Amino acids with aromatic side chain are

The correct option is phenylalanine, tryptophan, tyrosine.

To understand why this is the correct answer, we can look at the chemical structures of the amino acid side chains (R-groups):

1. Phenylalanine: This amino acid contains a phenyl ring (a benzene ring minus one hydrogen atom) attached to a methyl group. The presence of the benzene ring makes its side chain aromatic.
2. Tryptophan: The side chain of tryptophan contains an indole group, which consists of a benzene ring fused to a nitrogen-containing pyrrole ring. Since it contains a conjugated ring system with delocalized pi electrons, it is aromatic.
3. Tyrosine: Tyrosine has a structure very similar to phenylalanine, but it has a hydroxyl (-OH) group attached to the para position of the phenyl ring, forming a phenol group. The benzene ring structure makes its side chain aromatic.

Let's examine why the other options are incorrect by identifying the non-aromatic amino acids in them:

• Asparagine (in tryptophan, asparagine, tyrosine) has an aliphatic amide side chain (-CH₂-CO-NH₂), which is not aromatic.
• Serine (in phenylalanine, tryptophan, serine) has a polar, uncharged hydroxymethyl side chain (-CH₂-OH), which is not aromatic.
• Threonine (in tryptophan, threonine, tyrosine) has a polar, uncharged secondary alcohol side chain (-CH(OH)-CH₃), which is not aromatic.

Therefore, only the option containing phenylalanine, tryptophan, and tyrosine consists entirely of amino acids with aromatic side chains.