Zwitterionic behavior: When alanine (R = CH3) is placed in an aqueous solution at pH 7.3, what is its predominant ionic form?

Introduction / Context:
Amino acids are amphoteric and commonly exist as zwitterions in water near physiological pH. Recognizing their protonation states is key to predicting solubility, electrophoresis behavior, and interactions in proteins.

Given Data / Assumptions:

• Alanine pKa values: pKa_carboxyl ≈ 2–3; pKa_amino ≈ 9–10.
• Solution pH = 7.3, slightly above alanine’s isoelectric point (≈ 6.0).
• No extreme ionic strength or denaturants present.

Concept / Approach:
At pH between the two pKa values, the carboxyl is deprotonated (COO−) and the amino group remains protonated (NH3+), producing a dipolar ion (zwitterion) with roughly zero net charge. Slightly above the pI, the population may skew marginally toward the anionic side, but the dominant microstate remains zwitterionic.

Step-by-Step Solution:
Compare pH (7.3) to pKa_carboxyl (~2.3) → fully deprotonated COO−. Compare pH (7.3) to pKa_amino (~9.7) → predominantly protonated NH3+. Combine → NH3+ and COO− on the same molecule (zwitterion). Net charge near zero → consistent with electrophoretic behavior around neutral pH.

Verification / Alternative check:
Henderson–Hasselbalch calculations confirm species distribution: at pH between pKa values, the zwitterionic form dominates for simple amino acids like alanine.

Why Other Options Are Wrong:
Fully cationic form occurs near very low pH; fully anionic form near high pH; “nonpolar neutral without charges” ignores the internal ionic character; isotope is unrelated to acid–base chemistry.

Common Pitfalls:
Equating “net zero charge” with “uncharged”; the zwitterion is charged internally.

Final Answer:
A zwitterion (NH3+ and COO− present, net ~0).