Tryptophan

Tryptophan (Trp or W) is encoded by the codon UGG. It is an α-amino acid used in the biosynthesis of proteins. 
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.
Tryptophan is also a precursor to the neurotransmitters serotonin and melatonin.
People get tryptophan by eating protein-rich foods like meat, eggs, and dairy products.