The effects of a histidine (His) residue located on the C-terminal side of an asparaginyl (Asn) residue on the rate of deamidation were studied using Gly-Gln-Asn-X-His pentapeptides. The rates of deamidation of the pentapeptides were determined at 37 degrees C (I = 0.5) as function of pH, buffer species, and buffer concentration. A capillary electrophoresis stability-indicating assay was developed to monitor simultaneously the disappearance of the starting peptides and the appearance of the degradation products. The rates of degradation of the peptides were pH dependent, increasing with pH, and followed apparent first-order kinetics. At pH values <6.5, Gly-Gln-Asn-His-His degraded faster than Gly-Gln-Asn-Gly-His, suggesting that the N+1 His residue is catalyzing the deamidation of the Asn residue. The His side chain at these pH values could function as a general acid catalyst, stabilizing the oxyanionic transition state of the cyclic imide intermediate formation. In contrast, at pH values >6.5, Gly-Gln-Asn-Gly-His deamidates more rapidly than Gly-Gln-Asn-His-His. The bulk of the side chain of the N+1 His residue versus the N+1 Gly residue apparently inhibits the flexibility of the peptide around the reaction site and, consequently, reduces the rate of the reaction. The significance of this steric hindrance effect of the N+1 His residue on the rate of deamidation was examined further. It was observed that at pH >6.0, Gly-Gln-Asn-His-His undergoes deamidation faster than Gly-Gln-Asn-Val-His. This observation indicated that, at the higher pH values, the N+1 His residue is also acting as a catalyst. Thus, at basic pH, the N+1 His residue influences the rate of deamidation via two opposing effects; that is, general base catalysis and steric interference. The pentapeptide Gly-Gln-Asn-His-His, in addition to undergoing the deamidation reaction, also undergoes bond cleavage at the Asn-His peptide bond. The enhanced rate of Asn-His peptide bond cleavage can be attributed to the general base behavior of the His residue, leading to increased nucleophilicity of the Asn side-chain amide group. Finally, we have shown that the His residue that is two amino acids removed from the Asn, the N+2 position, has little or no effect on the rate of deamidation.