-t-butyloxycarbonyl-tyrosine (Boc-Tyr-OH) and N-benzyloxycarbonyl-tyrosine (Z-Tyr-OH) were esterified by alphachymotrypsin suspended in ethanol-buffer, 20.0:0.5 (v/v). The rates of esterification decreased in the order Ac-Tyr-OH > Z-Tyr-OH > Boc-Tyr-OH. For Z-Tyr-OH esterification the pH optimum depends on the nature of the buffer salt, being 4.0 for acetate and 6.0 for phosphate buffers.

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