In aqueous solution, amino acids exist predominantly as dipolar internal salts known as zwitterions. What is the electrical net charge of a neutral amino acid at its isoelectric point (pI)?

Answer and explanation

Correct answer: B. Exactly zero net charge

  1. Step 1: In aqueous neutral solution, the basic amino group () accepts a proton from the acidic carboxyl group () to form a dipolar ion (zwitterion): .
  2. Step 2: The isoelectric point () is defined as the specific pH of the aqueous medium at which the concentrations of the cationic form and anionic form are exactly equal.
  3. Step 3: At this unique pH, the molecule carries an equal number of positive and negative charges ( and ), resulting in a net electrical charge of zero.
  4. Step 4: Consequently, at the isoelectric point, the amino acid does not migrate toward either the cathode or the anode in an electric field.
Answer
Exactly zero net charge
Why not the others: '+1.0 net charge' occurs at pH below the (strongly acidic medium); '-1.0 net charge' occurs at pH above the (strongly alkaline medium); '+2.0 net charge' would require a diprotic basic side chain at very low pH.

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