CID 6306

C₆H₁₃NO₂

Isoleucine

C 6
H 13
N 1
O 2

Ball-and-stick Stick Space-filling

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22 atoms, 21 bonds.

IUPAC name: (2S,3S)-2-amino-3-methylpentanoic acid
Molar mass: 131.175(18) g/mol
Atoms: 22
SMILES: CCC(C)C(C(=O)O)N
InChIKey: AGPKZVBTJJNPAG-WHFBIAKZSA-N
PubChem CID: 6306

Made of

C H N O

Structural formula of isoleucine. Each unlabelled corner is a carbon atom, and the four 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.

Two neighbouring carbon atoms in isoleucine each carry four different groups, and the IUPAC name, (2S,3S)-2-amino-3-methylpentanoic acid, fixes the arrangement at both. The name describes a five-carbon acid with an amine group, –NH₂, on carbon 2 and a methyl group on carbon 3. Each is a chiral carbon, a term the lactic acid page explains.

Built from carbon, hydrogen, nitrogen and oxygen, it has exactly the formula and molar mass of leucine. MeSH, the US National Library of Medicine’s subject headings, calls it an isomer of leucine, with the same atoms differently arranged. It also files isoleucine as an essential amino acid with a branched chain.

Crystallised from alcohol, the solid forms waxy, shiny rhombic leaflets. The Hazardous Substances Data Bank (HSDB) records both 34.4 g and 41.2 g of it dissolving in a litre of water at 25 °C. HSDB also records the solid subliming, passing straight to vapour, at 168–170 °C.

Structure and bonding

The N and both O atoms gather at one end of an open chain, on carbon 2 and the carboxyl carbon. In the Ball-and-stick view, carbon 2 holds the NH₂, a hydrogen, the acid group and carbon 3. Carbon 3 holds a hydrogen, a methyl group, an ethyl group and carbon 2: the side chain forks right beside the amine-bearing carbon. In the two-dimensional structural formula, one wedge and one hashed bond mark these two chiral carbons.

In this computed model, angles around carbon 2 span 105.6° to 112.0°, and around carbon 3, 106.7° to 114.0°. Both ranges sit near the tetrahedral pattern that, according to Chem1, carbon adopts in most organic compounds. In the Space-filling view, the four side-chain carbons and their nine hydrogens form one bulky lump free of nitrogen and oxygen.

The model draws the uncharged molecule: a C=O and an O–H in the acid group, and a neutral NH₂. The glycine page covers the forms amino acids take in water. Isoleucine’s side-chain bonds are single, and single bonds can rotate, Chem1 notes, so the drawn chain is one computed arrangement.

Where it turns up

The US National Cancer Institute (NCI) finds isoleucine especially in meat and fish, in eggs and cheese, and in most nuts and seeds. NCI also reports it concentrated in human muscle tissue. The LOTUS natural-products database reports it in Humulus lupulus.

Isoleucine is obtained, HSDB records, by hydrolysis of the proteins zein and edestin, splitting them by reaction with water. A second route, amination, puts an amine group on alpha-bromo-beta-methylvaleric acid. HSDB lists medicine, nutrition and biochemical research as its uses, and the US Food and Drug Administration (FDA) permits L-isoleucine as a direct additive to food for people. US production reported to the Environmental Protection Agency (EPA) was 51,470 pounds in 2021 and 29,804 pounds in 2023. Pharmaceutical and medicine manufacturing is the industry that processes it.

Why it matters

Side chains set one amino acid apart from another, according to HSDB, and bear on how the higher orders of protein structure hold together. HSDB classes the branched side chains, isoleucine’s among them, as hydrophobic, or water-avoiding. Chem1 holds that the hydrophobic effect governs how proteins fold.

The enzymes that start breaking down branched-chain amino acids sit mainly outside the liver. On that route, isoleucine’s keto acid is alpha-keto-beta-methylvalerate (KMV). The group’s later products include succinyl CoA, propionyl CoA, acetyl CoA and acetoacetate. HSDB therefore calls these amino acids both ketogenic and glucogenic, able to lead to ketone bodies and to glucose. Maple syrup urine disease, covered on the leucine page, blocks this breakdown too.

Isoleucine is absent from adult human haemoglobin, HSDB notes, so Plasmodium falciparum, the malaria parasite living in red blood cells, must take it from outside the cell. In cells holding mature parasites, isoleucine enters about five times faster, and the parasite can swap it for leucine freed from haemoglobin. That traffic keeps the parasite’s protein building from being limited by its isoleucine supply.

