What Is Amidation? How a C-Terminal Amide Changes a Peptide

Published October 2026 · Patriot Labs Research Team · ~16 min read

The short answer

Amidation is a chemical change at the end of a peptide in which the C-terminal carboxylic acid (–COOH) is converted into a primary amide (–CONH₂). It removes the negative charge that a free acid carries, is the final step in making roughly half of all bioactive peptides, and in the laboratory is chosen at the first step of synthesis by picking an amide-forming resin.

The modification is small on paper — one oxygen is swapped for a nitrogen — but it changes the molecule in ways that matter. The amide is neutral where the acid was negative, it lets the end of the chain resemble an internal peptide bond rather than a charged terminus, and it blocks the enzymes that trim peptides from the C-terminal end. In the body a single enzyme carries out the reaction; in a synthesis laboratory it is built in directly. The rest of this guide takes each of those in turn, then covers how a certificate of analysis confirms that a peptide is amidated rather than left as the free acid.

What amidation is, atom by atom

Amidation replaces the hydroxyl (–OH) of the C-terminal carboxylic acid with an amino group (–NH₂), turning –C(=O)OH into –C(=O)NH₂, a primary carboxamide. Everything else about the peptide stays the same.

To see why that one swap is worth a whole guide, it helps to locate it. Every linear peptide has two ends: an N-terminus, where the first residue keeps a free amino group, and a C-terminus, where the last residue keeps a free carboxyl group. Along the chain, each residue is joined to the next by a peptide bond, which is itself an amide linkage — a carbonyl bonded to a nitrogen. In that sense a peptide is already a string of amides. What “amidation” refers to is not those internal bonds but the very last group on the chain: converting the terminal carboxylic acid into an amide so that the C-terminus looks, chemically, more like one of the internal linkages than like a free acid.

The consequence that drives everything downstream is charge. A carboxylic acid is ionizable: at the roughly neutral pH of body fluids it loses its proton and sits as a carboxylate, carrying a full negative charge. A primary amide is not ionizable in that range; it stays neutral. So the difference between a free-acid peptide and its amidated counterpart is not only an atom, it is the presence or absence of a fixed negative charge at the end of the molecule. That in turn affects the peptide’s net charge, its electrostatic interactions, its solubility behaviour and how it moves through a chromatography column.

How cells amidate a peptide: the PAM enzyme

In animals, amidation is carried out by a single enzyme, peptidylglycine α-amidating monooxygenase (PAM) — the only enzyme known to perform this reaction in humans. It works in two catalytic steps, housed in two domains of the same protein, and it needs copper, vitamin C and oxygen to run.

The first domain, peptidylglycine α-hydroxylating monooxygenase (PHM), hydroxylates the α-carbon of a glycine residue sitting at the C-terminus of the precursor. PHM is a copper enzyme: it carries two copper centres, held about 11 Ångström apart, and it uses molecular oxygen together with ascorbate (vitamin C) as the electron donor to activate that oxygen. The second domain, peptidyl-α-hydroxyglycine α-amidating lyase (PAL), then cleaves the hydroxylated intermediate, splitting the bond between the glycine nitrogen and its α-carbon. That cleavage releases two products: the mature peptide, now ending in an amide, and glyoxylate, the leftover two-carbon fragment of the glycine.

The key conceptual point is where the amide nitrogen comes from. It is not added from solution; it is donated by the backbone nitrogen of the C-terminal glycine. The enzyme effectively sacrifices that glycine, keeping its nitrogen to cap the preceding residue as an amide and discarding its carbon skeleton as glyoxylate. This is why amidation and a C-terminal glycine are so tightly linked, and it is the basis of the “glycine rule” covered next. PAM’s dependence on copper and ascorbate has made it a long-studied example of a copper monooxygenase, and the enzyme is active not only inside the secretory pathway of cells but also, in soluble forms, in the circulation.

The glycine-extended precursor and the amidation signal

Because the amide nitrogen is taken from a C-terminal glycine, every peptide destined to be amidated is first made one residue too long, ending in glycine. That extra glycine is the signal that the peptide should be amidated, and it is spelled out in the gene.

This was worked out in 1982, when Bradbury, Finnie and Smyth identified a pituitary enzyme that converted peptides ending in glycine into the corresponding one-residue-shorter peptide amide, studying the sequence around α-melanotropin. Their insight explained a pattern that had puzzled biochemists: when the amino-acid sequence of an amidated hormone was compared with the DNA that encoded it, the gene almost always carried a glycine codon immediately after the mature sequence, frequently followed by one or two basic residues such as lysine or arginine. Those basic residues mark where processing enzymes first cut the long precursor protein, and the exposed glycine is then handed to PAM.

