In an aqueous solution at its characteristic isoelectric point (pI), an -amino acid exists predominantly as which electrical species?

Answer and explanation

Correct answer: D. A dipolar ion or zwitterion carrying net zero electrical charge

  1. Step 1: The carboxylic acid group () is acidic and donates a proton, while the amino group () is basic and accepts a proton.
  2. Step 2: In aqueous solution, internal proton transfer yields a dipolar ion called a zwitterion: .
  3. Step 3: At the isoelectric point (pI), the concentrations of cationic and anionic species are exactly equal, so the zwitterion has a net electric charge of zero and does not migrate in an electric field.
Answer
A dipolar ion or zwitterion carrying net zero electrical charge ().
Why not the others: A completely un-ionized molecule does not exist in significant concentration in aqueous media; A net positive cation dominates only in strongly acidic solutions (); A net negative anion dominates only in strongly basic solutions ().

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