The amino acid with a benzene ring for a side chain — parent of dopamine, adrenaline, thyroid hormones, and melanin. Also the reason diet drinks have a warning label.
Symbol
Phe · F
Discovered
1879
Mol. Weight
165.19 g/mol
Essential
Yes
F
Discovery: From Lupine Seedlings
L-Phenylalanine
In 1879, German chemist Ernst Schulze — who would later also discover arginine and glutamine — isolated a new amino acid from the pressed juice of germinating lupine seedlings, together with his colleague J. Barbieri. The compound had an unusual structure: a standard amino acid backbone with a benzene ring attached via a methylene group. The benzene ring was the defining feature. Schulze named it phenylalanine — from phenyl (the benzene-derived group) and alanine, reflecting its structural relationship to alanine with a phenyl group added. Nearly a century later, a Japanese company began industrial production of phenylalanine by bacterial fermentation.
The phenyl group immediately made phenylalanine chemically interesting. Benzene rings are rigid, flat, and highly nonpolar — properties that give phenylalanine a character completely different from aliphatic amino acids. That aromatic ring absorbs ultraviolet light at 257–270 nm, making phenylalanine (along with tyrosine and tryptophan) one of the reasons proteins can be detected and quantified using UV spectrophotometry.
🧬 One Amino Acid, Four Hormones
Phenylalanine is the starting material for a remarkable biosynthetic cascade. A single enzyme (phenylalanine hydroxylase) converts phenylalanine to tyrosine. From tyrosine, a second enzyme produces DOPA. From DOPA, dopamine — the neurotransmitter of reward and motivation. From dopamine, noradrenaline (norepinephrine). From noradrenaline, adrenaline (epinephrine). And separately from DOPA, melanin — the pigment in skin and hair.
Every one of these molecules — two neurotransmitters, two hormones, and a pigment — traces its origin to a single dietary amino acid: phenylalanine. The cascade is one of the most consequential biosynthetic pathways in human physiology, controlling everything from how fast your heart beats under stress to what color your hair is.
Identifiers and Properties of Phenylalanine
Identity
IUPAC Name(2S)-2-Amino-3-phenylpropanoic acid
FormulaC₉H₁₁NO₂
Mol. Weight165.19 g/mol
CAS Number63-91-2
MDL NumberMFCD00064227
Physical
Melting point283 °C
Solubility27 g/L (20 °C)
pKa₁ (COOH)2.20
pKa₂ (NH₃⁺)8.76
pI5.48
Rf (BuOH/AcOH/H₂O = 12:3:5)0.68
Identifiers
Canonical SMILESC1=CC=C(C=C1)CC(C(=O)O)N
Isomeric SMILESC1=CC=C(C=C1)C[C@@H](C(=O)O)N
InChIKeyCOLNVLDHVKWLRT-QNWWWDJFSA-N
CategoryNonpolar
EssentialYes
PKU: When Phenylalanine Becomes Toxic
Phenylketonuria (PKU) is a rare but serious inherited metabolic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase, which is necessary for converting phenylalanine to tyrosine. Without this enzyme, phenylalanine accumulates in the blood and brain to toxic levels. Left untreated, this leads to severe and irreversible intellectual disability, behavioral problems, and seizures. PKU affects approximately 1 in 10,000–15,000 newborns in European populations.
Why Diet Drinks Have a Warning
⚠️ "Contains Phenylalanine"
Every product sweetened with aspartame carries the label "Phenylketonurics: Contains Phenylalanine." Aspartame is a dipeptide of aspartic acid and phenylalanine — when digested, it releases free phenylalanine. For most people this is harmless. For individuals with PKU, it can push blood phenylalanine to dangerous levels. The warning is a direct consequence of PKU biochemistry.
Dietary management: People with PKU must follow a strict low-phenylalanine diet for life, avoiding high-protein foods — meat, eggs, dairy, nuts, soybeans — and aspartame-containing products. They rely on special medical formulas low in phenylalanine to meet their protein needs without triggering toxicity.
