Deamidation via cyclic imide of asparaginyl peptides: dependence on salts, buffers and organic solvents.

The deamidation reaction of Asn side chain was studied in the presence of the chemicals usually used at high concentration in the purification and crystallization of peptides and proteins. All the experiments were performed on the model peptide Boc-L-Asn-Gly-Gly-NH2. The pathway of the reaction is not affected by the medium used; in all cases it proceeds through a succinimide intermediate giving a mixture of isoaspartyl and aspartyl peptide. However, the rate of the reaction significantly depends on the solvent: the addition of organic solvents to an aqueous solution of the peptide has the general effect of decreasing the reaction rate, which, on the other hand, is strongly enhanced by a high concentration of organic and inorganic buffers. Only a minor influence is exerted by aprotic salts and polyethylene glycol.