Diketopiperazines, formation

Marsden BJ, Nguyen TM-D, Schiller PW. Spontaneous degradation via diketopiperazine formation of peptides containing a tetrahydroisoquinoline-3-carboxylic acid residue in the 2-position of the peptide sequence. Int J Peptide Protein Res 1993 41 313-316. 

Diketopiperazine formation is a reaction that occurs specifically at the N-terminus of peptides and leads to cleavage of the first two residues. Glycylgly-cine N-methylamide (6.56) has proven to be particularly useful as a model compound in investigations of the mechanism and kinetics of the reaction as compared to the reactivity of analogues 6.40, 6.43, and 6.45 discussed in Sect. 

While we are not aware of any systematic investigation of the conditions and structural factors that influence diketopiperazine formation, the literature contains a number of examples of such reactions. Thus, cyclo(His-Trp), a degradation products of LHRH (Fig. 6.16, Reaction c), is a secondary product formed after cleavage of the Glp-His bond. After an incubation of 90 d at neutral pH, cyclo(His-Trp), represented 4 and 10% of the breakdown products obtained at 37° and 50°, respectively [74],... 

Diketopiperazine formation was also examined in the peptides 6.50, 6.51, and 6.52 shown in Fig. 6.17 [76a], Clear evidence for the formation of a diketopiperazine product was obtained only for Arg-Trp-Phe (6.50, R=OH). In this case, the product was cyclo(Arg-Trp) (Reaction b). The diketopiperazine formed from Phe-Trp-Arg (6.51, R=OH) was not seen directly, but the presence of Trp-Phe together with Phe-Trp afforded indirect evidence. Interestingly, diketopiperazine formation occurred during acid-catalyzed degradation but not under basic conditions, and, as explained, was restricted to the two deamidated tripeptides. 

Diketopiperazine formation occurred to a marked extent during the degradation of thyrotropin-releasing hormone in In HC1 at 60° (Fig. 6.21, Reaction d) [83]. After 6 h, cyclo(His-Pro) accounted for ca. 15% of the initial amount incubated. 

Several prenylated dipeptide methyl esters undergo rapid diketopiperazine formation upon N-terminal... 

C domains that oatalyzed the formation of multiple amide bonds Transglutaminases Chain Termination Strategies Thioesterase-catalyzed chain release Alternative chain release through reduction Condensation domains as chain termination catalysts Diketopiperazine formation Oxidative ohain termination... 

Golebiowski A, Jozwik J, Klopfenstein SR, Colson AO, Grieb AL, Russell AF, Rastogi VL, Diven CP, Portlock DE, Chen JJ (2002) Solid-supported synthesis of putative peptide P-tum mimetics via Ugi reaction for diketopiperazine formation. J Comb Chem 4 584—590... 

The main advantage of ferr-alkyl esters as linkers is their stability towards nucleophiles. For instance, no diketopiperazine formation is observed during the preparation of peptides containing carboxy-terminal proline, an otherwise common side reaction when using benzyl alcohol linkers (Section 15.22.1). 

Numerous examples of different variants of this cyclization/cleavage protocol have been reported. Diketopiperazine formation (Section 15.22.1), an unwanted side reaction in solid-phase peptide synthesis, is also an example of this type of compound release. Because intramolecular processes generally take place more readily than the corresponding intermolecular reactions, cyclization/cleavage can occur with alkyl... 

Diketopiperazine formation by intramolecular nucleophilic cleavage proceeds particularly smoothly if a linker prone to facile nucleophilic cleavage is used. In most of... 

Among the six-membered ring heterocycles, diketopiperazines are most commonly prepared by cyclative cleavage. Indeed, diketopiperazine formation is often observed as an undesirable by-product during peptide synthesis20 and the facile nature of this cyclization makes it an obvious choice for library generation. In an early example from Pfizer,21 a set of 10 immobilized a-amino acids was reductively alkylated with 10 aldehydes, followed by acylation with 10 a-amino acids and cyclization (Fig. 6). By... 

Other degradation mechanisms. Additional degradation reactions include N-terminal degradation to form pyroglutamic acid formation (Fig. 137) (193) and N-terminal degradation diketopiperazine formation (Fig. 138) (194). 

Battersby IE, Hancock WS, Canova-Davis E, Oeswein J, O Connor B. Diketopiperazine formation and N-terminal degradation in recombinant human growth hormone. Int J Peptide Protein Res 1994 44(3) 215-222. 

Gu L, Strickley RG. Diketopiperazine formation, hydrolysis, and epimeriza-tion of the new dipeptide angiotensin-converting enzyme inhibitor RS-10085. Pharm Res 1987 4(5) 392-397. 

Trityl-based resins are highly acid-labile. The steric hindrance of the linker prevents diketopiperazine formation and the resins are recommended for Pro and Gly C-terminal peptides. Extremely mild acidolysis conditions enable the cleavage of protected peptide segments from the resin. These resins are commercially available as their chloride or alcohol precursors. The trityl chloride resin is extremely moisture-sensitive, so reagents and glassware should be carefully dried before use to avoid hydrolysis into the alcohol form. It is necessary to activate the trityl alcohol precursor and it is highly recommended to reactivate the chloride just before use see Note 4). After activation, attachment of the first residue occurs by reaction with the Fmoc amino acid derivative in the presence of a base. This reaction does not involve an activated species, so it is free from epimerization. Special precautions should be taken for Cys and His residues that are particularly sensitive to epimerization during activation (Table 2). 

Alsina, J., Giralt, E., and Albericio, F. (1996) Use of A-tritylamino acids and PyAOP for the suppression of diketopiperazine formation in Fmoc/ Bu solid-phase peptide synthesis using alkoxybenzyl ester anchoring linkages. Tetrahedron Lett. 37, 4195-4198. 

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