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biochemistry · amide bond · condensation

What is a peptide bond?

A peptide bond is the amide bond that joins two amino acids: the carboxyl carbon of one bonds to the amino nitrogen of the next, and one molecule of water (18.02 g/mol) is released. A chain of n amino acids has n − 1 peptide bonds, so glycine and alanine join into a 146.15 g/mol dipeptide.

Open Peptide Calculator editorial teamUpdated 7 sources

The definition

A peptide bond is an amide bond between amino acids. The IUPAC–IUBMB nomenclature rules define peptides by amide formation between the carboxyl group of one amino acid and the amino group of another, and state that "the amide bonds in peptides may be called peptide bonds."[1] IUPAC's glossary describes peptides as amides formed from two or more amino carboxylic acid molecules with "formal loss of water".[2]

Peptide bond formation Glycine plus alanine gives glycylalanine plus water. The new bond joins the carbonyl carbon of glycine to the nitrogen of alanine. glycineH₂N–CH₂–COOH + alanineH₂N–CH(CH₃)–COOH glycylalanine (Gly-Ala)…CH₂–CO–NH–CH… + H₂O CO–NH = the peptide bond
Two amino acids, one peptide bond, one water molecule released.

How a peptide bond forms: condensation

A peptide bond forms by a condensation reaction: the carboxyl carbon of one amino acid bonds to the amino nitrogen of the next, and a molecule of water is lost.[3][5] What remains of each amino acid in the chain is called a residue.[1]

# one bond, one wateramino acid 1 + amino acid 2 → dipeptide + H2O# n residuespeptide bonds = n − 1 waters released = n − 1# example: Gly + Ala → Gly-Ala, C5H10N2O3, 146.15 g/mol

The counting rule is plain arithmetic: each bond joins two neighbours, so a chain of n residues has one bond fewer than it has residues. A dipeptide has 1 peptide bond, a tripeptide 2, and the 15-residue BPC-157 has 14. Check any chain with the peptide molecular weight calculator, which adds residue masses and one water back for the free ends.

Formula bookkeeping: amino acids minus water
PeptideBuilt fromWater lostResult (PubChem)
Gly-AlaGlycine C2H5NO2 + alanine C3H7NO21 H2OC5H10N2O3, 146.15 g/mol (calculated)
Carnosine[6]β-Alanine C3H7NO2 + histidine C6H9N3O21 H2OC9H14N4O3, 226.23 g/mol
Aspartame[7]Asp-Phe, then a methyl ester on phenylalanine1 H2O for the peptide bondC14H18N2O5, 294.30 g/mol

Carnosine uses β-alanine rather than a standard α-amino acid, and IUPAC's definition of peptides still covers it.[2]

Why the peptide bond is flat and rigid

The peptide bond has partial double-bond character, so it does not rotate freely and the atoms around it lie in one plane.[5][3]

  • Planar. Alberts' textbook: "The peptide bond is planar" and does not permit rotation, while the bonds on either side of it can rotate.[3]
  • Trans, usually. StatPearls notes that steric interference from side chains "favors the trans configuration", with the side chains on opposite sides.[5]
  • Why it matters. Because the peptide bond itself cannot rotate, the chain can only bend at the bonds on either side of it, which limits how the backbone folds.[3]

How a peptide bond breaks: hydrolysis

A peptide bond is broken by hydrolysis, the reverse of condensation, which consumes one water molecule per bond.[4] Peptide bonds are kinetically stable: they resist hydrolysis even under conditions that unfold a protein, such as heat.[5] When a protein unfolds, it is the folding that is lost, not the chain of peptide bonds.

Most peptide bonds join the α-carboxyl of one residue to the α-amino group of the next. IUPAC–IUBMB rules call these "eupeptide" bonds and allow amide links through side chains, called isopeptide bonds.[1] Glutathione is the classic example: its first link runs through glutamate's side-chain (γ) carboxyl. See the tripeptide page.

Keep in mindMolecular weight does not depend on which carboxyl forms the bond. γ-Glu-Cys-Gly and α-Glu-Cys-Gly have the same formula, so a sequence calculator gives the same mass for both.

Frequently asked questions

What is a peptide bond in simple terms?

It is the link that holds amino acids together in a chain: an amide bond from the carbon of one amino acid's carboxyl group to the nitrogen of the next amino acid's amino group.

How many water molecules are lost when a peptide forms?

One per peptide bond. A chain of n amino acids has n − 1 peptide bonds and releases n − 1 water molecules as it forms.

Is a peptide bond covalent?

Yes. It is a covalent amide bond with partial double-bond character, which makes it rigid and planar.[5]

What breaks a peptide bond?

Hydrolysis, which adds a water molecule back across the bond.[4]

Sources

  1. IUPAC-IUB JCBN: Nomenclature and symbolism for amino acids and peptides, 3AA-11 to 3AA-13 (peptides, residues, peptide bonds) · IUPAC–IUBMB (web version, Queen Mary University of London), 1983 recommendations
  2. IUPAC glossary of class names (1995): peptides, polypeptides, proteins · IUPAC, Pure Appl. Chem. 67:1307, 1995
  3. Alberts B et al. Molecular Biology of the Cell, 4th ed., chapter 3: The shape and structure of proteins · Garland Science, via NCBI Bookshelf, 2002
  4. Alberts B et al. Molecular Biology of the Cell, 4th ed., chapter 2: The chemical components of a cell · Garland Science, via NCBI Bookshelf, 2002
  5. Forbes Kaprive J, Merheb E. Protein structure and function · StatPearls, via NCBI Bookshelf, updated August 30, 2026
  6. PubChem CID 439224: carnosine · PubChem, NCBI, accessed November 13, 2026
  7. PubChem CID 134601: aspartame · PubChem, NCBI, accessed November 13, 2026