19F NMR spectroscopy monitors ligand binding to recombinantly fluorine-labelled b'x from human protein disulphide isomerase (hPDI).

We report a protein-observe F-19 NMR-based ligand titration binding study of human PDI b'x with Delta-somatostatin that also emphasises the need to optimise recombinant protein fluorination when using 5- or 6-fluoroindole. This study highlights a recombinant preference for 5-fluoroindole over 6-fluoroindole; most likely due to the influence of fluorine atomic packing within the folded protein structure. Fluorination affords a single 19F resonance probe to follow displacement of the protein x-linker as ligand is titrated and provides a dissociation constant of 23 +/- 4 mu M.