In biochemistry, the isoelectric pH is the pH value where a molecule, such as a protein or amino acid, carries no net electrical charge. According to the provided reference, it is "the pH at which no net migration takes place in an electric field." This means that at the isoelectric pH, if you were to put the molecule in an electric field, it would not move towards either the positive or negative electrode because the positive and negative charges on the molecule are perfectly balanced.
Understanding the Isoelectric Point (pI)
The isoelectric point (pI), which corresponds to the isoelectric pH, is a crucial property of amino acids and proteins. It's the pH at which the molecule is electrically neutral. This neutrality impacts several characteristics:
- Solubility: Proteins are typically least soluble at their isoelectric point. This is because the lack of net charge reduces the protein's ability to interact with water molecules.
- Migration in Electric Field: As the definition states, at the pI, a molecule won't migrate in an electric field during techniques like electrophoresis.
- Protein Purification: The pI can be utilized in protein purification techniques like isoelectric focusing.
Isoelectric Point vs. Isoionic Point
The provided reference also mentions the isoionic point: "the pH at which there is no net charge on the molecule." While often used interchangeably, the isoelectric point and the isoionic point are theoretically distinct. The isoelectric point is determined experimentally by observing the pH at which there's no migration in an electric field. The isoionic point, on the other hand, is the pH where the number of positive and negative charges on the molecule are equal in pure water. In practice, these values are often very close.
Calculating the Isoelectric Point
The method to calculate the isoelectric point depends on the molecule in question:
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For simple amino acids (without ionizable side chains): The pI is the average of the pKa values of the carboxyl group and the amino group.
pI = (pKa1 + pKa2) / 2
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For amino acids with ionizable side chains: The calculation becomes more complex, involving the pKa of the side chain as well. You need to identify the two pKa values that "bracket" the neutral species (zwitterion). The pI is the average of these two pKa values.
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For proteins: Predicting the pI of a protein is challenging due to the complex interactions of various amino acids. Computational tools are often used for estimations.
Example
Let's take glycine, a simple amino acid. Glycine has a pKa of ~2.34 for its carboxyl group and ~9.60 for its amino group. Therefore, the pI of glycine is:
pI = (2.34 + 9.60) / 2 = 5.97
This means that glycine will have no net charge at a pH of approximately 5.97.
