Dynamic Exchange of the Metal Chelating Moiety: A Key Factor in Determining the Rigidity of Protein–Tag Conjugates in Paramagnetic NMR

Site-specific labeling of proteins with a paramagnetic tag is an efficient way to provide atomic-resolution information about the dynamics, interactions, and structures of the proteins and protein-ligand complexes. The paramagnetic effects manifested in NMR spectroscopy generally contain paramagnetic relaxation enhancement, pseudocontact shifts (PCSs), and residual dipolar coupling (RDC), and these effects correlate closely with the flexibility of protein-tag conjugates. The rigidity of the paramagnetic tag is greatly important in decoding the structural details of macromolecular complexes, because paramagnetic averaging reduces the PCSs and RDCs. Here we show that the dynamic exchange of the metal chelating moiety is a key factor in determining the rigidity of the paramagnetic tag in the protein conjugates. Decreasing the conformational exchange rates in the metal chelating moiety greatly minimizes the paramagnetic averaging and thus increases PCSs and RDCs. This effect has been demonstrated in an open-chain tag, Py-l-Cys-DTPA, which generates large PCSs and RDCs that are comparable to those of the reported cyclic DOTA-like tags. The proposed route offers a unique way to design suitable paramagnetic tags for applications in biological systems.