Discussion
Which one of the following amino acids has more than two acid-base groups?
Answer: D
The amino acid is tyrosine.
Every amino acid has two ionisable groups on its backbone, the \(\alpha\)-carboxyl and the \(\alpha\)-amino group. Tyrosine also has an ionisable side chain: the phenolic hydroxyl, which loses a proton with a pKa of about 10. So it has three acid-base groups.
Alanine (methyl), leucine (isobutyl) and phenylalanine (benzyl) have non-polar side chains with no ionisable group, so each has only the two backbone groups. Phenylalanine is the closest trap, since it differs from tyrosine only by lacking that hydroxyl.
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