Short answer: A dipeptide is a peptide made of two amino-acid residues joined by one amide (peptide) bond. It forms when the carboxyl group of one amino acid and the amino group of another combine, with the elements of water removed [1][2]. Order matters: glycylalanine (Gly-Ala) and alanylglycine (Ala-Gly) contain the same two amino acids but are different compounds [1]. Add a third residue and you have a tripeptide [1].
This article is educational chemistry for laboratory readers. It does not describe biological effects, and it is not guidance for use in people or animals.
Dipeptide definition
The IUPAC-IUB Joint Commission on Biochemical Nomenclature's recommendations define a peptide as any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another; the amide bonds in peptides may be called peptide bonds [1]. A dipeptide is the smallest case: two amino acids, one bond.
When the amino acids combine, the elements of water are removed, and what remains of each amino acid is called an amino-acid residue [1]. So a dipeptide has two residues, not two complete amino acids.
The US National Human Genome Research Institute describes a peptide as a short chain of amino acids, typically 2 to 50, linked by peptide bonds [3]. A dipeptide sits at the very bottom of that range. For the bond itself — why it is planar and how stable it is — see our guide to the peptide bond.
How a dipeptide forms
The reaction is a condensation:
- The carboxyl group of the first amino acid reacts with the amino group of the second [2].
- An amide bond (–CO–NH–) links them, and the elements of water are removed [1][2].
- The product has one free amino end (the N-terminus) and one free carboxyl end (the C-terminus) [1][2].
Why order matters: Gly-Ala is not Ala-Gly
Two amino acids can be joined in two directions. IUPAC's own example: if glycine acylates alanine, the dipeptide is glycylalanine; if they condense in the reverse order, the product is alanylglycine [1]. OpenStax gives the same point with alanine and serine — alanylserine (Ala-Ser) and serylalanine (Ser-Ala) are two different dipeptides [2].
The same counting logic scales up quickly:
- OpenStax lists 20 amino acids commonly found in proteins [4].
- Because each of the two positions can hold any of those 20, there are 20 × 20 = 400 possible ordered dipeptides from that set (simple arithmetic, including pairs such as Gly-Gly).
- With three different amino acids, the count of orderings is larger again: OpenStax notes that six isomeric tripeptides contain valine, tyrosine, and glycine [2].
How dipeptides are named and written
IUPAC's naming rules are short and consistent [1]:
- The name begins with the N-terminal residue, written as an acyl name ending in "-yl" (glycyl, alanyl).
- Internal residues follow in order, also as "-yl" names.
- Only the C-terminal residue keeps the plain amino-acid name, and it ends the name — so glycine + alanine in that order is glycylalanine.
- Formulas are normally written in the same order, with the N-terminal residue on the left and the C-terminal residue on the right.
- Stereochemical prefixes go directly before the residue they describe, for example L-alanyl-L-leucine or glycyl-L-alanine.
Dipeptide, tripeptide, oligopeptide: the size words
IUPAC uses a multiplicative prefix before "peptide" to give the total number of residues [1]:
- Dipeptide — 2 residues
- Tripeptide — 3 residues
- Hexapeptide — 6 residues (IUPAC's own example)
- For large counts, numerals may replace the prefix, e.g. a 22-peptide [1]
Molecular weight of a dipeptide
Because one molecule of water is lost when the bond forms, a dipeptide's molecular weight is the sum of its two amino acids minus water. Glycylglycine is a clean check:
- Glycine: 75.07 g/mol [6]
- Water: 18.015 g/mol [7]
- 2 × 75.07 − 18.015 = 132.12 g/mol
- PubChem lists glycylglycine (C4H8N2O3) at 132.12 g/mol [5]
Examples of dipeptides
These well-characterized compounds show how varied a "two-residue" molecule can be. Descriptions are structural only.
- Glycylglycine — a dipeptide formed from two glycine residues; formula C4H8N2O3 [5].
