Protein Precipitation

What Is It?

Precipitation is widely used for product recovery of biomolecules especially proteins. Precipitation is usually induced by addition of a salt or an organic solvent, or by changing the pH to alter the nature of the solution.

How do the different methods work

The most common type of precipitation for proteins is salt induced precipitation. Protein solubility depends on several factors. It is observed that at low concentration of the salt, solubility of the proteins usually increases slightly. This is termed Salting in. But at high concentrations of salt, theo solubility of the proteins drops sharply. This is termed Salting out and the proteins precipitate out.

Another method is the addition of an organic solvent. If there is a medium decrease in the dielectric constant with the addition of an organic solvent, the the solubility should decrease also. Here we can expect precipitation.

A third method is precipitation by changing the pH of the protein solution. This effect is due to the different functional groups on a protein. There will be some pH, known as the isoeletric point where the net charge on the protein is zero. This is different for different proteins.

You can also precipitate proteins by the addition of a non-ionic polymer or metal ions

So, Why Is Protein Precipitation Special?

Proteins are different than most other molecules. The physical structure is just as important as the chemical structure. If the structure of the protein changes, the protein does not have the activity you want. This could even be harmful.

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