Phenylalanine

C 9
H 11
N 1
O 2

Ball-and-stick Stick Space-filling

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23 atoms, 23 bonds.

IUPAC name: (2S)-2-amino-3-phenylpropanoic acid
Molar mass: 165.192(23) g/mol
Atoms: 23
SMILES: C1=CC=C(C=C1)CC(C(=O)O)N
InChIKey: COLNVLDHVKWLRT-QMMMGPOBSA-N
PubChem CID: 6140

Made of

C H N O

Structural formula of phenylalanine. Each unlabelled corner is a carbon atom, and the eight hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Every US state now requires newborn babies to be screened for phenylketonuria (PKU), an inherited disorder that raises the level of one amino acid, phenylalanine, in the blood. The Chemical Entities of Biological Interest (ChEBI) database classes phenylalanine as an amino acid that cells build into proteins. It is also an essential amino acid for human development, a class the alanine page defines.

Its systematic name is (2S)-2-amino-3-phenylpropanoic acid, and the phenyl group in it is a benzene ring bonded to the chain through one of its six carbons. The L-form is an odourless white crystalline powder with a slightly bitter taste. Its other three-dimensional form, D-phenylalanine, acts as a painkiller according to the National Cancer Institute’s thesaurus (NCIt), while the L-form is the one built into proteins.

Structure and bonding

Twenty-three atoms make up the model: nine carbon, eleven hydrogen, one nitrogen and two oxygen. Six of the carbons close into a ring, which the Ball-and-stick and Stick views draw as three double bonds alternating with three single ones. That is one way of writing a benzene ring, whose carbons in fact share the double bonds’ electrons evenly, as the benzene page explains. In PubChem’s computed model all six ring bonds measure 1.385 to 1.395 Å. Viewed side-on, the ring is a flat plate, with its five hydrogens and the CH₂ carbon in its plane.

In a protein, the electron cloud of such a ring can accept a hydrogen bond, an interaction Chem1 describes as helping proteins hold their shape. Past the CH₂ link sits a carbon bonded to four different groups, which makes it chiral; the model is the L-form, the (2S) of the name. The Space-filling view packs the amino and acid groups into a knot beside the flat ring. Both groups are drawn uncharged, as –NH₂ and –COOH; the form an amino acid takes in water is on the glycine page.

Where it turns up

Meat, eggs, nuts and milk supply phenylalanine, and so do some artificial sweeteners, aspartame among them. It is a common part of many proteins, abbreviated Phe by the International Union of Pure and Applied Chemistry (IUPAC). Insulin is 8.1 per cent phenylalanine by mass, and serum albumin 7.0 per cent. Baking costs bread some of it: analyses of bread protein found 5 to 17 per cent of its phenylalanine degraded in the oven.

Outside the human body, ChEBI lists it as a metabolite of the yeast Saccharomyces cerevisiae, of the bacterium Escherichia coli, of algae and of plants. It has been reported in Salvia hispanica and Perilla frutescens among other organisms. Phenylalanine also absorbs ultraviolet light, and laboratories use that absorption to measure how much protein a sample holds, on the NCIt’s account.

Why it matters

An enzyme, phenylalanine hydroxylase, turns phenylalanine into another amino acid, tyrosine, working with tetrahydrobiopterin (BH₄). In PKU, variants of the enzyme’s gene, PAH, give altered versions that cannot handle phenylalanine effectively, and it builds up in the blood. The brain’s nerve cells are especially sensitive to it, which is why the excess can damage the brain. Without treatment, children with classic PKU, its most severe form, develop permanent intellectual disability, and untreated people may have a musty, mouse-like odour. MedlinePlus puts the disorder at 1 in 25,000 newborns in the United States.

The Hazardous Substances Data Bank (HSDB) lists two other products of phenylalanine in the human body, phenylpyruvic acid and phenylethylamine. After the Norwegian doctor Asbjörn Fölling discovered PKU in 1934, phenylpyruvic acid gave the first test for it, turning green with ferric chloride, iron(III) chloride, in untreated patients’ urine. In most babies only a few days old, the urine held too little of it for the test to work.

Properties

Properties of phenylalanine Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6140.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₉H₁₁NO₂
Molar mass	165.192(23) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	9
Hydrogen atoms	11
Nitrogen atoms	1
Oxygen atoms	2
Bonds	23
Single bonds	19
Double bonds	4
PubChem molecular weight	165.19 g/mol	g/mol	PubChem

Safety

Phenylalanine is not classified as hazardous under the Globally Harmonized System (GHS). In 338 of 344 reports to the European Chemicals Agency, it meets none of the hazard criteria. The hazard database CAMEO Chemicals notes that it gives off toxic nitrogen oxide fumes when heated to decomposition. The PubChem Laboratory Chemical Safety Summary for phenylalanine is the sheet for the pure substance.

Questions

What shape is a phenylalanine molecule?

No single shape name covers it, since it has several centres rather than one central atom. In PubChem's computed model the six-carbon ring is flat, its inside angles 119 to 121°, and the CH₂ carbon lies in its plane. The chain bends at that carbon through 113°, and the next carbon stands out of the plane, its four bonds in the tetrahedral arrangement usual for carbon in organic compounds.

How is phenylalanine made commercially?

The Hazardous Substances Data Bank (HSDB) names fermentation of carbohydrates as the cheapest route to L-phenylalanine, and bioengineered strains of the bacterium Escherichia coli are among the producers. The amino acid is also isolated commercially from proteins, among them ovalbumin, lactalbumin, zein and fibrin. HSDB gives the United States' consumption in 1985 as 2.2 × 10⁹ g, or 2,200 tonnes.

Who invented the newborn screening test for PKU?

In the early 1960s Dr Robert Guthrie devised a cheap, simple blood test. It showed whether a baby's blood held enough phenylalanine to let a certain kind of bacteria grow. He and a colleague published the method in 1963, and that year Massachusetts passed the first state law making PKU testing compulsory for every newborn. NICHD, the US child-health institute, credits screening plus prompt treatment with all but ending intellectual disability from PKU in the US.

Is phenylketonuria inherited?

Yes. Research in 1939 showed that PKU is passed down in families and comes from the body's inability to break phenylalanine down. Its pattern is autosomal recessive: the disorder appears only when both copies of the PAH gene in each cell carry a variant. Hundreds of different variants of that gene have been identified in people with PKU. Newborn screening picks up most cases shortly after birth, and treatment then begins promptly.

Made of the same elements

C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.
C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₆H₁₃NO₂ Isoleucine 22 atoms, 21 bonds.

All 25 compounds made of carbon, hydrogen, nitrogen and oxygen

C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.