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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