Selective Displacement of a Scorpionand Triazole Ligand from Metallocyclam Complexes Visualised with NMR Spectroscopy

Target-activated metal complexes (TAMCs) - complexes that remain benign until reaching a specific biomolecular target, binding to which then effects structural change that turns on cytotoxicity or other activity - 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 metallocyclam 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 for zinc(II). Triazole displacement may be visualised by monitoring the H-1 NMR resonance of the single triazole C-H proton environment. Using H-2 NMR spectroscopy with a specifically deuterated complex enables the same change to be monitored in a noisy H-1 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 the stripping of the metal ion from the complex.