2

The procedures (see Section 1.7) used to combine two amino acid residues to form a peptide are referred to as coupling methods. Coupling involves nucleophilic attack by the amino group of one residue at the electrophilic carbonyl carbon atom of the carboxy-containing component that has been activated by the introduction of an electron-withdrawing group Y. Activation may be carried out either in the presence of the N-nucleophile or in the absence of the N-nucleophile, which may be by choice or by necessity. Activation in the absence of the nucleophile is referred to as preactivation. When a coupling is effected by the addition of a single compound to a mixture of the two reactants, the compound is referred to as a coupling reagent. In some cases, the coupling reagent requires a subsequent deprotonation of one of the reactants to effect the reaction. The common activated forms of the acid appear in Figure 2.1 in the order of increasing complexity, which also corresponds — with the exception of the mixed anhydride — to the order in which the methods became available. The activating moiety Y is composed of either a halide atom or an azide group or an oxygen atom linked to a double-bonded carbon atom (O-C=), a cationic

carbon (O-C+) or phosphorus (O-P+) atom, or a nitrogen atom adjacent to a doublebonded atom (O-N-X=). Some activated forms are much more stable than others. Three different types can be distinguished. The activated form may be a shelf-stable reagent such as an activated ester, a compound of intermediate stability such as an acyl halide or azide or a mixed or symmetrical anhydride that may or may not be isolated, or a transient intermediate, indicated in Figure 2.1 by brackets, that is neither isolable nor detectable. The latter immediately undergoes aminolysis to give the peptide, or it may react with a second nucleophile that originates from the reactants or was added for the purpose, to give the more stable activated ester or symmetrical anhydride R-C(=O)–Y′, whose aminolysis then generates the peptide.