Protein Precipitation Methods

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Abstract
There are many methods employed to precipitate proteins out of solution. In this experiment we manipulated many physical and chemical variables in order to achieve purification of a protein via precipitation. In the first part of the experiment we purified the protein casein by modifying it’s pH. In the second part of the experiment we manipulated the ionic strength of albumin in egg whites, in a process called salting out. By manipulating these chemical properties we were able to precipitate casein from skim milk and albumin from egg whites. The final product was weighed, and recorded. Final mass of albumin per egg was 0.142 grams. The final mass of casein from skim milk was 5.522 grams. These methods were both successful at separating the protein out of the solution, but the technique that produced the highest percent yield was acidification and modification of a dielectric constant with ethanol. Intro

Proteins can be precipitated out of solution. By modifying the physical and chemical properties, it is possible to selectively precipitate proteins. For example, proteins precipitate from solution when the pH of the solution equals the pI. So by adding acid to reduce the pH to the value to the pI, we can achieve precipitation of specific proteins, depending on their individual pI’s. Another method for precipitating proteins is the salting-in and salting-out method. This process achieves protein separation by using the ionic strength and solubility differences associated with adding salt to the solution. The salt that is most commonly employed in this method is ammonium sulfate. A third variable that can be changed in order to precipitate a protein is heat. Heat application interferes with the noncovalent chemical bonds that keep protein in a folded conformation. The fourth property that is manipulated for protein precipitation is the dielectric constant, which is a variable associated with Coulomb’s Law. Certain chemical...
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