# Peptide bond
A peptide bond is an amide covalent bond that links the carbonyl carbon of one amino acid to the nitrogen atom of another, formed with the formal loss of a water molecule; in proteins these bonds join amino acids into polypeptide chains[1](https://www.old.goldbook.iupac.org/html/P/P04479.html). Because the nitrogen lone pair delocalizes into the carbonyl, the C–N bond acquires partial double-bond character, which makes it rigid, planar, and central to the geometry of protein structure[2](https://www.ncbi.nlm.nih.gov/books/NBK6824/) • [3](https://www.nature.com/articles/nsb0301_201).
| Key fact | Detail |
|---|---|
| Definition | Amide bond from the carbonyl carbon of one amino carboxylic acid to the nitrogen of another, with formal loss of water (IUPAC)[1](https://www.old.goldbook.iupac.org/html/P/P04479.html) |
| Bond length | C–N distance of 1.32 Å, shorter than a normal single bond and longer than a normal double bond[2](https://www.ncbi.nlm.nih.gov/books/NBK6824/) |
| Double-bond character | About 40% N–C′ double-bond character from two dominant resonance structures[3](https://www.nature.com/articles/nsb0301_201) |
| Conformation | ω dihedral angle of 180° (trans) or 0° (cis); more than 99.5% of non-proline peptide bonds in proteins are trans[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5977977/) |
| Cis at proline | About 5–6% of Xaa–Pro bonds in folded proteins are cis, versus 0.03–0.05% for other bonds[5](https://www.sciencedirect.com/science/article/pii/S0022283698924597) |
| Hydrolysis stability | Uncatalyzed half-times of roughly 350–600 years at 25 °C and neutral pH[6](https://pubs.acs.org/doi/10.1021/ja954077c) |
| Biological formation | Catalyzed in the peptidyl transferase center of the large ribosomal subunit, which is composed entirely of RNA[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC1370378/) |
## Formation
A peptide bond forms by condensation (dehydration synthesis): the amino group of one amino acid attacks the carboxyl group of another, and a water molecule is eliminated[8](https://en.khanacademy.org/science/biology/macromolecules/v/peptide-bond-formation). In cells the reaction is carried out in several steps by the ribosome and additional RNA and protein factors[9](https://www.mohp.gov.eg/theducation/FirstGroup/Fany/Eligible_materials/Biochemistry/Biochemistry/Biochemistry.pdf). The reverse reaction, cleavage by water, is hydrolysis[10](https://prod-cat-files.macmillan.cloud/CMS/SampleChapters/7b43eb5d27dc48738c5ceb9c4847929a/Chapter_3-Amino-Acids-Peptides-and-Proteins.pdf).
In the ribosome, the peptidyl transferase center of the large subunit positions a P-site peptidyl-tRNA and an A-site aminoacyl-tRNA so that the α-amino group of the A-site substrate attacks the carbonyl carbon of the ester bond linking the nascent peptide to the P-site tRNA[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC129327/). Atomic-resolution structures published in 2000 showed that this catalytic site is composed entirely of RNA, making the ribosome a ribozyme[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC1370378/). Kinetic measurements indicate the ribosome accelerates the reaction mainly by eliminating the entropic contribution to the activation barrier, through substrate positioning rather than acid-base catalysis[12](https://pubmed.ncbi.nlm.nih.gov/16257828/).
Chemists also form peptide bonds in the laboratory. [Solid-phase peptide synthesis](https://www.edgepedia.org/solid-phase-peptide-synthesis), proposed by Merrifield in the early 1960s, anchors the growing chain to an insoluble polystyrene resin and extends it stepwise with Nα-protected amino acids, using excess soluble reagents that are removed by filtration; the first reported synthesis of a peptide and the term "peptide" itself date to Fischer and Fourneau in 1901[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC3564544/).
## Planarity and backbone geometry
Resonance between the two dominant structures of the amide group gives the C–N bond roughly 40% double-bond character[3](https://www.nature.com/articles/nsb0301_201). The bond is therefore not freely rotatable, and the α-carbons of adjacent residues, together with the carbonyl C=O and N–H groups, lie in a single plane[2](https://www.ncbi.nlm.nih.gov/books/NBK6824/). The torsion angle about this bond, ω, is 180° in the trans arrangement and 0° in the cis arrangement[2](https://www.ncbi.nlm.nih.gov/books/NBK6824/).
