Phenylalanine
Properties
| State | Solid (white crystalline flakes or powder) |
| Color | White |
| Solubility | Slightly soluble in water (27 g/L at 25°C); soluble in dilute mineral acids |
| Melting Point | 283°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Phenylalanine
Phenylalanine is one of the nine amino acids the human body cannot synthesize, so every gram has to come from dietary protein — about 0.5 g per 100 g of beef, soy, or eggs. The benzyl side chain absorbs UV at 257 nm with a small molar extinction coefficient (about 200 M⁻¹cm⁻¹), which is why protein chemists doing A280 measurements lean mostly on tryptophan and tyrosine but pick up a small Phe contribution that matters for accurate concentration calls. Metabolically, phenylalanine sits one enzymatic step upstream of tyrosine: phenylalanine hydroxylase (PAH), a tetrameric BH4-dependent monooxygenase, hydroxylates the para position of the ring to give tyrosine, which then feeds into dopamine, norepinephrine, epinephrine, and melanin biosynthesis. Loss-of-function mutations in PAH cause phenylketonuria — first described by Asbjørn Følling in 1934, now caught in every U.S. newborn-screening panel via the Guthrie heel-prick card. Untreated PKU produces severe intellectual disability; managed PKU requires lifelong restriction of dietary Phe to roughly 250-500 mg/day, with synthetic Phe-free amino acid formulas filling the protein gap. The famous "contains phenylalanine" warning on every diet soda is there because aspartame is L-aspartyl-L-phenylalanine methyl ester and metabolizes back to free Phe in the gut.
Where you'll encounter it
If you've ever read the side of a Diet Coke can and seen "phenylketonurics: contains phenylalanine," that warning exists because aspartame hydrolyzes in the small intestine to release exactly the amino acid PKU patients can't process. A 12-oz can delivers about 100 mg of free Phe — enough to push a strict PKU patient over their daily allowance. In a biochemistry lab, the standard way to measure protein concentration on a NanoDrop relies on UV absorbance at 280 nm, where Trp dominates, Tyr contributes, and Phe adds a small but non-zero baseline that throws off readings for Phe-rich, Trp-poor proteins like collagen. Crystallographers also love Phe residues: their flat aromatic faces stack with neighboring rings (face-to-edge T-shaped or face-to-face offset), and these stacking interactions stabilize hydrophobic cores in nearly every globular protein structure deposited in the PDB.
Common Uses
- Required dietary amino acid for ribosomal protein synthesis in humans
- Substrate for phenylalanine hydroxylase converting Phe to tyrosine and downstream catecholamines
- L-aspartyl-L-phenylalanine methyl ester (aspartame) intermediate for tabletop sweeteners
- Newborn-screening Guthrie-card analyte for phenylketonuria detection
- UV absorbance contributor at 257 nm for protein concentration measurement on NanoDrop
Safety Information
Generally Recognized as Safe (GRAS) by the FDA for dietary use in non-PKU individuals at typical food levels. Phenylketonurics must restrict intake to physician-prescribed levels (typically 250-500 mg/day) to prevent neurotoxic plasma Phe accumulation above 360 µmol/L. FDA mandates aspartame-containing products carry a "Phenylketonurics: Contains Phenylalanine" warning under 21 CFR 172.804. No OSHA PEL or ACGIH TLV established for the pure compound; standard dust-handling PPE applies in bulk powder operations. GHS classification: not a hazardous substance per OSHA HazCom 2012.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.