- Carnosine — the N-(beta-alanyl) derivative of L-histidine, formula C9H14N4O3, 226.23 g/mol [8]. Its beta-alanine unit is not one of the 20 alpha-amino acids commonly found in proteins [4][8].
- Anserine — a dipeptide of beta-alanine and 3-methyl-L-histidine units, formula C10H16N4O3 [9].
- Aspartame — a dipeptide obtained by condensing the alpha-carboxy group of L-aspartic acid with the amino group of methyl L-phenylalaninate (L-aspartyl-L-phenylalanine methyl ester) [10]. FDA describes it as a dipeptide composed primarily of two amino acids, phenylalanine and aspartic acid; it is approved for use in food as a sweetener and is about 200 times sweeter than table sugar [11].
- Cyclo(Gly-Gly) — piperazine-2,5-dione, also listed as glycine anhydride, is classed as a cyclic peptide and a member of the 2,5-diketopiperazines [12]. Here the two residues are joined head-to-tail in a ring, so there is no free N- or C-terminus.
A tripeptide example with a twist: glutathione
Glutathione is a useful next step up because its linkage is unusual. IUPAC writes it as γ-glutamylcysteinylglycine [1], and PubChem's ChEBI description says it consists of glutamic acid attached via its side chain to the N-terminus of cysteinylglycine [13].
IUPAC explains that "peptide" usually refers to amide bonds between C-1 of one amino acid and N-2 of another (sometimes called eupeptide bonds), but the term also includes residues linked by other amide bonds, sometimes called isopeptide bonds [1]. Glutathione's glutamyl–cysteine link runs through the side-chain (gamma) carboxyl rather than the alpha carboxyl, which is why it is written with the γ prefix [1][13]. For the compound itself, see our glutathione research overview.
Why this matters for reading lab documents
- Sequence notation is directional. "Gly-Ala" and "Ala-Gly" name different compounds, so a sequence on a label or COA should be read N-terminus to C-terminus [1][2].
- Molecular weight reflects water loss. A listed mass should equal the residue sum, not the free-amino-acid sum [5][6][7].
- Prefixes like L-, D-, β- and γ- carry real structural information about which isomer or which linkage is present [1][8][13].
Frequently Asked Questions
What is a dipeptide in simple terms?
Two amino acids joined by one peptide (amide) bond, with the elements of water removed when the bond forms [1][2].
How many peptide bonds does a dipeptide have?
One, in a linear dipeptide. A cyclic dipeptide such as cyclo(Gly-Gly) (piperazine-2,5-dione) has two amide groups in its six-membered ring [12].
What is the difference between a dipeptide and a tripeptide?
A dipeptide has two amino-acid residues; a tripeptide has three. IUPAC's multiplicative prefix gives the total residue count [1].
Is Gly-Ala the same as Ala-Gly?
No. They are named glycylalanine and alanylglycine, and they are different compounds because the order of the residues differs [1].
How do you calculate a dipeptide's molecular weight?
Add the molecular weights of the two amino acids and subtract one water (18.015 g/mol). For glycylglycine: 2 × 75.07 − 18.015 = 132.12 g/mol, matching PubChem [5][6][7].
Is a dipeptide a protein?
No. NHGRI describes peptides as short chains of typically 2 to 50 amino acids, with longer chains called polypeptides [3]; a dipeptide is the shortest possible peptide.
Bottom line
A dipeptide is the simplest peptide: two amino-acid residues and one amide bond, formed with loss of water [1][2]. Its name and notation run from the N-terminus to the C-terminus, so Gly-Ala and Ala-Gly are different compounds [1]. Examples range from glycylglycine to carnosine, aspartame and the cyclic diketopiperazines [5][8][10][12], and one more residue takes you to tripeptides such as glutathione [1][13]. For lab readers, these rules are the basis for reading sequences, masses and labels correctly.
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