19F: A small probe for a giant protein

Herein we describe a method for the efficient production (-90% fluorination) of 5-F-Trp human H ferritin via the selective incorporation of 19F into the side chain of W93 using 5-fluoroindole as the fluorinated precursor of the amino acid. Human H ferritin is a nanocage composed of 24 identical subunits, each containing a single Trp belonging to a loop exposed on the external surface of the protein nanocage. This makes 5-F-Trp a potential probe for the study of intermolecular interactions in solution by exploiting its intrinsic fluorescence. More interestingly, albeit the large size of the cage (12 nm external diameter, -500 kDa molecular mass) we observe a broad but well defined NMR 19F resonance that can be used for the dual purpose of detecting solution intermolecular interactions via chemical shift perturbation mapping and monitoring the uptake of ferritin by cells treated with ferritin-based drug carriers, the latter being an application area of increasing importance.