November 20, 2017
Do you get sleepy after Thanksgiving dinner? Don’t blame me!
What molecule am I?

L-Tryptophan is one of nine essential amino acids for humans—“essential” in that the body cannot synthesize them, so they must be part of the diet. Fortunately, tryptophan is a constituent of many common foods, especially ones that are high in protein.

In 1901, British biochemists Frederick Gowland Hopkins and Sydney W. Cole discovered tryptophan in the milk derivative casein. (In 1929, Hopkins was a co-winner of the Nobel Prize in Physiology or Medicine for the discovery of vitamins.) The first lab synthesis of tryptophan was reported by German chemist/pharmacologist Alexander Ellinger in 1906.

In the body, tryptophan is a biochemical precursor to serotonin and melatonin, a neurotransmitter and a hormone, respectively. Low serotonin levels are associated with depression, which has prompted the marketing of tryptophan as an over-the-counter depression treatment. The scientific evidence for this use, however, is sketchy. Most commercial tryptophan is produced by microbial fermentation.

Legend has it that melatonin produced from large amounts of tryptophan in turkey meat causes drowsiness after a Thanksgiving dinner. It turns out, however, that turkey contains no more tryptophan than other types of meat. It’s more likely that the consumption of copious quantities of all kinds of food prompts our post–turkey feast naps.

Originally the Molecule of the Week for November 20, 2006

L-Tryptophan fast facts

CAS Reg. No. 73-22-3
Molar mass 204.23 g/mol 
Formula C11H12N2O2
Appearance White crystals or powder 
Melting point ≈290 ºC (dec.)
Water solubility 11.4 g/L
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