Intrinsic anisotropy parameters of a series of lanthanoid complexes deliver new insights into the structure-magnetism relationship

Summary Lanthanoid chelating tags (LCTs) are widely used for advanced paramagnetic NMR of biomacromolecules. The magnitude of their induced pseudocontact shifts (PCSs) and residual dipolar couplings (RDCs) depends critically on the anisotropy of the magnetic susceptibility tensor, which is usually determined by the resonances of the conjugated protein, inevitably reducing the effect by motional averaging. Here, for the first time, we present the intrinsic anisotropy parameters for the full lanthanoid series determined experimentally from resonances on the ligand itself. The strongly shifted proton spectra could only be assigned by extensive, site-specific isotope labeling. The extremely large anisotropies obtained deliver an upper limit for future PCS applications. To our great surprise, at least at room temperature, we observed an unprecedented correlation between the oblate or prolate f-electron distribution of the lanthanoid and the orientation of the main magnetic axis as well as the size of the magnetic anisotropy.