At its specific isoelectric point (pI), an amino acid exists in which ionization state?
Answer and explanation
Correct answer: A. Zwitterion with an equal number of positive and negative charges and net charge zero
- Step 1: Amino acids contain both basic amino groups () and acidic carboxyl groups ().
- Step 2: At the isoelectric point (pI), internal proton transfer yields a dipolar ion called a zwitterion: and .
- Step 3: The positive and negative charges balance identically, resulting in a net electrical charge of zero, meaning the molecule does not migrate in an electric field.
Answer
Zwitterion with an equal number of positive and negative charges and net charge zero.
Why not the others: Cationic form occurs in strongly acidic solutions (); Anionic form occurs in strongly alkaline solutions (); Completely uncharged non-ionic molecule does not exist in aqueous biological solutions.
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