For coupling peptides the carboxyl group is usually activated. This speeds up the reaction. There are two main types of activating groups: carbodiimides and triazolols. However the use of pentafluorophenyl esters and BOP-Cl are useful for cyclising peptides.
These activating agents were the first developed. Most common are dicyclohexylcarbodiimide (DCC) and diisopropylcarbodiimide (DIC).The C-terminus is often used as the attachment site on which the amino acid monomers are added in peptide synthesis. To enhance the electrophilicity of carboxylate group, the negatively charged oxygen must first be "activated" into a better leaving group. DCC is used for this purpose. The negatively charged oxygen will act as a nucleophile, attacking the central carbon in DCC. DCC is temporarily attached to the former carboxylate group (which is now an ester group), making nucleophilic attack by an amino group (on the attaching amino acid) to the former C-terminus (carbonyl group) more efficient. The problem with carbodiamides is that they are too reactive and that they can therefore cause racemization of the amino acid.
To solve the problem of racemization, triazoles were introduced. The most important ones are 1-hydroxy-benzotriazole (HOBt) and 1-hydroxy-7-aza-benzotriazole (HOAt). Others have been developed. These substances can react with the O-acylurea to form an active ester which is less reactive and less in danger of racemization. HOAt is especially favourable because of a neighbouring group effect. Recently, HOBt has been removed from many chemical vendor catalogues; although almost always found as a hydrate, HOBt may be explosive when allowed to fully dehydrate and shipment by air or sea is heavily restricted. Alternatives to HOBt and HOAt has been introduced. One of the most promising and inexpensive is ethyl 2-cyano-2-(hydroxyimino)acetate (trade name Oxyma Pure), which is not explosive and has a reactivity of that in between HOBt and HOAt.