You have accidentally mixed a bunch of amino acids (tryptophan, tyrosine, alanine, selenocysteine, proline, arginine) together...

You have accidentally mixed a bunch of amino acids (tryptophan, tyrosine, alanine, selenocysteine, proline, arginine) together and once your boss finds out, you will be fired on the spot. Your only hope of saving your job is to separate them before anyone finds out. You decide that the only viable option is to use ion exchange chromatography. Explain, in detail, how you would do this?

Expert Solution

Ion exchange chromatography involves the separation of ionizable molecules based on their total charge. This technique enables the separation of similar types of molecules that would be difficult to separate by other techniques because the charge carried by the molecule of interest can be readily manipulated by changing buffer pH. The amino acids that form proteins are zwitterionic compounds that contain both positively and negatively charged chemical groups. Depending on the pH of their environment, amino acids may carry a net positive charge, a net negative charge, or no charge. The pH at which a molecule has no net charge is called its isoelectric point, or pI.

The pI value can be calculated based on the primary sequence of the molecule.

In a buffer with a pH greater than the pI of the amino acid, the amino acid will carry a net negative charge; therefore, a positively charged anion exchange resin is chosen to capture this amino acid.

In a buffer with a pH lower than the pI of the amino acid, then it will carry a positive net charge; thus a negatively-charged cation exchange resin is chosen.

When an ion exchange chromatography column is loaded with a sample at a particular pH, all amino acids that are appropriately charged will bind to the resin. For example, if an anion exchange resin is chosen, all amino acids that are negatively charged at the loading buffer pH will bind to the positively charged column resin. A good rule of thumb for choosing a buffer pH is the following:

Anion exchanger — 0.5–1.5 pH units greater than the pI of the amino acids
Cation exchanger — 0.5–1.5 pH units less than the pI of the amino acid

queen_honey_blossom answered 2 years ago

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