When the pH of a solution is greater than the isoelectric point (pI) of a protein, the protein will have a net negative charge.
Understanding pI and pH Relationship
The isoelectric point (pI) is the pH at which a molecule, such as a protein, carries no net electrical charge or is electrically neutral. pH, on the other hand, is a measure of the acidity or basicity of a solution.
Here's a breakdown of what happens when pH > pI:
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Deprotonation: At a pH above the pI, there are more hydroxide ions (OH-) in the solution. These hydroxide ions tend to deprotonate (remove protons from) the acidic groups (e.g., -COOH) on the protein.
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Negative Charge: The deprotonation of acidic groups results in negatively charged groups (e.g., -COO-). The protein gains more negative charges than positive charges.
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Net Negative Charge: Consequently, the overall net charge of the protein becomes negative. The protein will migrate towards a positive electrode if placed in an electric field (electrophoresis).
Consequences of Negative Charge
The net negative charge of the protein when pH > pI can have several effects:
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Solubility: The negative charge can increase the protein's solubility in aqueous solutions due to electrostatic repulsion between protein molecules, preventing aggregation.
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Interactions: The negatively charged protein will interact differently with other molecules in the solution. It will repel negatively charged molecules and attract positively charged molecules.
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Conformation: The charge distribution can also affect the protein's conformation (shape) as the molecule adjusts to minimize electrostatic repulsion and maximize attraction within the protein.
Example
Consider a protein with a pI of 6.0.
- At pH 5.0 (pH < pI), the protein will have a net positive charge.
- At pH 6.0 (pH = pI), the protein will have no net charge.
- At pH 7.0 (pH > pI), the protein will have a net negative charge.
In summary, when the pH of a solution exceeds a protein's isoelectric point (pI), the protein acquires a net negative charge due to deprotonation of acidic residues, influencing its solubility, interactions with other molecules, and potentially its conformation.