So the biosynthetic route to an amidated peptide runs in a fixed order. A gene encodes a large precursor protein. Processing enzymes (prohormone convertases and carboxypeptidases) cut it down to a peptide that ends in –Xaa–Gly. PAM then hydroxylates and cleaves that terminal glycine, leaving –Xaa–NH₂. The glycine is never part of the finished hormone; it exists only to supply the nitrogen and to tell the cell’s machinery that this particular peptide is one that must be capped. Reading a precursor sequence, a researcher can often predict amidation simply by spotting a glycine followed by basic residues at the right position.

Why amidation matters: charge, shape and receptor fit

Amidation matters because, for a large number of peptide hormones, the amide is required for the molecule to be recognised by its receptor. The free-acid version of the same sequence is often far less active, or inactive. Three linked effects explain why.

The first is the charge change already described. Removing the terminal negative charge changes the local electrostatics at the C-terminus, which is exactly the region many receptors read. Where a binding pocket is shaped to accept a neutral amide and to form hydrogen bonds with it, a negatively charged carboxylate is a poor fit. The second is mimicry: a C-terminal amide resembles an internal peptide bond more than a free acid does, so the end of an amidated peptide presents the kind of surface a protein is used to contacting. The third is conformation. The terminal group participates in hydrogen bonding, and an amide can help stabilise a C-terminal α-helix or a defined end structure, whereas a charged carboxylate can disrupt it. Because receptor recognition depends on both the chemistry and the shape of the ligand’s end, these effects compound.

None of this means amidation always increases activity, and the size of the effect depends on the sequence and on which receptor is involved. What is reliable is the general principle: the identity of the C-terminal group is part of the information a peptide carries, and for amidated hormones that information is “amide, not acid.” This is why a research peptide intended to match a natural amidated sequence must itself be amidated; the free-acid analogue is a different molecule with different properties.

Amidation and protease resistance

Amidation also makes a peptide harder to degrade from its C-terminal end, because it removes the handle that a whole class of enzymes depends on. Carboxypeptidases are exopeptidases that chew residues off a peptide one at a time starting at the C-terminus, and they recognise the free C-terminal carboxylate to do it.

Replace that carboxylate with an amide and the recognition element is gone. Carboxypeptidase A and carboxypeptidase B, the classic examples, lose their grip, and the peptide becomes resistant to C-terminal trimming. Studies comparing amidated peptides with their free-acid counterparts report slower degradation and longer persistence in plasma in laboratory and animal models, with the exact difference depending on sequence context. This is one reason amidation is a routine choice when a peptide is being designed for research on stability.

The protection is specific, not general. Amidation does nothing to stop endopeptidases, which cut in the middle of a chain, or aminopeptidases, which trim from the N-terminal end. A peptide can be fully amidated and still be cleaved internally within minutes if it contains a sequence that an endopeptidase targets. Amidation is therefore best understood as closing one degradation route — the C-terminal exopeptidase route — rather than as a shield around the whole molecule. It is frequently paired with an N-terminal cap, discussed below, to close the opposite end as well.

Amidated peptides in nature

A striking number of well-known peptide hormones are amidated, which is the best evidence of how important the modification is. Estimates commonly cited in the literature put the figure at roughly half of all known bioactive peptides, and more than seventy neural and endocrine peptides are reported to require a C-terminal amide for full activity.

The table below lists representative examples. The pattern cuts across systems — pituitary, gut, nervous system — which is why amidation is treated as a general feature of peptide signalling rather than a quirk of one family.

A useful illustration of how much the C-terminus matters is GLP-1, which exists in the body in two mature forms: a glycine-extended free-acid form and an amidated form that ends one residue earlier. The two are closely related sequences that differ chiefly at the C-terminus, and both are biologically active — a reminder that amidation is a defined, specified property of a given peptide rather than an optional finish. α-MSH is the other instructive case: it is capped at both ends, acetylated at the N-terminus and amidated at the C-terminus, the “double-cap” arrangement discussed below.

How a C-terminal amide is made in the laboratory

In chemical synthesis, a peptide’s C-terminus is decided before the first amino acid is added, by the choice of resin. Making an amide does not mean converting the acid at the end; it means starting the synthesis on a support that releases an amide.

