Practice question

Polyhistidine tags, typically hexahistidine, exploit coordination chemistry between imidazole side chains of histidine and immobilized divalent transition metal ions such as Ni2+ or Co2+ chelated by nitrilotriacetic acid resin. At physiological pH, histidine residues donate electrons to vacant orbitals of metal, forming stable complex that allows selective retention of tagged protein from crude lysate. Elution is achieved with high imidazole concentration or low pH. Avidin, GST, and Protein A are affinity matrices for biotinylated, GST-tagged, and antibody Fc-containing proteins respectively, unrelated to metal affinity chromatography principle.