Affinity chromatography

Affinity chromatography separates a target through its reversible, selective binding to a ligand immobilized on a chromatographic stationary phase. The chromatographic mobile phase carries a mixture through the material; components with little binding tend to pass through, while the target is retained. Washing removes unbound material, and changing suitable solution conditions or adding a competitor can release the target. Strong binding and recoverable binding are different design requirements.^[ref]

For a concrete experimental comparison, prepare two otherwise similar columns: one bearing the intended ligand, the other lacking it. Load the same protein mixture, collect the flow-through, wash, and then collect the released material. Target enrichment specifically on the ligand-bearing material supports the intended binding mechanism. Retention on both columns instead suggests an additional interaction with the support; disappearance from the flow-through alone does not prove successful purification.

An original numerical example separates recovery from purity. Start with $10$ milligrams of target in $100$ milligrams of total protein. If the collected fraction contains $8$ milligrams of target in $10$ milligrams total, its target purity rises from $10\%$ to $80\%$, while target recovery is $8/10=80\%$. An activity assay may answer a further question: whether the recovered target remains functional.

In his 1970 report, Pedro Cuatrecasas described agarose derivatives for presenting binding partners on an insoluble support. The arrangement makes molecular recognition usable as a preparative tool: retain a chosen partner in one place, pass a complex mixture over it, and recover material selected by that interaction.^[ref]