P-Aspartyl peptides

Deamidation. Can occur in aqueous media, particularly under basic conditions, leading to formation of a- and p-aspartyl peptides. Most problematic with peptides containing Asn-Gly and Asn-Ser... 

CC Yang, RB Merrifield. The P-phenacyl ester as a temporary protecting group to minimize cyclic amide formation during subsequent treatment of aspartyl peptides with HF. J Org Chem 41, 1032, 1976. 

M Bodanszky, JZ Kwei. Side reactions in peptide synthesis. VI1. Sequence dependence in the formation of aminosuccinyl derivatives from P-benzyl-aspartyl peptides. 

M Bodanszky, J Martinez. Side reactions in peptide synthesis. 8. On the phenacyl group in the protection of the p-carboxyl function of aspartyl peptides. J Org Chem 43, 3071, 1978. 

Cycloalkyl esters have been used to protect the P-CO2H group in aspartyl peptides to minimize aspartimide formation during acidic or basic reactions. Aspartimide formation is limited to 2-3% in TFA (20h, 25°C), 5-7% with HF at 0°C, and 1.5-4% TfOH (thioanisole in TFA). Cycloalkyl esters are also stable to Et N, whereas use of the benzyl ester leads to 25% aspartimide formation during EtsN treatment. Cycloalkyl esters are stable to CF3COOH, but are readily cleaved with HF or TfOH. ... 

Optically active compounds. p-Nitrocarbobenzoxy-L-allylglycine added at 20° to 0.115 M Na-metaperiodate, the pH adjusted to 7.5 with 0.5 M Na-carbonate, some 0.01 M K-permanganate added to initiate the oxidation, and the product isolated after 1 hr. p-nitrocarbobenzoxy-L-aspartic acid. Y 95%. — This method can be used in the synthesis of a-aspartyl peptides to avoid the complications, attending the presence of two carboxyl groups. F. e. and limitations s. E. Sondheimer, J. Org. Ghem. 30, 665 (1965). 

Itoh, M. Peptides. I. Selective protection of a- or sidechain carboxyl groups of aspartic and glutamic acid. A facile synthesis of p-aspartyl and y-glutamyl peptides. Chem. Pharm. Bull. 17, 1679 (1969). 

Aspartimide formation. Can result in formation of a- and p-piperidides -aspartyl peptides. Most problematic with Asp(OfBu>-X (X = Gly, Asn(Trt), Ser, Thr, Arg(Pmc) sequences... 

At temperatures of around 100 °C, asparagine and glutamine side chains react with the side chain of bound lysine (Figure 2.50) with the ehmination of ammonia (the reaction is termed deamidation) and the peptide chains are connected by intermolecular and intramolecular transverse covalent bonds. These bonds, known as isopeptide bonds, consist of dipeptides -N-(P-aspartyl)-L-lysine and e-N-iy- glutamyl)-L-lysine, respectively. About 15% of lysyl residues can react in this way. The digestive proteases of some animals (e.g. chickens and rats) can spHt these bonds, but for humans the so-called bound lysine is not available. The result is the certain, but usually not very significant, reduction of the nutritional value of proteins. Reduction of the nutritional value can be important in cases where a diet is low in protein and the hmiting amino acid of the protein is lysine. The Maillard reaction often leads to an extensive loss of lysine. 

Figure 7. Complete proteolytic stability of all types of P-and y-peptides towards a variety of peptidases. The P-peptides ranged in size from dimer to ISmer. The enzymes include all common types of peptidases (endo/exo, metallo, serine, threonine, and aspartyl proteases). After 40 hours there was no observable cleavage of any of the homologated peptides and no inhibition of the enzymes [41].

The cyclization of aspartic acid residues to form aspartimide is the most likely side-reaction observed in routine SPPS (Fig. 10). This is a sequence-dependent side-reaction that occurs either during chain elongation or during final TFA cleavage when Asp(OtBu)-AA sequence (AA = Ala, Gly, Ser, Asn(Trt)) is present in the peptide. Hydrolysis of the aspartimide ring leads to a mixture of both a- and P-peptides. Its reaction with piperidine used for Fmoc removal also leads to the formation of a- and p-piperidides. Normally, in Fmoc-based SPPS, Asp (OtBu) provides sufficient protection. However, for particular sequences such as Asp(OtBu)-Asn(Trt) particularly sensitive to aspartimide formation, addition of HOBt to the piperidine solution or protection of the aspartyl amide bond with the 2-hydroxy-4-methoxybenzyl (Hmb) group should be considered (36). 

Cyclization of aspartic acid and asparagine to form aspartimides, and to a lesser extent of glutamic acid and glutamine to form glutarimide is an acid- and base-catalyzed common side reaction in peptide synthesis (see also Section 2.2.2). In SPPS it is particularly troublesome when Asp-Gly, Asp-Ala, and Asp-Ser sequences are present,but also with Asp-Asn.P P Piperidine-catalyzed aspartimide formation can be very rapid,and in this context DBU is even worse than piperidine.P The formation of aspartimide is reduced by the addition of HOBt or 2,4-dinitrophenol, but more efficiently it is reduced by protecting the aspartyl amide bond with the 2-hydroxy-4-methoxybenzyl (Hmb) group (see Section 2.3.2).P 1... 

C. Oliyai, J. P. Patel, L. Carr, and R. T. Borchardt, Chemical pathways of peptide degradation. VII. Solid state chemical instability of an aspartyl residue in a model hexapeptide. Pharm. Res. 7/.-901-908 (1994). 

PhSeH, DMF, rt, 48 h, 79% yield. Under basic coupling conditions an as-partyl peptide that has a (5-phenacyl ester is converted to a succinimide. The use of PhSeH prevents the a,P-rearrangement of the aspartyl residue during deprotection. 

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