CuAAC click reaction between an azide and alkyne takes place in presence of a Cu (I) catalyst under mild conditions, resulting in the formation of a triazole link connecting the two molecules. In peptide chemistry the increasing popularity of the CuAAC is largely a result of the unique properties of both azides and the resulting triazoles. Azide groups are easy to introduce, stable to water and oxidative conditions, and orthogonal to many functional groups in peptide synthesis. For applications in vitro and in vivo, azides are virtually absent from any naturally occurring species (bioorthogonal). Interestingly, the triazole moiety formed by click reaction has unique similarity to an amide bond. The relative planarity, strong dipole moments, and hydrogen bonding ability of triazole linkage make it as attractive as an amide bond with the added advantage that it is less prone to hydrolytic cleavage.
Click chemistry provides a number of avenues for peptide synthesis and modifications and could be combined with other techniques to make complex structures and multicomponent functionalized systems with ease. For example, peptides can be converted postsynthetically to an azido derivative which can be clicked with an appropriate substrate containing a clickable alkynyl group or vice versa. Peptides can also be made by inter- and intramolecular click reactions using azide or alkyne containing amino acids or building blocks during peptide synthesis. Building blocks containing clickable moieties are instrumental in constructing side-chain modified peptides, interside-chain peptide chimera, peptide small molecule conjugates, and cyclic peptides. Solid phase resins modified with clickable groups can also be used for making clickable/modified peptides. Click chemistry is compatible with various protected amino acid side chains used in peptide synthesis.
A number of reagents and building blocks can be utilized for peptide click chemistry. These include azido-amino acids (e.g. Fmoc- protected for solid phase synthesis), propargyl amino acids, PEG azide and alkynes (Maleimide-PEG3-Azide and acetylene PEG maleimide for pegylation), alkyne and azide containing chemical modification reagents (propargyl amine, 1-(2-nitrophenyl) propargyl alcohol, succinidyl-hex-5-ynoate, succinimidyl-4-azovalerate, pentynoic acid, 2-azido-3-methyl propanoic acid) and Diazo transfer reagents (imidazole-1-sulfonyl azide).