Nitriloacetic acid (NTA) is a chelating agent that forms coordination compounds with metal ions. Nickel (Ni²⁺) is most used. The resulting complex interacts with histidines due to the intrinsic ability of the imidazole groups to chelate nickel (II) nitriloacetate.

This fact is harnessed for the purification of His-tag proteins via immobilized Ni-NTA residues, e.g. in affinity columns. In addition, this interaction can be used to label proteins with Fluorophores.