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Which of the following statements about the isoelectric point (pI) of amino acids is NOT correct?

1. Solubility of an amino acid is at its minimum at pI.
2. Aspartic acid has a lower pI than glycine.
3. An amino acid exists with no net charge at pI.
4. Basic amino acids exist in cationic form at pH = 7.

Correct answer: An amino acid exists with no net charge at pI.

Solution

At pI, an amino acid exists as a zwitterion (H3N+-CHR-COO-), which carries both a positive and a negative charge but has zero net charge. The statement 'exists with no charge' is incorrect — it should be 'exists with no NET charge'. All other statements are correct: pI corresponds to minimum solubility, Asp (pI~2.77) < Gly (pI~5.97), and basic amino acids (pI > 7) are cationic at pH 7.

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