306-14-9Relevant articles and documents
The effects of a histidine residue on the C-terminal side of an asparaginyl residue on the rate of deamidation using model pentapeptides
Goolcharran, Chimanlall,Stauffer, Lori L.,Cleland, Jeffrey L.,Borchardt, Ronald T.
, p. 818 - 825 (2000)
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 °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 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. (C) 2000 Wiley-Liss, Inc. and the American Pharmaceutical Association.
Further Studies on the Protection of Histidine Side Chains in Peptide Synthesis: The Use of the ?-Benzyloxymethyl Group
Brown, Tom,Jones, John H.,Richards, John D.
, p. 1553 - 1562 (2007/10/02)
Further studies on the use in peptide synthesis of N(?)-phenacyl protection for histidine side chains have shown that whilst this prevents the side chain-induced racemization which can occur if there is a lone pair of electrons available at the ?-nitrogen, there are concomitant drawbacks.As an alternative approach to the racemisation problem, the effect of halogenation of the heterocyclic ring carbons (to diminish the availability of the ?-nitrogen lone pair) has been investigated.This gives derivatives which are convenient in both classical and solid-phase applications, the halogen modifying groups being removed at the last stage by catalytic hydrogenolysis over a rhodium catalyst.Racemization is suppressed as expected, but it is not eleminated completely: direct blockade of the ?-nitrogen appears to be indispensable for its complete prohibition.Protection of the ?-nitrogen with a benzyloxymethyl group has now been found to be much more satisfactory than the use of the phenacyl group for this purpose.A ?-benzyloxymethyl substituent not only prohibits side chain-induced racemisation but also gives derivatives with convenient physical properties which can be incorporated into well estblished classical and solid-phase strategies without the need for any novel or additional operations or changes in protocol.The protecting group is stable to basic conditions, to trifluoroacetic acid, and to aqueous solutions of carboxylic acids, but is cleaved cleanly and rapidly by hydrogen bromide in trifluoroacetic acid or by catalytic hydrogenolysis.N(α)-t-Butoxycarbonyl-N(?)-benzyloxymethyl-L-histidine has been prepared in good yield by a simple procedure from an easily accessible intermediate and isolated as a crystalline solid; its use has been demonstrated by a number of exercises including a solid-phase synthesis of 5-isoleucine-angiotensin II and a classical synthesis of trihistidine.
