Aromatic amino acids and hydrophobicity: Which listed amino acid is aromatic but not purely hydrophobic under physiological conditions (due to a polar substituent)?

Introduction / Context:
Aromatic amino acids (phenylalanine, tyrosine, tryptophan) contribute to protein spectroscopy, stacking interactions, and hydrophobic cores. However, not all aromatics behave identically with respect to polarity and hydrogen bonding—substituents on the ring can change solubility and reactivity.

Given Data / Assumptions:

• Phenylalanine has a benzyl side chain (nonpolar).
• Tyrosine has a phenolic side chain (ring plus –OH).
• Tryptophan has an indole ring (largely hydrophobic with a weakly polar NH).

Concept / Approach:
Hydrophobicity is reduced by polar groups that can hydrogen bond with water. Tyrosine’s phenolic –OH increases polarity and enables hydrogen bonding, making it less hydrophobic (often categorized as aromatic, polar). Phenylalanine lacks heteroatoms in the ring and is strongly hydrophobic. Tryptophan is mostly hydrophobic despite its indole NH because the large aromatic system dominates.

Step-by-Step Solution:
Evaluate side chains: Phe → nonpolar benzyl; Tyr → phenol; Trp → indole. Assess polarity: Tyr’s –OH contributes measurable polarity and H-bonding. Rank hydrophobicity: Phe ≈ Trp > Tyr (most polar of the three aromatics). Select amino acid that is aromatic yet not purely hydrophobic → Tyrosine.

Verification / Alternative check:
Hydropathy index commonly lists Tyr as less hydrophobic than Phe and Trp. Tyrosine often locates at protein surfaces or interfaces where its –OH can interact with solvent.

Why Other Options Are Wrong:
Phenylalanine is strongly hydrophobic; tryptophan is largely hydrophobic; “None of these/All of these” contradict the polarity differences.

Common Pitfalls:
Assuming all aromatics are equally hydrophobic; ignoring the phenolic –OH in tyrosine.

Final Answer:
Tyrosine.