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Ion exchange chromatography separates molecules based on differences between the overall charge of the proteins. It is usually used for protein purification but may be used for purification of oligonucleotides, peptides, or other charged molecules, The protein of interest must have a charge opposite that of the functional group attached to the resin in order to bind. For example, immunoglobulins, which generally have an overall positive charge, will bind well to cation exchangers, which contain negatively charged functional groups. Because this interaction is ionic, binding must take place under low ionic conditions. Elution is achieved by increasing the ionic strength to break up the ionic interaction, or by changing the pH of the protein.
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