Q.37 Which of the following groups consists of only aromatic compounds?
1. Tyrosine, Threonine and Phenylalanine
2. Phenol, Tyrosine and Phenylalanine
3. Phenol, Histidine and Tryptophan
4. Tyrosine, Phenylalanine and Proline
Aromatic compounds contain stable, planar rings with delocalized electrons (e.g., benzene-like structures), and in amino acids, this refers to side chains with phenyl, phenolic, indole, or imidazole rings.
Correct Answer
2. Phenol, Tyrosine and Phenylalanine
All three possess aromatic rings: phenol (hydroxybenzene), tyrosine (phenol-derived side chain), phenylalanine (benzene ring)—no non-aromatic exceptions.
Option Analysis
Tyrosine, Threonine, Phenylalanine (Option 1)
Threonine has an aliphatic alcohol side chain—no aromatic ring—disqualifying the group.
Phenol, Tyrosine, Phenylalanine (Option 2)
Correct: Phenol’s benzene ring, tyrosine’s para-hydroxyphenyl, phenylalanine’s benzyl—all classically aromatic.
Phenol, Histidine, Tryptophan (Option 3)
Histidine’s imidazole is aromatic (6π electrons, Huckel’s rule), tryptophan has indole (aromatic), phenol qualifies—but option 2 fits without debate.
Tyrosine, Phenylalanine, Proline (Option 4)
Proline’s pyrrolidine ring is aliphatic (saturated, no delocalized π system)—not aromatic.
| Option | Compound 1 Ring | Compound 2 Ring | Compound 3 Ring | All Aromatic? |
|---|---|---|---|---|
| 1 | Tyrosine (Yes) | Threonine (No) | Phe (Yes) | No |
| 2 | Phenol (Yes) | Tyr (Yes) | Phe (Yes) | Yes |
| 3 | Phenol (Yes) | His (Yes) | Trp (Yes) | Yes |
| 4 | Tyr (Yes) | Phe (Yes) | Proline (No) | No |
Clinical Relevance
Aromatic amino acids absorb UV light (key for protein quantification at 280 nm); vital for biochemistry exams like CSIR-NET.