Properties

Properties of isoleucine Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 6306.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₆H₁₃NO₂
Molar mass	131.175(18) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	6
Hydrogen atoms	13
Nitrogen atoms	1
Oxygen atoms	2
Bonds	21
Single bonds	20
Double bonds	1
PubChem molecular weight	131.17 g/mol	g/mol	PubChem

Safety

In food protein and as a permitted additive, isoleucine is essentially harmless, and it carries no classification under the Globally Harmonized System (GHS). HSDB notes toxic fumes of nitric oxide when it is heated to decomposition. The PubChem Laboratory Chemical Safety Summary for isoleucine holds the full record.

Questions

Are isoleucine and leucine isomers?

Yes. Both are C₆H₁₃NO₂ with a molar mass of 131.18 g/mol, and MeSH calls isoleucine an isomer of leucine. What separates them is where the side chain branches. Isoleucine carries its methyl branch on carbon 3, the carbon right next to the one bearing the amine; leucine carries it on carbon 4. Isoleucine's IUPAC name carries two chiral-carbon labels, (2S,3S); leucine's carries one, (2S).

Why does isoleucine's name start with (2S,3S)?

The two labels belong to carbons 2 and 3, the only carbons in the molecule that hold four different groups each. S records the arrangement at each of them, and L-isoleucine, the form cells build into protein, is the (2S,3S) one. Other arrangements are separate compounds: ChEBI, the Chemical Entities of Biological Interest database, names D-isoleucine as its enantiomer, or mirror-image form. HSDB describes an L-allo form with a sweet taste, while L-isoleucine tastes bitter.

What happens if the body cannot break down isoleucine?

Two disorders single it out. In SBCAD deficiency, named for the enzyme short/branched chain acyl-CoA dehydrogenase, isoleucine is the one amino acid people cannot process, yet most have no health problems from it; it is most common among Hmong people, and newborn screening finds most cases. In beta-ketothiolase deficiency, the body handles both isoleucine and ketones poorly, and related compounds can reach toxic levels in the blood.

Is isoleucine used in weight-loss products?

Isoleucine is named in a US Food and Drug Administration rule, 21 CFR 310.545, on ingredients of over-the-counter weight control drug products. For those ingredients, the rule holds that the data are inadequate to recognise them generally as safe and effective for that purpose. Outside drug products, Haz-Map, an occupational health database, records isoleucine as a nutritional supplement for foods, citing the FDA.

Made of the same elements

C₆H₁₃NO₂ Leucine 22 atoms, 21 bonds.
C₈H₁₁NO₂ Dopamine 22 atoms, 22 bonds.
C₉H₁₃NO₃ Epinephrine 26 atoms, 26 bonds.
C₉H₁₁NO₂ Phenylalanine 23 atoms, 23 bonds.
C₈H₉NO₂ Acetaminophen 20 atoms, 20 bonds.
C₉H₁₁NO₃ Tyrosine 24 atoms, 24 bonds.
C₅H₉NO₄ Glutamic acid 19 atoms, 18 bonds.
C₁₀H₁₂N₂O Serotonin 25 atoms, 26 bonds.

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

C₁₁H₁₂N₂O₂ Tryptophan 27 atoms, 28 bonds.
C₈H₁₀N₄O₂ Caffeine 24 atoms, 25 bonds.
C₃H₇NO₂ Alanine 13 atoms, 12 bonds.
C₅H₆N₂O₂ Thymine 15 atoms, 15 bonds.
C₁₂H₁₇NO DEET 31 atoms, 31 bonds.
C₁₃H₁₆N₂O₂ Melatonin 33 atoms, 34 bonds.
C₂H₅NO₂ Glycine 10 atoms, 9 bonds.
C₄H₄N₂O₂ Uracil 12 atoms, 12 bonds.
C₄H₅N₃O Cytosine 13 atoms, 13 bonds.
C₅H₅N₅O Guanine 16 atoms, 17 bonds.
C₅H₄N₄O₃ Uric acid 16 atoms, 17 bonds.
C₁₃H₂₁NO₃ Albuterol 38 atoms, 38 bonds.
CO(NH₂)₂ Urea 8 atoms, 7 bonds.
C₁₄H₁₈N₂O₅ Aspartame 39 atoms, 39 bonds.
C₁₄H₂₂N₂O Lidocaine 39 atoms, 39 bonds.
C₁₇H₂₁NO Diphenhydramine 40 atoms, 41 bonds.
C₂₀H₂₄N₂O₂ Quinine 48 atoms, 51 bonds.