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Ch.13 - Properties of Solutions
All textbooksBrown 14th EditionCh.13 - Properties of SolutionsProblem 89
Chapter 13, Problem 89

Proteins can be precipitated out of aqueous solution by the addition of an electrolyte; this process is called “salting out” the protein. 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?

Verified step by step guidance
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Step 1: Understand the concept of 'salting out' in protein chemistry. 'Salting out' is a process where proteins precipitate out of solution upon the addition of high concentrations of salt. This is due to changes in the solubility of the protein in the presence of the salt.
Step 2: Analyze the first hypothesis: The waters of hydration surrounding the protein prefer to surround the electrolyte. This suggests that the electrolyte competes with the protein for water molecules, leading to dehydration of the protein and subsequent precipitation.
Step 3: Analyze the second hypothesis: The incoming ions adsorb onto the protein surface, forming ion pairs and neutralizing the protein's charge. This implies that the protein's solubility decreases because it loses its charge, which is essential for maintaining solubility in water.
Step 4: Consider experimental measurements to test these hypotheses. For the first hypothesis, you could measure the hydration shell of the protein using techniques like nuclear magnetic resonance (NMR) or infrared spectroscopy to observe changes in water-protein interactions.
Step 5: For the second hypothesis, you could use electrophoretic mobility measurements or zeta potential analysis to determine changes in the protein's net charge as salt is added. This would help identify if the protein's charge is being neutralized by ion adsorption.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Hydration Shell

A hydration shell is a layer of water molecules that surrounds a solute particle, such as a protein, in an aqueous solution. This shell stabilizes the solute by forming hydrogen bonds and electrostatic interactions with the water molecules. When an electrolyte is added, the water molecules may preferentially interact with the ions of the electrolyte, potentially disrupting the hydration shell of the protein and leading to its precipitation.
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Ionic Interactions and Charge Neutralization

Ionic interactions refer to the electrostatic forces between charged particles, such as proteins and ions. When ions from an electrolyte adsorb onto the surface of a protein, they can neutralize the protein's charge, which can alter its solubility in water. If the protein achieves a zero net charge, it may precipitate out of solution due to reduced repulsion between protein molecules, leading to aggregation.
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Experimental Measurements for Hypothesis Testing

To distinguish between the two hypotheses regarding salting out, specific experimental measurements can be conducted. Techniques such as zeta potential measurements can assess the net charge of the protein in the presence of electrolytes, while dynamic light scattering can provide information on particle size and aggregation. Additionally, measuring the solubility of the protein at varying electrolyte concentrations can help elucidate the mechanism behind the precipitation process.
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Related Practice
Textbook Question

Aerosols are important components of the atmosphere. Does the presence of aerosols in the atmosphere increase or decrease the amount of sunlight that arrives at the Earth's surface, compared to an 'aerosol-free' atmosphere? Explain your reasoning.

Textbook Question

The 'free-base' form of cocaine (C17H21NO4) and its protonated hydrochloride form (C17H22ClNO4) are shown below; the free-base form can be converted to the hydrochloride form with one equivalent of HCl. For clarity, not all the carbon and hydrogen atoms are shown; each vertex represents a carbon atom with the appropriate number of hydrogen atoms so that each carbon makes four bonds to other atoms

(a) One of these forms of cocaine is relatively water-soluble: which form, the free base or the hydrochloride?

Textbook Question

The 'free-base' form of cocaine (C17H21NO4) and its protonated hydrochloride form (C17H21NO4) are shown below; the free-base form can be converted to the hydrochloride form with one equivalent of HCl. For clarity, not all the carbon and hydrogen atoms are shown; each vertex represents a carbon atom with the appropriate number of hydrogen atoms so that each carbon makes four bonds to other atoms (e) How many mL of a concentrated 18.0 M HCl aqueous solution would it take to convert 1.00 kilograms (a 'kilo') of the free-base form of cocaine into its hydrochloride form?