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.