This unnatural L-phenylalanine derivative, 4-Azido-L-phenylalanine, is used e.g. for genetic code expansion in protein biosynthesis as it can be incorporated into the proteins. Due to the azide-moiety, this molecule allows for specific bioconjugation of various functionalities such as e.g. PEGs, antibodies or fluorescent labels, by click chemistry or Staudinger ligation.