Kinetics and mechanism of the reversible isomerization of aspartic acid residues in tetrapeptides

The influence of pH and buffer concentration on the rate of the isomerization of Asp residues has been analysed using model aspartic acid-containing tetrapeptides, in the pH range 1.5–10 at 37 °C and, µ= 1 mol dm–3. The reaction involves the reversible formation of an amino-succinimide intermediate.Kinetic evidence indicates that of the various forms of the Asp-peptide in acid–base equilibrium, only one, having the carboxylic Asp side chain in the neutral state and the N–H peptide group next to the Asp residue in the deprotonated state, reacts at an appreciable rate. The reaction involves nucleophilic attack by the nitrogen atom of the peptide bond on the carbonyl carbon of the Asp side chain, giving a cyclic tetrahedral intermediate. At pH values lower than about 3 the cyclization step is rate-determining, but at higher pH the rate-determining step is the general acid-catalysed departure of the leaving group. This change of rate-determining step, together with the ionization of the carboxylic side chain of the Asp residue, produces a bell-shaped curve in the pH–rate profile for the cyclization reactions.