L-tryptophan

Tryptophan (abbreviated as Trp or W; encoded by the codon UGG) is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), an α-carboxylic acid group (which is in the deprotonated –COO− form under biological conditions), and a side chain indole, classifying it as a non-polar, aromatic amino acid. It is essential in humans, meaning the body cannot synthesize it and thus it must be obtained from the diet.

Tryptophan is also a precursor to neurotransmitters serotonin and melatonin.