Discovery: The Secret in Casein
In 1901, two British scientists — Frederick Gowland Hopkins and Sydney Cole — were working with casein, the main protein in milk. They had been systematically breaking it down using the enzyme trypsin (which cuts proteins apart at specific sites), then carefully analyzing what fragments remained. The goal was methodical: find what's in there.
What they found was a new substance that defied initial analysis. It had a complex ring structure unlike anything seen in an amino acid before. They called it tryptophan, a name that reflects the method of its discovery: from the Greek trypein (to wear down, as trypsin does to proteins) and phainein (to appear). It had "appeared" through tryptic digestion.
Hopkins would go on to win the Nobel Prize in Physiology or Medicine in 1929, partly for his discovery of vitamins — but tryptophan was one of his significant earlier achievements. The identification of amino acids in the early twentieth century was slow, painstaking work, and tryptophan was among the most chemically challenging to characterize.
"Tryptophan's indole ring — a fused benzene and pyrrole — is the most complex side chain of any standard amino acid. In all of protein biology, nothing else looks quite like it."
The Turkey Myth: Setting the Record Straight
"Turkey makes you sleepy because of tryptophan"
Every Thanksgiving this claim circulates, and every year the science gets simplified to the point of being wrong. Turkey contains about 250–330 mg of tryptophan per 100g — similar to chicken, beef, and many other proteins. The post-Thanksgiving drowsiness is overwhelmingly caused by eating a large, carbohydrate-heavy meal, which diverts blood flow to digestion and triggers a real physiological response. The turkey is not uniquely responsible.
The irony is that tryptophan is biochemically interesting in ways the myth doesn't capture. It's the precursor to serotonin (a neurotransmitter involved in mood and sleep regulation) and to melatonin (involved in circadian rhythm). But these conversion pathways are slow and tightly regulated — eating a meal doesn't meaningfully change brain serotonin levels in the short term. The chemistry is real; the causal link to turkey specifically is not.
One Codon: The Rarest Genetic Signal
Here is something genuinely remarkable about tryptophan: of all the 20 standard amino acids, only two are encoded by a single codon in the genetic code. One is methionine (ATG, which also serves as the universal start signal for protein synthesis). The other is tryptophan: TGG, and only TGG.
Why does this matter? The genetic code is often described as "redundant" — most amino acids can be specified by multiple three-letter sequences of DNA. This redundancy provides a buffer against mutations: changing one letter in a codon often still results in the same amino acid. But tryptophan has no such safety net. Any mutation that changes TGG to TGA, TAA, or TAG creates a stop codon — and protein synthesis halts immediately. Tryptophan sits on a genetic knife's edge.
Perhaps unsurprisingly, tryptophan is the rarest amino acid in most proteins — typically appearing less than 1.5% of the time. Its single codon, its complex structure, and its relative rarity in proteins are all connected threads in the same story.
The Most Complex Structure
Look at the side chain of tryptophan and you'll see something unique: an indole ring. This is a bicyclic system — a benzene ring fused to a pyrrole ring — that doesn't appear in any other standard amino acid. It gives tryptophan its unusual ultraviolet absorption properties (proteins absorb UV strongly at 280nm largely because of tryptophan and tyrosine), and it makes tryptophan the heaviest of the 20 amino acids at 204.23 g/mol.
The indole ring is also found in many important biological and chemical compounds. Indigo — the ancient blue dye used in textiles for millennia, from Egyptian linen to Levi's jeans — shares the indole core with tryptophan. So does the plant hormone auxin (which controls plant growth), and so does the venom of some insects.
Interesting Facts
Where to Find Tryptophan in Food
Tryptophan is an essential amino acid — humans cannot synthesize it and must obtain it through diet. It's present in all protein-containing foods, but some sources are particularly rich:
