Assessing multiple conformations of lanthanide binding tags for proteins using a sensitive 19F-reporter

Distinct stereospecific effect of chiral tether between a tag and protein on the rigidity of paramagnetic tag

Flexibility between the paramagnetic tag and its protein conjugates is a common yet unresolved issue in the applications of paramagnetic NMR spectroscopy in biological systems. The flexibility greatly attenuates the magnetic anisotropy and compromises paramagnetic effects especially for pseudocontact shift and residual dipolar couplings. Great efforts have been made to improve the rigidity of paramagnetic tag in the protein conjugates, however, the effect of local environment vicinal to the protein ligation site on the paramagnetic effects remains poorly understood. In the present work, the stereospecific effect of chiral tether between the protein and a tag on the paramagnetic effects produced by the tag attached via a D- and L-type linker between the protein and paramagnetic metal chelating moiety was assessed. The remarkable chiral effect of the D- and L-type tether between the tag and the protein on the rigidity of paramagnetic tag is disclosed in a number of protein-tag-Ln complexes. The chiral tether formed between the D-type tag and L-type protein surface minimizes the effect of the local environment surrounding the ligation site on the averaging of paramagnetic tag, which is helpful to preserve the rigidity of a paramagnetic tag in the protein conjugates.

Paramagnetic Chemical Probes for Studying Biological Macromolecules

Paramagnetic chemical probes have been used in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy for more than four decades. Recent years witnessed a great increase in the variety of probes for the study of biological macromolecules (proteins, nucleic acids, and oligosaccharides). This Review aims to provide a comprehensive overview of the existing paramagnetic chemical probes, including chemical synthetic approaches, functional properties, and selected applications. Recent developments have seen, in particular, a rapid expansion of the range of lanthanoid probes with anisotropic magnetic susceptibilities for the generation of structural restraints based on residual dipolar couplings and pseudocontact shifts in solution and solid state NMR spectroscopy, mostly for protein studies. Also many new isotropic paramagnetic probes, suitable for NMR measurements of paramagnetic relaxation enhancements, as well as EPR spectroscopic studies (in particular double resonance techniques) have been developed and employed to investigate biological macromolecules. Notwithstanding the large number of reported probes, only few have found broad application and further development of probes for dedicated applications is foreseen.

Rigid, Highly Reactive and Stable DOTA-like Tags Containing a Thiol-Specific Phenylsulfonyl Pyridine Moiety for Protein Modification and NMR Analysis*

Site specific installation of a paramagnetic ion with magnetic anisotropy in a biomolecule generates valuable structural restraints, such as pseudocontact shifts (PCSs) and residual dipolar couplings (RDCs). These paramagnetic effects can be used to characterize the structures, interactions and dynamics of biological macromolecules and their complexes. Two single-armed DOTA-like tags, BrPSPy-DO3M(S)A-Ln and BrPSPy-6M-DO3M(S)A-Ln, each containing a thiol-specific reacting group, that is, a phenylsulfonyl pyridine moiety, are demonstrated as rigid, reactive and stable paramagnetic tags for protein modification by formation of a reducing resistant thioether bond between the protein and the tag. The two tags present high reactivity with the solvent exposed thiol group in aqueous solution at room temperature. The introduction of Br at the meta-position in pyridine enhances the reactivity of 4-phenylsulfonyl pyridine towards the solvent exposed thiol group in a protein, whereas the ortho-methyl group in pyridine increases the rigidity of the tag in the protein conjugates. The high performance of these two tags has been demonstrated in different cysteine mutants of ubiquitin and GB1. The high reactivity and rigidity of these two tags can be added in the toolbox of paramagnetic tags suitable for the high-resolution NMR measurements of biological macromolecules and their complexes.