Paramagnetic effects from lanthanide ions present powerful tools for protein studies by nuclear magnetic resonance (NMR) spectroscopy provid-ed that the lanthanide can be site-spe-cifically and rigidly attached to the pro-tein. A new, particularly small and rigid lanthanide-binding tag, 3-mercap-to-2, 6-pyridinedicarboxylic acid (3MDPA), was synthesized and at-tached to two different proteins via a disulfide bond. The complexes of the N-terminal domain of the E. coli argi-nine repressor (ArgN) with seven dif-ferent paramagnetic lanthanide ions and Co2+ were analyzed in detail by NMR spectroscopy. The magnetic sus-ceptibility anisotropy (ΔX) tensors and metal position were determined from pseudocontact shifts. The 3MDPA tag generated very different Ax tensor ori-entations compared to the previously studied 4-mercaptomethyl-DPA tag, making it a highly complementary and useful tool for protein NMR studies.