4,4'-dithiobisdipicolinic acid: a small and convenient lanthanide binding tag for protein NMR spectroscopy.

Pseudocontact shifts (PCS) from paramagnetic lanthanide ions present powerful long-range structure restraints for studies of proteins by nuclear magnetic resonance spectroscopy. To elicit PCSs, the lanthanide must be attached site-specifically to the target protein. In addition, it needs to be attached rigidly to avoid averaging of the PCSs due to mobility with respect to the protein and it must not interfere with the function of the protein. Here, we present a dipicolinic acid reagent that spontaneously forms a disulfide bond with thiol groups of accessible cysteine residues. A minimal number of rotatable bonds between the cysteine side chain and the tag helps to minimise mobility. Combined with the small size of the tag and quantitative tagging yields, these features make it a highly attractive tool for generating structure restraints by paramagnetic lanthanides.