HIS-Tagged Bacterial Protein Purification Kit (Gravity Flow) is supplied by HiMedia. It is a fast and convenient purification of recombinant polyhistidine-tagged proteins by immobilized metal ion affinity chromatography. Binding of protein is based on the interaction between the polyhistidine tag of the recombinant protein and immobilized Ni2+ ions. The chelating group of Ni-TED is based on TED (tris-carboxymethyl ethylene diamine), a strong pentadentate metal chelator. TED occupies five of the six binding sites in the coordination sphere of the Ni2+ ion, the remaining coordination site of Ni2+ is available for protein binding. Compared to TED, other chelating groups such as NTA (nitrilotriacetic acid) have four binding sites available for the Ni2+ ion, the remaining two sites of Ni2+ are available for protein binding.