Effect of the Competing Ligand on Ni-NTA/Histag Strength Revealed by Click Chemistry-Based Force Spectroscopy.

The specific interaction between Ni-nitrilotriacetic acid and the six-histidine tag may be one of the most important coordination bonds utilized in biological research because of its wide application for recombinant protein purification. The complex stability is critical for target protein binding. Thus, measurement of the mechanical stability of the system was attempted soon after the invention of atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) two decades ago. Moreover, the competing ligand imidazole and protons are the two critical factors for target protein elution. However, the mechanochemistry between the system and the imidazole/proton has not been determined. Here, an AFM-SMFS system using strain-promoted alkyne-azide cycloaddition and Cu-free click chemistry was used to characterize the system. Consequently, the destabilizing effect of the imidazole and proton on the interaction was revealed quantitatively, leading to a 3-fold increase in the bond dissociation rate.