Selective displacement of a scorpionand triazole ligand from metallocyclam complexes visualised using NMR spectroscopy.

13 December 2019

Target-activated metal complexes (TAMCs) ā€“ complexes that remain benign until reaching a specific biomolecular target, binding to which then affects structural change that turns on cytotoxicity or other activity ā€“ hold considerable allure. The successful development of TAMCs requires analytical methods that allow clear and unequivocal visualisation of changes in the coordination geometry of these systems in solution. Towards this goal, we report an NMR-based method to monitor coordination/ de-coordination of a pendant triazole ligand to/ from metal-cyclam complexes of zinc(II) and mercury(II). This scorpionand ligand can be displaced from the metal, which remains bound to the macrocyclic ligand, using an appropriate competing ligand: chloride for the mercury(II) complexes, piperidine or citrate at zinc(II). Triazole displacement may be visualised by monitoring the 1H NMR resonance of the single triazole Cā€“H proton environment. Using 2H NMR with a specifically deuterated complex enables the same change to be monitored in a noisy 1H landscape, as would be encountered at a protein binding site or other biological context. MALDI-TOF mass spectrometry experiments provide confirmation that the changes observed by NMR spectroscopy are due to changes in triazole coordination, rather than stripping of the metal ion from the complex.