Question

Which one of the following amino acids has more than two acid-base groups?

• A. Alanine
• B. Leucine
• C. Phenylalanine
• D. Tyrosine

Hint:

To identify amino acids with more than two acid-base groups, look for those with ionizable side chains (e.g., Asp, Glu, Lys, Arg, His, Cys, Tyr).

Answer 1

The Correct Option is D

Step 1: General acid-base groups in amino acids
- Every amino acid has at least two acid-base groups: the $\alpha$-amino group (–NH$_3^+$) and the $\alpha$-carboxyl group (–COOH).
- Some amino acids have an additional ionizable side chain, which contributes a third acid-base group. Step 2: Analyze each option
- Alanine: only –NH$_3^+$ and –COOH groups → 2 groups.
- Leucine: only –NH$_3^+$ and –COOH groups → 2 groups.
- Phenylalanine: only –NH$_3^+$ and –COOH groups → 2 groups (benzyl group is not ionizable).
- Tyrosine: has –NH$_3^+$, –COOH, and a phenolic –OH group in the side chain, which is weakly acidic (p$K_a$ $\approx$ 10.1). → 3 ionizable groups. Therefore, the amino acid with more than two acid-base groups is Tyrosine.