L-Tryptophan is the L-stereoisomer of the essential amino acid tryptophan. All amino acids consist of a basic structure which is different only in their special groups called functional groups. The amino acid tryptophan has an indole functional group. Tryptophan is an essential amino acid. This means that it must be ingested from an outside source. An example of a food item rich in this amino acid is banana. Basically, all amino acids are important for the body because they are the building blocks of proteins. Amino acids also possess some special features which vary according to the type of amino acid. Tryptophan also serves as a biochemical precursor of various substances in various reactions. It is a precursor of the neurotransmitter serotonin which is involved in regulating mood and gut movements. It can also act as a precursor of niacin.