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.