Salt coagulates proteins by disrupting the interactions that keep them dissolved in solution, leading to aggregation and precipitation.
Here's a more detailed explanation:
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Disruption of Protein-Water Interactions: Proteins are typically surrounded by a layer of water molecules that interact with the charged and polar amino acid residues on the protein's surface. These interactions help keep the protein soluble.
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Charge Shielding by Salt Ions: When salt is added to the solution, the salt ions (e.g., Na+ and Cl-) interact with the charged amino acid residues on the protein's surface. This effectively shields these charges, reducing the protein's ability to interact with water. In essence, the protein interacts with the salt ions rather than the water molecules.
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Exposure of Hydrophobic Patches: As the water molecules are displaced by salt ions, hydrophobic regions on the protein surface, which were previously hidden from the aqueous environment, become exposed.
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Aggregation and Precipitation: Hydrophobic regions of multiple protein molecules now attract each other to minimize their contact with water. This leads to the formation of aggregates. As the aggregates grow larger, they become less soluble and eventually precipitate out of the solution, forming a visible coagulated mass. This precipitation is also known as salting out.
In summary, salt coagulates proteins by shielding charges, exposing hydrophobic patches, and promoting protein-protein interactions over protein-water interactions, leading to aggregation and precipitation.
