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ion exchange chromatography (IEX) is a common protein purification technique in which we separate proteins based on charge. Proteins are made up of amino acid letters, some of which are sometimes charged (depending on pH). Different proteins have different amino acid spellings and thus different charges and we can exploit these different charges to separate them using columns filled with charged resin (basically little beads).  You chose a resin with an opposite charge to that of your protein of interest. Your protein will stick and then you can was off the other stuff and, once all that stuff’s far away,  push your protein off with salt. Since a salt (such as table salt, NaCl) is made up of a positive part (e.g. Na⁺ in table salt) and a negative part (e.g. Cl⁻ in table salt) that come apart when you dissolve them (it both dissolves and dissociates), it provides a source of both positively-charged & negatively-charged competitors. The more opposite the protein & bead charges are, the stronger the charge-opposite charge interactions, and thus the more salt it’ll take to get pushed off. So if you gradually increase the salt you can push proteins off based on their charge, separating them in the process. ⠀

I always had a problem remembering the terminology, but when it comes to ion exchange chromatography – the flavors refer to the type of ion (charged particle) that’s being exchanged – NOT the one that the resin – so anion exchange chromatography uses positively-charged resin to bind negatively-charged (anionic) proteins – and cation exchange chromatography uses negatively-charged resin to bind positively-charged (cationic) proteins. ⠀