Aminolysis peptides

Protease Ester, amide hydrolysis, ester aminolysis, peptide synthesis Many stable proteases No D-proteases... 

Leaving group (Thio-)acyl enzyme Aminolysis Peptide product... 

There are two basic strategies for enzyme-catalyzed peptide synthesis equiUbrium- and kineticaHy controlled synthesis. The former is the direct reversal of proteolysis and involves the condensation of an amino component with unactivated carboxyl component. The latter proceeds by the aminolysis of an activated peptide ester. 

Thus, the family of azolides represents a versatile system of reagents with graduated reactivity, as will be shown in the following section by a comparison of kinetic data. Subsequent chapters will then demonstrate that this reactivity gradation is found as well for alcoholysis to esters, aminolysis to amides and peptides, hydrazinolysis, and a great variety of other azolide reactions. The preparative value of azolides is not limited to these acyl-transfer reactions, however. For example, azolides offer new synthetic routes to aldehydes and ketones via carboxylic acid azolides. In all these reactions it is of special value that the transformation of carboxylic acids to their azolides is achieved very easily in most cases the azolides need not even be isolated (Chapter 2). 

For the mechanism of azolide hydrolysis under specific conditions like, for example, in micelles,[24] in the presence of cycloamyloses,[25] or transition metals,[26] see the references noted and the literature cited therein. Thorough investigation of the hydrolysis of azolides is certainly important for studying the reactivity of those compounds in chemical and biochemical systems.[27] On the other hand, from the point of view of synthetic chemistry, interest is centred instead on die potential for chemical transformations e.g., alcoholysis to esters, aminolysis to amides or peptides, acylation of carboxylic acids to anhydrides and of peroxides to peroxycarboxylic acids, as well as certain C-acylations and a variety of other preparative applications. 

Peptides can be formed by aminolysis of iV-aminoacyl-3,5-dimethylpyrazole with free amino acid esters 303 as shown in Table 5-5. 

Benesch, R., and Benesch, R.E. (1956) Formation of peptide bonds by aminolysis of homocysteine thiol-actones./. Am. Chem. Soc. 78, 1597. 

Peptide-bond formation between an A -alkoxycarbonylamino acid and an amino-containing component usually proceeds in the same way as described for coupling an Y-acylamino acid or peptide (see Section 1.9), except for the side reaction (Figure 1.7, path B) of oxazolone formation. Aminolysis of the activated component (Figure 1.10, path A) gives the desired peptide. There are three aspects of the side reaction... 

M Bodanszky. Synthesis of peptides by aminolysis of nitrophenyl esters. Nature (London) 75, 685, 1955. 

JH Jones, GT Young. Anchimeric acceleration of aminolysis of esters and its application to peptide synthesis. Chem Commun 35, 1967. 

FIGURE 2.17 Peptide-bond formation from benzotriazol-l-yl-oxy-m s(dimethy-lamino)phosphonium hexafluorophosphate-mediated reactions of Af-alkoxycarbonylamino acids.48 The peptide can originate by aminolysis of either of two precursors the acyloxyphos-phonium cation and the benzotriazolyl ester. 

FIGURE 2.21 Experimental evidence indicates that products from BOP-mediated reactions do not originate from the benzotriazolyl ester. The use of BOP allows successful coupling of A-alkoxycarbonyl-A-methylamino acids, whereas the benzotriazolyl esters of these acids undergo aminolysis only with great difficulty. The higher ratio of products obtained from the BOP-mediated reaction in the competing reactions described implies a compound other than the benzotriazolyl ester as the precursor of the peptides. 

Mixed anhydrides (see Section 2.6) The mixed-anhydride method provides efficient coupling of peptides with minimal isomerization if the established protocol is strictly adhered to. This includes a short activation time at low temperature, isopropyl chloroformate as the reagent, and A-methylmorpho-line or /V-mcthylpipcridinc as the tertiary amine (Figure 2.25, path D). In what is an apparent anomaly with respect to conventional wisdom, a polar solvent such as dimethylformamide seems to be preferable to apolar solvents for minimizing isomerization. Aminolysis at the wrong carbonyl of the anhydride of a peptide (path F) is less than that for the anhydride from the corresponding /V-alkoxycarbonylamino acid. 

The indole group of tryptophan is inert to activation and aminolysis reactions, so it does not have to be protected for constructing a peptide. It is sensitive, however, to... 

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