The heaviest standard amino acid, encoded by a single codon, and wrongly blamed for every post-Thanksgiving nap since 1950.
Symbol
Trp · W
Discovered
1901
Mol. Weight
204.23 g/mol
Essential
Yes
W
Discovery: The Secret in Casein
L-Tryptophan
In 1901, British scientists Frederick Gowland Hopkins and Sydney Cole were systematically breaking down casein (milk protein) using trypsin, then carefully analyzing what fragments remained. They isolated 4–8 grams of a new substance from 600 grams of crude casein — a new compound with a complex ring structure unlike anything seen in an amino acid before. They called it tryptophan, 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 among his significant earlier contributions. The identification was slow, painstaking work, and tryptophan was among the most chemically challenging amino acids to characterize because of its unusual bicyclic indole side chain.
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.
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 — comparable to chicken, beef, and many other proteins. Chicken breast, for comparison, provides around 180–200 mg per 3-ounce (85g) serving. 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 is 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
Of all the 20 standard amino acids, only two are encoded by a single codon. One is methionine (AUG, which also serves as the universal start signal for protein synthesis). The other is tryptophan: UGG, and only UGG.
This matters because the genetic code is usually redundant — most amino acids can be specified by multiple codons, providing a buffer against mutations. Tryptophan has no such safety net. Any single-base mutation that changes UGG to UGA or UAG creates a stop codon — and protein synthesis halts immediately. Tryptophan sits on a genetic knife's edge, which may explain why it is the rarest amino acid in most proteins, typically appearing less than 1.5% of the time.
The Most Complex Structure
Tryptophan's side chain is an indole ring — a bicyclic system consisting of a benzene ring fused to a pyrrole ring — that doesn't appear in any other standard amino acid. This gives tryptophan its unusual ultraviolet absorption properties (proteins absorb UV strongly at 280 nm largely because of tryptophan and tyrosine residues), and makes tryptophan the heaviest of all 20 standard 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 toward light), and so does the venom of certain insects.
Functions of L-Tryptophan in the Body
Tryptophan is an essential amino acid, a building block for all proteins, and the starting material for three distinct biosynthetic pathways of significant physiological importance.
Serotonin synthesis
Tryptophan is the direct dietary precursor to serotonin — the neurotransmitter involved in the regulation of mood, sleep, appetite, pain perception, and temperature. The conversion occurs in two steps via 5-hydroxytryptophan (5-HTP). Serotonin plays a role in a wide range of physiological processes, and tryptophan availability in the brain — which competes with other large neutral amino acids for transport across the blood-brain barrier — is a genuine factor in serotonin synthesis. This is why high-carbohydrate meals (which divert competing amino acids into muscle) can modestly increase tryptophan entry into the brain.
Melatonin synthesis and sleep regulation
Serotonin is further converted to melatonin in the pineal gland, under conditions of darkness. Melatonin is the primary regulator of the body's circadian rhythm — signaling nighttime and promoting sleep onset. This serotonin → melatonin pathway makes adequate tryptophan intake relevant to normal sleep patterns and circadian function. Clinical studies have examined tryptophan supplementation for sleep latency and quality, with modest but consistent positive results.
Niacin (Vitamin B3) synthesis
Tryptophan can be converted to niacin (nicotinic acid, vitamin B3) via the kynurenine pathway. This is nutritionally significant: approximately 60 mg of dietary tryptophan yields 1 mg of niacin equivalent. While this conversion is inefficient and cannot fully substitute for dietary niacin, it means that adequate tryptophan intake provides some protection against pellagra — the niacin-deficiency disease characterized by the "four Ds": dermatitis, diarrhea, dementia, and death. Populations subsisting on corn-based diets without tryptophan-rich foods historically suffered from pellagra for precisely this reason.
Did You Know?
The ancient blue dye indigo — used to colour Pharaoh's robes and modern denim alike — shares its core indole ring with tryptophan. Under the right conditions, bacteria convert tryptophan from their growth medium directly into indigo blue.
Interesting Facts
🧬
One codon only. UGG is tryptophan's sole genetic code — any single mutation destroys the signal and creates a stop codon. Only methionine shares this distinction among the 20 standard amino acids.
⚖️
Heaviest amino acid. At 204.23 g/mol, tryptophan is the largest and heaviest of the 20 standard amino acids — more than twice the weight of glycine (75.07 g/mol).
🔵
Connected to indigo. The indole ring in tryptophan's structure is chemically related to the indigo molecule — the blue compound used as a dye since ancient Egypt. Bacteria can convert tryptophan to indigo under the right conditions.
🌱
A plant hormone precursor. Tryptophan is the starting material for auxin (indole-3-acetic acid), the primary plant growth hormone. Without tryptophan, plants cannot grow toward light or develop properly.
📡
A UV detector. Tryptophan absorbs strongly at 280 nm ultraviolet light. Scientists exploit this to measure protein concentration — the more tryptophan (and tyrosine) present, the stronger the UV absorption. This is a standard laboratory technique used daily worldwide.
🌿
Target of glyphosate. Herbicides like glyphosate work by blocking an enzyme in the tryptophan (and other aromatic amino acid) synthesis pathway in plants. Animals lack this pathway entirely — which is why glyphosate selectively affects plants and microbes, not humans.
Where to Find Tryptophan in Food
Tryptophan is an essential amino acid — humans cannot synthesize it. It is present in all protein-containing foods, but concentrations vary considerably. Values are approximate:
