4-Azido-L-phenylalanine

4-Azido-L-phenylalanine is an unnatural derivative of L-phenylalanine that can be incorporated by recombinant methods into proteins allowing site-specific modification, e.g. by ‘click chemistry’, Staudinger reaction, or by UV-irradiation at 254 nm. The azido-modified protein can be detected with either fluorescent alkyne or biotin alkyne. Detection sensitivity with these reagents in 1-D gels and Western blots is in the low femtomole range and compatible with downstream LC-MS⁄MS and MALDI MS analysis.