Localising nuclear spins by pseudocontact shifts from a single tagging site

Rational Design of Lanthanide-Binding Tags to Optimize Magnetic Anisotropy in Paramagnetic Protein NMR

Due to their exceptional anisotropic magnetic properties, lanthanide ion (Ln3+) complexes are of great utility in many fields of chemistry, including magnetic materials, biomedical imaging, and nuclear magnetic resonance (NMR) spectroscopy. How to achieve large magnetic anisotropies in the Ln3+ complexes coordinated with open-chain ligands is still a challenge. In this study, we started from the open-chain 4PS-PyMTA ligand and assessed the magnetic anisotropy using installed Ln3+ coordinating pendants by increasing size and rigidity. This approach yielded Ln3+ complexes with increasing magnetic anisotropies. The magnetic anisotropy and conformational dynamics of these open-chain 4PS-PyMTA-based Ln3+ complexes were evaluated by NMR spectroscopy. The impact of the coordination arms on the magnetic anisotropy was further characterized by the ligand field parameters derived from europium luminescence spectra. These data show that the design strategy yielded an efficient way to enhance the conformational rigidity of the Ln3+ chelating moiety and the ligand field strength, which underpins magnetic anisotropy. The magnitudes of pseudocontact shifts and residual dipolar couplings obtained with these 4PS-PyMTA-derived tags installed on a protein rival those obtained previously with synthetically less accessible cyclen-based Ln3+ tags. Our work provides a practical strategy to open-chain Ln3+ complexes with large magnetic anisotropies and straightforward synthetic protocols.

A Set of Three GdIII Spin Labels with Methanethiosulfonyl Groups for Bioconjugation Covering a Wide Range of EPR Line Widths

Spin labels based on GdIII complexes are important tools for the elucidation of the structure, dynamics and interaction of biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Their EPR spectroscopic properties line width and relaxation times influence their performance in a particular application. To be able to apply a complex well-suited for a specific application, a set of GdIII complexes with different EPR spectroscopic properties ready-made for spin labeling will be highly useful. We prepared three GdIII complexes with DO3APic, NO3Pic, and PyMTA as the basic ligand units. They cover a wide range of EPR line widths but have in common a cysteine-targeting methanethiosulfonyl (MTS) group connected to a pyridine ring, which is an intrinsic part of the ligand. The reaction with a cysteine-containing pentapeptide (0.45 mM in the peptide, pH ∼ 7) was complete within 90 s and chemoselective. The MTS group hydrolyzed with half-lives of >24, 8, 2, and 1 h at pH 5, 6, 7, and 8, respectively. The structurally related nicotinic acid-substituted disulfide (NDS) group was found to be hydrolytically much more stable. However, the MTS spin label clearly won the competition for the pentapeptide over the NDS spin label. If high reactivity is essential, MTS is clearly the better choice.

A lanthanide tag for a complementary set of pseudocontact shifts

Pseudocontact shifts (PCS) generated by paramagnetic lanthanide ions deliver powerful restraints for protein structure analysis by NMR spectroscopy. We present a new lanthanide tag that generates different PCSs than that of a related tag, which differs in structure by a single oxygen atom. It is highly reactive towards cysteine and performs well in turn-on luminescence and in EPR spectroscopy.

Ligand-Capped Cobalt(II) Multiplies the Value of the Double-Histidine Motif for PCS NMR Studies

In structural studies by NMR, pseudocontact shifts (PCSs) provide both angular and distance information. For proteins, incorporation of a di-histidine (diHis) motif, coordinated to Co2+, has emerged as an important tool to measure PCS. Here, we show that using different Co(II)-chelating ligands, such as nitrilotriacetic acid (NTA) and iminodiacetic acid (IDA), resolves the isosurface ambiguity of Co2+-diHis and yields orthogonal PCS data sets with different Δχ-tensors for the same diHis-bearing protein. Importantly, such capping ligands effectively eliminate undesired intermolecular interactions, which can be detrimental to PCS studies. Devising and employing ligand-capping strategies afford versatile and powerful means to obtain multiple orthogonal PCS data sets, significantly extending the use of the diHis motif for structural studies by NMR.

Site-selective generation of lanthanoid binding sites on proteins using 4-fluoro-2,6-dicyanopyridine

The paramagnetism of a lanthanoid tag site-specifically installed on a protein provides a rich source of structural information accessible by nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. Here we report a lanthanoid tag for selective reaction with cysteine or selenocysteine with formation of a (seleno)thioether bond and a short tether between the lanthanoid ion and the protein backbone. The tag is assembled on the protein in three steps, comprising (i) reaction with 4-fluoro-2,6-dicyanopyridine (FDCP); (ii) reaction of the cyano groups with α -cysteine, penicillamine or β -cysteine to complete the lanthanoid chelating moiety; and (iii) titration with a lanthanoid ion. FDCP reacts much faster with selenocysteine than cysteine, opening a route for selective tagging in the presence of solvent-exposed cysteine residues. Loaded with Tb 3 + and Tm 3 + ions, pseudocontact shifts were observed in protein NMR spectra, confirming that the tag delivers good immobilisation of the lanthanoid ion relative to the protein, which was also manifested in residual dipolar couplings. Completion of the tag with different 1,2-aminothiol compounds resulted in different magnetic susceptibility tensors. In addition, the tag proved suitable for measuring distance distributions in double electron-electron resonance experiments after titration with Gd 3 +  ions.