This rigidity simplifies protein geometry. Of the three repeating backbone dihedral angles, only φ (rotation about the N–Cα bond) and ψ (rotation about the Cα–C bond) vary; ω is generally treated as fixed at 180°[3](https://www.nature.com/articles/nsb0301_201) • [14](https://www.ebi.ac.uk/training/online/courses/foundations-protein-structure/fundamentals-of-protein-composition/the-peptide-bond-and-primary-structure/the-nature-of-the-peptide-bond/). [Linus Pauling](https://www.edgepedia.org/linus-pauling)'s prediction of the α-helix, made with Robert Corey, rested on assuming a planar peptide bond, equivalent residues, and hydrogen bonding of each amide proton to a carbonyl oxygen at an N–O distance of 2.72 Å; their standard planar trans peptide unit dates to 1951[3](https://www.nature.com/articles/nsb0301_201).
The planar ideal is an approximation. Large-scale surveys found ω distributions in high-resolution proteins averaging 179.6° with a standard deviation of 4.7°, and later gas-phase ab initio calculations showed the isolated peptide bond can rotate as much as 40° with little energetic cost[15](https://pmc.ncbi.nlm.nih.gov/articles/PMC3258596/) • [3](https://www.nature.com/articles/nsb0301_201). Significant deviations from planarity do occur in real protein structures, with consequences for their stability[16](https://www.sciencedirect.com/science/article/pii/S0010854516300169).
## Cis and trans conformations
The trans form is favored because it minimizes repulsion between non-bonded atoms attached to the two Cα centers[14](https://www.ebi.ac.uk/training/online/courses/foundations-protein-structure/fundamentals-of-protein-composition/the-peptide-bond-and-primary-structure/the-nature-of-the-peptide-bond/). In folded proteins, more than 99.5% of peptide bonds not involving proline are trans; the cis state costs roughly 2–6 kcal/mol more, and rotation about the partial double bond faces a barrier of about 20 kcal/mol[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5977977/).
Proline is the exception. Its side chain loops back and bonds covalently to its own backbone nitrogen, forming a five-membered pyrrolidine ring, so neither cis nor trans is strongly favored and the steric difference between the two states is much smaller[2](https://www.ncbi.nlm.nih.gov/books/NBK6824/) • [17](https://pmc.ncbi.nlm.nih.gov/articles/PMC10376262/). Surveys of protein structures find about 5–6% of Xaa–Pro bonds cis, against 0.03–0.05% of other bonds[5](https://www.sciencedirect.com/science/article/pii/S0022283698924597). In small peptides the cis fraction of X–Pro bonds can reach 10–40%, and in cyclic peptides it can be higher still; one database analysis found 57.4% of prolines in cyclic peptides cis versus 5.6% in acyclic ones[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC5128737/).
Because of the high rotational barrier, cis–trans isomerization is slow at room temperature and can limit the rate of protein folding; molecular chaperones exist that catalyze cis–trans proline isomerization[4](https://pmc.ncbi.nlm.nih.gov/articles/PMC5977977/) • [19](https://www.sciencedirect.com/science/article/pii/S0022283699932175). Cis prolines also serve structural and functional roles, such as positioning active-site residues, and are often conserved within protein families[20](https://pmc.ncbi.nlm.nih.gov/articles/PMC12102755/).
## Hydrolysis and cleavage
Peptide bonds are kinetically stable in water. In neutral solution at 25 °C, uncatalyzed hydrolysis half-times are approximately 500 years for the C-terminal bond of acetylglycylglycine, 600 years for the internal bond of acetylglycylglycine N-methylamide, and 350 years for the dipeptide glycylglycine; these rates are insensitive to pH and ionic strength[6](https://pubs.acs.org/doi/10.1021/ja954077c). This stability is why enzymes are needed to break them at biological timescales: proteases facilitate nucleophilic attack on the otherwise unreactive carbonyl group, using a catalytic triad or an activated water molecule[21](https://pure.au.dk/ws/files/87239444/Ph_d_thesis_Nanna_Villumsen_final_print.pdf). Trypsin, for example, cleaves specifically after the basic residues lysine and arginine[22](https://pubmed.ncbi.nlm.nih.gov/22485945/).
