The interactions of an unpaired electron and the biomolecular nuclear spins provide rich sources of structural restraints, usually represented in paramagnetic relaxation enhancement (PRE), pseudocontact shift (PCS) and residual dipolar coupling (RDC)1. These paramagnetic effects are generally achieved via site-specific labeling of proteins with paramagnetic metal ions since most proteins don’t have paramagnetic centers. How to increase the rigidity and stability of the paramagnetic tag is still challenging in site-specific labeling of proteins for NMR study. In the present report, we show that the non-covalent interaction of paramagnetic complex and protein provides an interesting way of generating fine-tuned paramagnetic effects on proteins.