Isoelectric point (pI) — By definition, at which pH condition does an amino acid exhibit no net electric charge and therefore shows minimal electrophoretic mobility in an electric field?

Introduction / Context:
The isoelectric point (pI) is fundamental to techniques such as isoelectric focusing, protein purification, and prediction of solubility. At pI, a molecule's positive and negative charges balance to give zero net charge.

Given Data / Assumptions:

• Amino acids exist as zwitterions over a range of pH values.
• At a specific pH (pI), net charge becomes zero.
• Mobility in an electric field depends on net charge.

Concept / Approach:
The pI is the pH at which the sum of positive charges equals the sum of negative charges; the molecule still bears internal charges but migrates minimally in an electric field because the net force is near zero. For simple amino acids, pI approximates the midpoint between relevant pKa values; for polyionic proteins, pI depends on the distribution of all ionizable side chains.

Step-by-Step Solution:

Verification / Alternative check:
In isoelectric focusing, proteins stop migrating when they reach the gel region where pH equals their pI, confirming minimized mobility at zero net charge.

Why Other Options Are Wrong:

• “Carboxyl group uncharged” or “amino group uncharged”: oversimplify; multiple ionizable groups influence pI.
• “Maximum electrolytic mobility”: incorrect; mobility is minimized near pI.

Common Pitfalls:
Confusing “uncharged groups” with “no net charge”; even at pI, groups can remain ionized in a zwitterionic state.

Final Answer:
where the molecule carries no electric charge