Carnosine can chelate divalent metal ions. ATP forms chelate complexes with metal ions. Magnesium citrate has been reported as more bioavailable than oxide or amino-acid chelate forms. Amino acids form very stable chelate complexes with copper(II). Siderophores are usually classified by the ligands used to chelate the ferric iron. Chemical indicators are small molecules that can chelate calcium ions. The acid has been observed to chelate metal ions such as calcium and magnesium. Siderophores, natural or synthetic, can chelate metal ions other than iron ions. Metal chelate compounds are common components of fertilizers to provide micronutrients. Complexes of polydentate ligands are called "chelate" complexes. It is the precursor of phytochelatins, glutathione oligomers that chelate heavy metals such as cadmium. The ATP molecule is normally found in a chelate with a magnesium ion. In addition and due to its ability to chelate metal ions, it provides corrosion inhibition. They chelate Zn ion, thereby inhibiting MMP activity. An acid-base reaction takes place with the formation of zinc eugenolate chelate.