The isoelectric point (pI) is the pH at which an amino acid has no net charge. For acidic residues, how is the pI determined?

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Multiple Choice

The isoelectric point (pI) is the pH at which an amino acid has no net charge. For acidic residues, how is the pI determined?

Explanation:
The key idea is that the isoelectric point is defined by the pH at which the molecule carries no net charge. For acidic residues, there are two acidic sites that can lose protons—the α-carboxyl group and the side-chain carboxyl group. As pH rises, the molecule transitions from positively charged to negatively charged, and there is a pH between those two deprotonation steps where the overall charge sums to zero. That zero-charge point is what the pI represents. In practice, you can estimate its value by averaging the pKa values of those two acidic groups, but the fundamental criterion remains the pH at which the molecule has no net charge.

The key idea is that the isoelectric point is defined by the pH at which the molecule carries no net charge. For acidic residues, there are two acidic sites that can lose protons—the α-carboxyl group and the side-chain carboxyl group. As pH rises, the molecule transitions from positively charged to negatively charged, and there is a pH between those two deprotonation steps where the overall charge sums to zero. That zero-charge point is what the pI represents. In practice, you can estimate its value by averaging the pKa values of those two acidic groups, but the fundamental criterion remains the pH at which the molecule has no net charge.

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