KINETICS AND MECHANISM OF SUCCINIMIDE RING FORMATION IN THE DEAMIDATION PROCESS OF ASPARAGINE RESIDUES

The cyclization of Ac-Gly-Asn-Gly-Gly-NHMe to the succinimide derivative has been studied in the pH range 5.5-10.4 at 37-degrees-C and mu = 1 mol dm-3. Kinetic evidence indicates that the reaction is a multistep process with a change in the rate-determining step at pH 6.5-7.0. The suggested mechanism involves the pre-equilibrium deprotonation of the NH group next to the Asn residue, followed by nucleophilic attack of the nitrogen atom on the carbonyl carbon of the Asn side chain giving a cyclic tetrahedral intermediate. At acidic pH the cyclization step is rate determining, whereas, the removal of the leaving group by apparent general-base catalysis is the rate-determining step at neutral and basic pH. The literature data on the deamidation rate are discussed in light of the proposed mechanism.