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