Deamidation of the asparaginyl‐glycyl sequence

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Abstract
The deamidation of Ac‐Asn‐Gly‐NHMe and Ac‐Isn‐Gly‐NHMe has been studied as a model for the facile deamidation of the Asn‐Gly sequence in proteins. At alkaline pH, the product in each case is an identical mixture of Ac‐α‐Asp‐Gly‐NHMe (˜ 22%) and Ac‐β‐Asp‐Gly‐NHMe (˜ 78%) as determined by n.m.r. spectroscopy. Because this same ratio is obtained from both Ac‐Asn‐Gly‐NHMe and Ac‐Isn‐Gly‐NHMe, the postulated mechanism, that deamidation proceeds through a cyclic imide intermediate, is confirmed. Unlike peptides of aspartyl esters, cyclization does not occur under nonaqueous conditions or at low pH in aqueous solution.