Tryptophan
Tryptophan has an α-amino group (which is in the –NH3+ form under biological conditions), and an α-carboxylic acid group (which is in the deprotonated –COO− form under biological conditions). It also has a side chain indole, which makes it a non-polar aromatic amino acid. Tryptophan is essential to humans: the body cannot synthesize it, so it must be got from the diet.