Problem 89 - Chemistry: The Central Science

Question

Proteins can be precipitated out of aqueous solution by the addition of an electrolyte; this process is called "salting out" the protein. (a) Do you think that all proteins would be precipitated out to the same extent by the same concentration of the same electrolyte? (b) If a protein has been salted out, are the protein-protein interactions stronger or weaker than they were before the electrolyte was added? (c) A friend of yours who is taking a biochemistry class says that salting out works because the waters of hydration that surround the protein prefer to surround the electrolyte as the electrolyte is added; therefore, the protein's hydration shell is stripped away, leading to protein precipitation. Another friend of yours in the same biochemistry class says that salting out works because the incoming ions adsorb tightly to the protein, making ion pairs on the protein surface, which end up giving the protein a zero net charge in water and therefore leading to precipitation. Discuss these two hypotheses. What kind of measurements would you need to make to distinguish between these two hypotheses?

Step-by-Step Solution

Answer

Different proteins have different amino acid compositions, sizes, and shapes, so it is unlikely that all proteins would be precipitated out to the same extent by the same concentration of the same electrolyte. When a protein is salted out, protein-protein interactions become stronger due to the disruption of the protein's hydration shell. To distinguish between the two hypotheses of salting out (hydration shell removal vs. ion adsorption), measurements such as zeta potential, spectroscopic techniques, and solvent structure analysis can be performed. These measurements may reveal changes in protein charge, the presence of ion pairs, or alterations in solvent structure, helping to distinguish between the two hypotheses.

Step 1: (a) Precipitation of Different Proteins by the Same Electrolyte

Different proteins have different amino acid compositions, sizes, and shapes. The degree of their interaction with the electrolyte and the extent to which they are precipitated out can vary from one protein to another. Therefore, it is unlikely that all proteins would be precipitated out to the same extent by the same concentration of the same electrolyte.

Step 2: (b) Protein-Protein Interactions in the Presence of Electrolytes

When a protein is salted out, the protein's hydration shell is disrupted, and the protein molecules come closer together. As a result, the protein-protein interactions become stronger, since the attractive forces between the proteins now dominate their interaction with the solvent.

Step 3: (c) Two Hypotheses of Salting Out

The first hypothesis suggests that waters of hydration prefer to surround the electrolyte ions, leading to the removal of the protein's hydration shell and protein precipitation. The second hypothesis states that the incoming ions adsorb tightly to the protein, creating ion pairs on the protein surface. The protein ends up with zero net charge in water, leading to precipitation.

Step 4: (c) Measurements to Distinguish Between Hypotheses

To distinguish between these two hypotheses, several measurements could be performed: 1. We could analyze the changes in electrolyte concentration and protein charge by measuring the zeta potential (the potential difference between the dispersion medium and the stationary layer of the fluid) of the proteins. A decrease in protein charge would support the ion adsorption hypothesis, while no significant change would support the hydration shell hypothesis. 2. We could investigate the presence of ion pairs on the protein surface using spectroscopic techniques, such as infrared spectroscopy or X-ray crystallography. The presence of ion pairs would support the ion adsorption hypothesis. 3. We could examine the solvent structure around the protein and the electrolyte using techniques like small-angle X-ray scattering (SAXS) or neutron scattering. Changes in solvent structure could indicate the removal of the protein's hydration shell or the formation of ion pairs, helping to distinguish between the two hypotheses.