Non-enzymatic cleavage also occurs by pH-dependent pathways: direct hydrolysis and intramolecular aminolysis by the N-terminal amine, and bonds next to aspartate are preferentially hydrolyzed under acidic conditions[23](https://pubmed.ncbi.nlm.nih.gov/31807746/) • [24](https://pmc.ncbi.nlm.nih.gov/articles/PMC4287292/). Metal ions can assist peptide bond hydrolysis, a chemistry explored for [artificial proteases](https://www.edgepedia.org/artificial-protease) in biotechnology[16](https://www.sciencedirect.com/science/article/pii/S0010854516300169).
## References
1. [IUPAC Gold Book – peptides](https://www.old.goldbook.iupac.org/html/P/P04479.html)
2. [Protein Structure, Modelling and Applications, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK6824/)
3. [Arthur S. Edison (2001). Linus Pauling and the planar peptide bond. Nature Structural & Molecular Biology 8, 201–202.](https://www.nature.com/articles/nsb0301_201)
4. [Propensity for cis-proline formation in unfolded proteins, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5977977/)
5. [Non-proline cis peptide bonds in proteins, Journal of Molecular Biology](https://www.sciencedirect.com/science/article/pii/S0022283698924597)
6. [Radzicka & Wolfenden (1996). Rates of uncatalyzed peptide bond hydrolysis in neutral solution. JACS.](https://pubs.acs.org/doi/10.1021/ja954077c)
7. [After the ribosome structures: How does peptidyl transferase work? PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC1370378/)
8. [Peptide bond formation, Khan Academy](https://en.khanacademy.org/science/biology/macromolecules/v/peptide-bond-formation)
9. [Biochemistry teaching text on polypeptide formation](https://www.mohp.gov.eg/theducation/FirstGroup/Fany/Eligible_materials/Biochemistry/Biochemistry/Biochemistry.pdf)
10. [Nelson & Cox, Chapter 3: Amino Acids, Peptides and Proteins (sample chapter)](https://prod-cat-files.macmillan.cloud/CMS/SampleChapters/7b43eb5d27dc48738c5ceb9c4847929a/Chapter_3-Amino-Acids-Peptides-and-Proteins.pdf)
11. [Structural insights into peptide bond formation, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC129327/)
12. [The ribosomal peptidyl transferase center: structure, function, evolution, inhibition, PubMed](https://pubmed.ncbi.nlm.nih.gov/16257828/)
13. [Introduction to Peptide Synthesis, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3564544/)
14. [The nature of the peptide bond, EBI Foundations of protein structure](https://www.ebi.ac.uk/training/online/courses/foundations-protein-structure/fundamentals-of-protein-composition/the-peptide-bond-and-primary-structure/the-nature-of-the-peptide-bond/)
15. [Nonplanar peptide bonds in proteins are common and conserved, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3258596/)
16. [Metal assisted peptide bond hydrolysis, ScienceDirect](https://www.sciencedirect.com/science/article/pii/S0010854516300169)
17. [Proline Isomerization: From Chemistry and Biology to Therapeutic Opportunities, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10376262/)
18. [Natural Proline-Rich Cyclopolypeptides from Marine Organisms, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5128737/)
19. [Cis peptide bonds in proteins: residues involved, conformations, interactions and locations, ScienceDirect](https://www.sciencedirect.com/science/article/pii/S0022283699932175)
20. [Cis-nonProline peptides: Genuine occurrences and their functional roles, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC12102755/)
21. [Chapter 4. Hydrolysis of Peptide Bonds, Aarhus University PhD thesis](https://pure.au.dk/ws/files/87239444/Ph_d_thesis_Nanna_Villumsen_final_print.pdf)
22. [Fragmentation of protein using trypsin, PubMed](https://pubmed.ncbi.nlm.nih.gov/22485945/)
23. [The pH dependent mechanisms of non-enzymatic peptide bond cleavage reactions, PubMed](https://pubmed.ncbi.nlm.nih.gov/31807746/)
24. [Early Engineering Approaches to Improve Peptide Developability, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4287292/)
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*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Organic substances › Carbonyl and carboxyl chemistry › Carboxylic acid derivatives › Amides › Amide reactions and synthesis*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license