Short answer: A peptide bond is the amide bond that joins one amino acid to the next. It forms when the carboxyl group of one amino acid combines with the amino group of another and a molecule of water is released [1][2]. Chains of amino acids linked this way are peptides; long chains are polypeptides, and proteins are built from one or more polypeptides [2][3].
This article is educational chemistry for laboratory readers. It is not guidance for use in people or animals.
How a peptide bond forms
Every amino acid has an amino group and a carboxyl group. When two amino acids are joined, the –OH from the carboxyl group of the first and a hydrogen from the amino group of the second leave as water. What remains is a –CO–NH– link between them: the peptide bond [1][2]. Because water is lost, chemists call this a condensation (or dehydration) reaction [1][2].
Order matters. Alanine and serine can combine in two ways: if the alanine carboxyl reacts with the serine amino group you get alanylserine (Ala-Ser); the reverse gives serylalanine (Ser-Ala), a different molecule [4].
Each new amino acid adds one more peptide bond, so a linear chain of n amino acids contains n − 1 peptide bonds. The repeating –N–CH–CO– atoms form the chain's backbone [4].
Peptides, polypeptides, and proteins
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; a chain of 51 or more is a polypeptide [3]. OpenStax notes that "polypeptide" and "protein" are sometimes used interchangeably, but technically a polypeptide is the amino acid polymer, while "protein" refers to one or more polypeptides that have folded and combined into a functional molecule [2]. These cutoffs are conventions, not chemistry: the bond joining the residues is the same in a dipeptide and in a large protein.
Reading a sequence: N-terminus to C-terminus
Every linear peptide has two different ends. One carries a free amino group (the N-terminus), and the other carries a free carboxyl group (the C-terminus) [2]. By convention, sequences are written from left to right starting at the N-terminal amino acid, using three-letter or one-letter abbreviations (Ala-Ser, or A-S) [4].
Why the peptide bond is flat
A peptide bond behaves differently from an ordinary single bond. The nitrogen's lone pair of electrons is shared, by resonance, with the neighboring carbonyl group. That gives the C–N bond partial double-bond character and restricts rotation around it, so the amide group is planar [4].
Two practical consequences follow:
- The backbone is a chain of rigid, flat amide units connected through the alpha carbons of the repeating –N–CH–CO– backbone [4].
- Hydrogen bonding. The –CO–NH– group provides an N–H that can donate a hydrogen bond and a carbonyl oxygen that can accept one [5]. These backbone hydrogen bonds underlie regular structures such as the helices that Pauling, Corey, and Branson described in their 1951 paper on hydrogen-bonded helical configurations of the polypeptide chain [6].
Trans and cis peptide bonds
Because the peptide group is planar, its two alpha carbons can sit on opposite sides of the C–N bond (trans) or on the same side (cis). In OpenStax's description, the N–H points 180° away from the C=O [4], which is the trans arrangement. Cis bonds do occur. A survey of 571 non-redundant proteins in the Brookhaven Protein Data Bank identified 43 cis peptide bonds not involving proline, most of them in functionally important regions such as near active sites, and the authors suggested such bonds may be more abundant than previously thought [7].
How stable is a peptide bond?
Very stable without a catalyst. Radzicka and Wolfenden measured uncatalyzed hydrolysis (breakdown by water) of model compounds in neutral solution and estimated half-times at 25 °C of about 350 years for glycylglycine, 500 years for the C-terminal bond of acetylglycylglycine, and 600 years for an internal peptide bond [8]. The study used these uncatalyzed rates as a baseline for judging how proficient protein-cleaving enzymes are [8]. Peptide bonds can also be broken deliberately in the lab by partial or complete hydrolysis, yielding smaller peptides and eventually free amino acids [5].
A long backbone half-life does not mean a peptide sample is stable indefinitely. Other reactions can be much faster than backbone hydrolysis. In the same study, one model dipeptide derivative cyclized into a diketopiperazine with a half-time of about 35 days at pH 7 and 37 °C [8], and synthetic peptide products can also contain oxidation products and other related impurities [9]. That is why storage conditions matter; see how to store research peptides and lyophilized peptides explained.
Why peptide bonds matter for purity testing
The peptide bond absorbs ultraviolet light at 214 nm. Kuipers and Gruppen measured a molar extinction coefficient of 923 M⁻¹ cm⁻¹ per peptide bond at that wavelength, in conditions chosen for reversed-phase HPLC with mass spectrometry. They also found that tryptophan absorbs about 30 times more strongly, and phenylalanine, tyrosine, and histidine about six times more strongly, than a single peptide bond [10].
Two practical points for anyone reading an analytical report:
1. Peptides are visible to UV detection even without aromatic residues, because every residue linkage contributes absorbance at 214 nm [10]. Learn more in what is HPLC peptide testing.
2. UV peak area is not a molecule count. Absorbance depends on the number of peptide bonds and on which amino acids are present [10], so an impurity with a different sequence or length will not absorb exactly like the target peptide. Mass spectrometry is what confirms identity; see mass spectrometry peptide testing and how to read a peptide COA.
Peptide bonds vs. disulfide bonds
Peptide bonds are not the only covalent bonds in peptides. Two cysteine residues can be joined by a disulfide bond (–S–S–), formed by mild oxidation and broken by mild reduction. A disulfide can link two separate chains or create a loop within one chain. Insulin, for example, has two chains totaling 51 amino acids, held together by disulfide bridges [4].
FAQ
What is a peptide bond in simple terms?
The amide link (–CO–NH–) that joins the carboxyl group of one amino acid to the amino group of the next, formed with the loss of one water molecule [1][2].
Is a peptide bond covalent?
Yes. OpenStax describes it as a covalent bond formed by a dehydration reaction [2].
What type of reaction forms a peptide bond?
A condensation (dehydration) reaction, because water is released [1][2].
Why can't a peptide bond rotate freely?
Resonance between the nitrogen lone pair and the carbonyl gives the C–N bond partial double-bond character, so the amide group is planar and rotation is restricted [4].
How many peptide bonds are in a peptide with 10 amino acids?
Nine, for a linear chain: each new residue adds one bond [4].
What breaks a peptide bond?
Hydrolysis. Without a catalyst it is extremely slow in neutral water [8]; enzymes such as proteases and laboratory hydrolysis conditions break it much faster [5][8].
Bottom line
Every research peptide is a chain of amino acids held together by peptide bonds. The bond's flat, rigid structure and hydrogen-bonding groups help shape how chains fold, its slow uncatalyzed hydrolysis means other reactions can matter more for sample stability, and its UV absorbance at 214 nm is part of how HPLC methods see peptides at all. For how Dynamite Research Peptides documents identity and purity for each batch, see our quality page and the research peptide glossary.
All products are for research use only — not for human or animal consumption.