What is a common example of affinity chromatography?

Affinity chromatography is a powerful technique used to purify proteins based on their specific interactions with other molecules. The choice involving His tags binding to nickel-NTA resin is a prime example of this technique. In this process, recombinant proteins that have been genetically engineered to include a polyhistidine (His) tag are exposed to a nickel-NTA resin. The imidazole side chains of the histidine residues in the protein form strong, specific interactions with the nickel ions immobilized on the resin. This allows for the selective capture of His-tagged proteins from a mixture, enabling effective purification. After the binding step, unbound contaminants can be washed away, and the desired protein can be eluted by competing off the His tag, typically using a solution with high imidazole concentration.

In contrast, the other options do not represent affinity chromatography. Utilizing size exclusion to separate proteins is a method based on the size of molecules rather than specific interactions. Using carboxymethyl to bind positively charged peptides refers to ion exchange chromatography, where the charge of the molecules is the basis of separation. Employing dialysis to remove excess salts is a separation based on diffusion through a semi-permeable membrane, rather than a specific affinity interaction between a ligand and a target molecule