Which of the following best describes the type of molecule tryptophan is?

• A. Sulfur-containing amino acid
• B. Aromatic amino acid
• C. Aliphatic amino acid
• D. Basic amino acid

Answer: (B)

Verified step by step guidance

1. Understand the structure of tryptophan: Tryptophan is an amino acid with a side chain that contains an indole group, which is a bicyclic structure consisting of a benzene ring fused to a pyrrole ring.
2. Classify the side chain: The indole group in tryptophan is aromatic because it contains conjugated π-electrons that follow Huckel's rule for aromaticity (4n+2 π-electrons, where n is an integer).
3. Compare tryptophan's side chain to the options: Sulfur-containing amino acids, such as cysteine and methionine, have sulfur atoms in their side chains, which tryptophan does not. Aliphatic amino acids, such as alanine and leucine, have non-aromatic hydrocarbon side chains. Basic amino acids, such as lysine and arginine, have positively charged side chains at physiological pH, which tryptophan does not.
4. Identify the correct classification: Since tryptophan's side chain is aromatic due to the indole group, it is classified as an aromatic amino acid.
5. Conclude the reasoning: Based on the structural and chemical properties of tryptophan, the correct answer is 'Aromatic amino acid'.