Figure 1. Chemical structures of flavone-8-acetic acid (FAA) and azido-FAA. The blue circle denotes the locus of chemical substitution that still retains the full activity of the parent FAA molecule. Azido-FAA can undergo efficient coupling via a modified Staudinger ligation (Saxon E and Bertozzi CR. Science 287: 2007–10, 2000) with a FLAG peptide-phosphine tag under mild, biologically relevant conditions (pH 7.0–7.4, 25°C) to generate a peptide-tagged probe for locating and isolating proteins that bind to FAA. The red portions of the peptide-phosphine tag and the azido-FAA conjugate represent the FLAG peptide epitope used for immunorecognition.