Medical treatment: Some individuals with PKU benefit from sapropterin dihydrochloride (Kuvan), a synthetic form of tetrahydrobiopterin (BH4), a natural cofactor for phenylalanine hydroxylase. In responsive patients, it can reduce phenylalanine levels significantly, sometimes allowing a less restrictive diet. Regular blood tests are required to monitor phenylalanine levels and adjust management accordingly.
Emerging therapies: Ongoing research is exploring enzyme substitution therapy (pegvaliase, already approved in some countries) and gene therapy approaches aimed at addressing the underlying enzyme deficiency. These hold promise for reducing the burden of lifelong dietary restriction for PKU patients.
PKU is detected at birth through newborn screening programs — mandatory in many countries — using a simple heel-prick blood test taken within the first days of life. Early diagnosis and dietary management from the first weeks of life can effectively prevent neurological damage, allowing individuals with PKU to lead healthy lives.
Functions and Benefits of L-Phenylalanine
As an essential amino acid, phenylalanine's primary biological role is as a biosynthetic precursor for the catecholamine pathway. This has practical implications for neurotransmitter balance and mood.
Catecholamine synthesis and mood
Phenylalanine is the direct precursor of tyrosine, which in turn is converted to DOPA, dopamine, noradrenaline, and adrenaline. Noradrenaline in particular plays a major role in mental alertness, memory consolidation, and mood regulation. Because phenylalanine is the dietary entry point for this entire pathway, adequate intake directly supports the brain's capacity to produce these neurotransmitters. Phenylalanine supplementation has been investigated in clinical research for its potential to support mood — particularly in studies examining the racemic DL-phenylalanine form, which inhibits the breakdown of natural pain-relieving and mood-elevating compounds.
Vitiligo treatment
One of the more clinically established uses of phenylalanine is in the treatment of vitiligo — a skin condition characterized by loss of pigmentation. Phenylalanine is the metabolic precursor to melanin (via tyrosine and DOPA), and topical or oral phenylalanine combined with UVA radiation therapy has been studied and shown to stimulate repigmentation in affected skin areas. This approach is used in some clinical protocols as an adjunct to standard vitiligo management.
Did You Know?
Glyphosate (Roundup) kills plants by blocking a key enzyme in the shikimate pathway — the only route by which plants, fungi, and bacteria can synthesize phenylalanine, tyrosine, and tryptophan. Animals are unaffected because we lack this pathway entirely and must obtain these amino acids from food.
Interesting Facts
🔤
Why F? Phenylalanine's one-letter code is F — not P (taken by proline) or A (alanine). The F is thought to come from the Ph in Phenylalanine, with ph pronounced like f in German, where much amino acid nomenclature originated. It remains one of the more phonetically logical single-letter assignments in the system.
🌡️
A notably high decomposition point. Phenylalanine decomposes at around 283°C — well above many other standard amino acids, reflecting the strong intermolecular stacking interactions between its flat aromatic rings. Like most amino acids, it does not truly melt but breaks down before it can transition into a liquid state.
🌿
The shikimate pathway. Plants, fungi, and bacteria synthesize phenylalanine via the shikimate pathway — a metabolic route that doesn't exist in animals. Because we lack this pathway, we depend entirely on dietary phenylalanine. The herbicide glyphosate kills plants by blocking a key enzyme in this pathway, preventing aromatic amino acid synthesis. Animals are unaffected because they don't have the pathway to block.
🎨
Lignin — the world's second most abundant polymer. In plants, phenylalanine is the precursor to the phenylpropanoid pathway, which produces lignin — the rigid structural polymer that gives wood its strength. Lignin is the second most abundant organic polymer on Earth, after cellulose. Every wooden structure, every tree trunk, every piece of timber ultimately traces its backbone to phenylalanine chemistry.
Where Phenylalanine Is Found
As an essential amino acid, phenylalanine must come from food. It is present in all complete proteins. Note: individuals with PKU must carefully restrict phenylalanine from all dietary sources. Values below are approximate per 100g:
