Tryptophan is not classified as a basic amino acid even though it has a heterocycle containing a nitrogen atom. Why is the N atom in the five-membered ring of tryptophan not readily protonated by acid?

Amino acid comprising a second basic group, an amino acid, is termed a basic amino acid. Lysine and arginine are examples of a few basic amino acids.

Tryptophan comes under the category of essential amino acids. The body cannot generate this amino acid, and it must be acquired via diet.

The electron pair indicated by the arrow in the first figure is delocalized on the bicyclic ring system, providing 10$\mathrm{\pi }$electrons. This makes the electron pair less available for donation and less basic.

Protonation of the nitrogen atom in tryptophan

The ring structure on tryptophan is aromatic as each atom comprises a p orbital. The aromaticity is disrupted due to the protonation of the nitrogen atom, thereby making it a less favorable reaction