Fig. 3.

Possible reaction pathways for spontaneous isomerization of aspartic acid (Asp) and deamidation of asparagine (Asn) residues in protein. The simultaneous formation of β- and d-Asp residues in the protein could be explained in the following four steps. (i) When the carbonyl group of the side chain of the lα-aspartyl residue/lα-Asn is attacked by the nitrogen of the amino acid residue following the Asp residue, l-succinimide is formed by intramolecular cyclization. (ii) l-succinimide may be converted to d-succinimide through an intermediate [I] that has the prochiral α-carbon in the plane of the ring. (iii) Protonation of the intermediate [I] may proceed from the upper or lower side of the plane in an ordinary peptide or protein. (iv) d- and l-succinimide are hydrolyzed at either side of their two carbonyl groups, yielding both β- and α-Asp residues, respectively. Thus, four isomers, lα-Asp, lβ-Asp, dα-Asp and dβ-Asp, are simultaneously formed in the protein.