De novo protein biosynthesis can be monitored by feeding of metabolite analogues (so-called metabolic labeling) and subsequent click reaction. Azido-homoalanine for example is recognized as a methionine analogue and is incorporated into de novo synthesized proteins in methionine-free medium conditions. The resulting proteins contain azide moieties and thus can be detected after click to an alkyne-containing reporter molecule (e.g. a fluorescent dye). This non-radioactive method has major practical advantages compared to traditional 35S amino acid incorporation methods.
Alternatively, O-propargyl-puromycin is efficiently incorporated into proteins during de novo protein biosynthesis and can be used in complete medium. The resulting alkyne protein fragments can be detected via click to azide-containing reporter molecules.

A catalyst system based on CuSO4 and sodium ascorbate is recommended in combination with dye azides to label alkyne-modified proteins. Please also refer to our general Click protocols for more details.
Due to the 20 (21) amino acids that are the building blocks of proteins, the physicochemical properties of proteins are more diverse compared to oligonucleotides, which are just composed of 5 major building blocks. Therefore, finding the optimal click conditions is more difficult compared to oligonucleotides and labeling rates are usually lower. Please note that despite these difficulties detection applications (e.g. de novo protein biosynthesis detection) are easily feasible.