Question

What are the different forms of Alanine?

• A. Zwitterionic forms \
• B. Zwitterion as acid \
• C. Zwitterion as base \
• D. All of the above

Hint:

Amino acids are amphoteric buffers. Their zwitterion forms can donate a proton (acting as an acid) or accept a proton (acting as a base) depending on the pH.

Answer 1

The Correct Option is D

Step 1: Concept Amino acids are amphoteric molecules containing both acidic carboxyl groups ($-\text{COOH}$) and basic amino groups ($-\text{NH}_{2}$). Depending on the pH of the surrounding solution, they exchange protons and change their ionic form.

Step 2: Meaning A zwitterion is a dipolar ion that contains an equal number of positively and negatively charged functional groups, leaving it with a net charge of zero.

Step 3: Analysis * Around neutral pH, alanine exists predominantly as a zwitterion ($^{+}\text{H}_{3}\text{N}-\text{CH}(\text{CH}_{3})-\text{COO}^{-}$) $\rightarrow$ Option A describes a valid form. * If acid is added (low pH), the carboxylate group accepts a proton ($\text{COO}^{-} \rightarrow \text{COOH}$). In this scenario, the zwitterion acts as a base by accepting a proton $\rightarrow$ Option C is a valid behavior/form. * If base is added (high pH), the ammonium group loses a proton ($\text{NH}_{3}^{+} \rightarrow \text{NH}_{2}$). Here, the zwitterion acts as an acid by donating a proton $\rightarrow$ Option B is a valid behavior/form. Because alanine can adopt all of these ionic states and act as either an acid or a base, all options are correct.

Step 4: Conclusion Since statements A, B, and C all accurately reflect the chemistry of alanine, option D is the correct choice. Final Answer: (D)