Protein Lab Write Up

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
When the pH=pI, protein will precipitate out. The pH of milk is around 6.5. The casein pI is 4.6. By adding acid to the milk until the pH is reduced from 6.5 to 4.6, we successfully forced the protein to separate out. (See Equation #2) If skim milk had another protein that had a pI of 3.5, we would have to add more acid in order to get that secondary protein to precipitate out with the casein. At the isoelectric point the protein is uncharged and displays minimal water solubility. At low pH the protein carries a positive charge, at high pH the protein carries a negative charge. We would need the pH of the solution to be 3.5 if we wanted to precipitate a protein out of it with as isoelectric point of 3.5. This is a nice feature of proteins, because it allows us to selectively precipitate the proteins, when several are present. The final mass of casein recovered was 5.522 grams. The percent yield was calculated to be 69.025%. (See Calculation #1) This is a reasonable percent yield of recovered casein from skim milk. Milk actually contains 3 main types of protein, casein, lactalbumins, and lactoglobulins. It is possible that the other 2 proteins did not precipitate out, and that’s why only approximately 70% was