Tryptophan
Properties
| State | Solid (white to slightly yellowish crystalline powder) |
| Color | White to yellowish |
| Solubility | Slightly soluble in water (11.4 g/L at 25°C); soluble in hot water and dilute acids |
| Melting Point | 289°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Tryptophan
Tryptophan is the bulkiest of the 20 standard amino acids and the rarest in proteins, typically present at about 1 mol% of total residues. Its defining feature is the indole side chain, which gives Trp the highest molar extinction coefficient of any natural amino acid at 280 nm (around 5,500 M^-1 cm^-1). This single property is why every Nanodrop, every UV-Vis spectrophotometer, and every FPLC chromatogram in a protein lab measures absorbance at A280 — you are essentially counting the tryptophans in your sample. Biochemically, Trp sits at the head of two important pathways: hydroxylation by tryptophan hydroxylase plus decarboxylation by AADC produces serotonin, and pineal acetylation and methylation of serotonin yields melatonin. A separate route, the kynurenine pathway, accounts for roughly 95% of dietary Trp catabolism and ends in NAD+ biosynthesis, which is why severe niacin deficiency only develops on diets simultaneously low in tryptophan and B3. The fluorescence emission of Trp residues shifts from around 350 nm in water to 320 nm when buried in a hydrophobic protein core, making intrinsic Trp fluorescence one of the most-used handles for tracking folding, ligand binding, and conformational change without needing an external probe.
Where you'll encounter it
If you have ever blanked the spectrophotometer with buffer and pipetted 2 microliters of your protein prep onto a Nanodrop pedestal, the number that comes back is dominated by Trp. The standard "1 A280 unit equals 1 mg/mL" rule of thumb is calibrated against an average Trp content, which is why low-Trp proteins like collagen read deceptively low and Trp-rich proteins like lysozyme read high. In stopped-flow folding experiments, you watch a Trp residue go from solvent-exposed to buried over the course of milliseconds, with the emission max sliding blue and the intensity changing by a factor of two or three. The turkey-tryptophan story is folklore — turkey contains roughly the same Trp as chicken or beef. Post-Thanksgiving drowsiness has more to do with the carbohydrate-driven insulin spike that pulls competing branched-chain amino acids out of plasma, briefly raising the Trp:LNAA ratio at the blood-brain barrier.
Common Uses
- Reference chromophore for protein quantitation by A280 absorbance on Nanodrop and FPLC inline detectors
- Intrinsic fluorescence probe for tracking protein folding, ligand binding, and conformational change
- Precursor for chemical and enzymatic synthesis of serotonin, melatonin, and kynurenine pathway metabolites
- Essential dietary amino acid added to animal feed and human nutritional supplements at gram-scale
- Substrate in fermentation routes for industrial production of indole-derived fine chemicals and dyes
- Probe residue introduced by site-directed mutagenesis to install fluorescence reporters at specific sites
- Active pharmaceutical ingredient in over-the-counter sleep aids marketed alongside 5-HTP and melatonin
- Calibration standard for amino acid analyzers using ninhydrin or OPA post-column derivatization
Safety Information
Tryptophan is GRAS for dietary use and not classified as hazardous under GHS for laboratory handling. The cautionary historical case is the 1989 eosinophilia-myalgia syndrome (EMS) outbreak in the United States, traced to a contaminated bacterial fermentation batch from a single Japanese manufacturer, which produced over 1,500 cases and at least 37 deaths. Pure pharmaceutical-grade Trp is not implicated. Pharmacologically the main concern is serotonin syndrome when Trp or 5-HTP is combined with SSRIs, MAOIs, or triptans, presenting as hyperthermia, clonus, and autonomic instability; the FDA recommends discontinuing supplements 14 days before starting an MAOI. No OSHA PEL is established. Standard lab practice: nitrile gloves, dust mask for bulk weighing, and avoid inhalation of fine powder.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.