Lanthanide-binding peptides for NMR measurements of residual dipolar couplings and paramagnetic effects from multiple angles

Lanthanide-binding peptide tags (LBTs) containing a single cysteine residue can be attached to proteins via a disulfide bond, presenting a flexible means of tagging proteins site-specifically with a lanthanide ion. Here we show that cysteine residues placed in different positions of the LBT can be used to expose the protein to different orientations of the magnetic susceptibility anisotropy (Δχ) tensor and to generate different molecular alignments in a magnetic field. Δχ tensors determined by nuclear magnetic resonance (NMR) spectroscopy for LBT complexes with Yb³⁺, Tm³⁺, and Er³⁺ suggest a rational way of producing alignment tensors with different orientations. In addition, knowledge of the Δχ tensor of LBT allows modeling of the protein-LBT structures. Despite evidence for residual mobility of the LBTs with respect to the protein, the pseudocontact shifts and residual dipolar couplings displayed by proteins disulfide-bonded to LBTs are greater than those achievable with most other lanthanide binding tags.