Which of the following amino acids is classified as aromatic?

The classification of amino acids as aromatic is based on the presence of a distinct cyclic structure that contains alternating double bonds, which can result in delocalized electrons. The amino acid identified in the correct answer, tryptophan (often denoted by the letter W), is classified as aromatic due to its indole side chain. This structure features a benzene ring fused to a five-membered nitrogen-containing ring, enabling the delocalization of electrons that is characteristic of aromatic compounds.

In contrast, glutamate is a negatively charged, polar amino acid with a side chain containing a carboxylic acid group, which does not possess an aromatic ring. Leucine belongs to the aliphatic amino acids, having a branched-chain structure without any aromatic character. Similarly, alanine, with its simple aliphatic side chain, also lacks any aromatic properties. Therefore, tryptophan stands out among the options provided due to its aromatic structure and characteristics.