Biomolecules will bind themselves to oppositely charged groups due to the presence of charged
moieties, on an ion-exchange matrix. Separation of the bound substances occurs when elution of
the matrix causes the strength of the binding to decrease. Molecules with very small differences in
charge can be separated, making ion exchange a very high-resolution technique.
lon-exchange columns packed with, say, carboxylate groups (-CO0H) are available. Such
a column is negatively charged when ionized (CO0-) and a protein with a net positive charge
will bind itself to the column, whereas a protein with a negative charge will not do so. The bound
protein molecules can be 'exchanged' with positively charged ions (such as NaCl) and this releases
the negatively charged protein. In addition to a high sample capacity, the ability to process large
volumes and the elution of dilute sample components in a concentrated form make ion exchange
extremely useful.