Almost all research peptides are built by solid-phase peptide synthesis (SPPS), the method R. B. Merrifield introduced in 1963, in which the chain is assembled one protected amino acid at a time while anchored to an insoluble resin bead. The link between the growing chain and the bead is a chemical handle called the linker, and the linker determines what the C-terminus becomes when the finished peptide is cleaved off. In the widely used Fmoc approach, amide-forming resins — Rink amide (in its Rink amide AM and Rink amide MBHA versions), PAL, NovaSyn TGR and Ramage resins — are built around a nitrogen-based handle, so that treating the finished peptide with trifluoroacetic acid (TFA) cleaves the linker and leaves a primary amide on the last residue. Choose a different resin — Wang, or 2-chlorotrityl — and the same synthesis yields a free carboxylic acid instead.

The Rink amide linker, introduced by Rink in 1987, is an acid-labile dimethoxy-substituted benzhydrylamine-type handle designed specifically for making C-terminal peptide amides by the Fmoc method. The table summarises the usual choices.

Amides can also be generated enzymatically — carboxypeptidase-Y and recombinant PAM have both been used in vitro — but for routine synthesis the resin choice is the standard route. The practical takeaway is that “amidated” is a design decision fixed at the start, and a peptide ordered as the amide is a genuinely different product from the same sequence ordered as the free acid. For the broader chemistry of how the chain itself is assembled, see What Is Solid-Phase Peptide Synthesis?

How amidation is detected: the one-dalton signature

Amidation is confirmed by mass. Because the modification swaps a hydroxyl for an amino group — replacing –OH with –NH₂ — the amide is lighter than the free acid by about 0.984 daltons, so an amidated peptide weighs roughly one dalton less than its free-acid counterpart.

That “one-dalton” difference is small, but high-resolution mass spectrometry resolves it cleanly, and it is the most direct way to tell an amide from an acid. On a certificate of analysis, the measured monoisotopic mass of a peptide amide should match the amidated calculation, not the free-acid value; a one-dalton discrepancy in the wrong direction is a sign the wrong C-terminus was made. Amidation also shifts the molecule slightly in reversed-phase HPLC, because removing the polar, ionizable carboxylate makes the peptide marginally less polar, so it tends to elute a little later than the corresponding acid.

One subtlety is worth stating to avoid confusion with the biosynthetic literature. Measured against the free-acid analogue of the same final sequence, amidation is a loss of about 0.98 Da. Measured against the glycine-extended precursor — the longer chain the enzyme actually starts from — the net change is a loss of roughly 58 Da, because the whole terminal glycine, minus the retained nitrogen, is removed. Both figures describe the same event from different starting points; for quality-control purposes on a finished peptide, the relevant comparison is the one-dalton difference from the free acid. Patriot Labs reports each lot’s identity and purity from an independent US laboratory by HPLC and mass spectrometry, with the certificate of analysis on the product page, and for an amidated peptide the reported mass is what shows the amide was installed. For more on reading those documents, see Understanding Peptide COA Testing.

Amidation and acetylation: the two end-caps

C-terminal amidation is one of a pair of common “capping” modifications; the other is N-terminal acetylation, at the opposite end of the chain. Together they are sometimes called a double cap because they close both ends against the exopeptidases that attack from each terminus.

N-terminal acetylation adds an acetyl group (CH₃CO–) to the free amino group at the start of the peptide, converting a positively charged amine into a neutral amide and adding about 42 daltons to the mass. Where amidation blocks carboxypeptidases at the C-terminus, acetylation blocks aminopeptidases at the N-terminus, so a peptide capped at both ends is protected from exopeptidase trimming from either direction. α-MSH is the textbook natural example of a doubly capped peptide. The two modifications are independent — a peptide can carry either, both or neither — but they are often requested together when the goal is to make a synthetic sequence resemble its native, fully processed form. Amidation and acetylation both work by the same underlying trick: replacing a charged, enzyme-readable terminus with a neutral amide.

What this guide does not establish

This guide is an explanation of a chemical and biological modification. It is not a claim about any product’s effects, and several boundaries are worth stating plainly.

• Nothing here describes any use in a person or animal. Biology is described to explain how peptides are amidated and why the modification changes a molecule’s properties, not to suggest that any compound should be administered or that it produces any effect in a living subject.
• Amidation is a structural fact, not a benefit. That amidation can increase receptor binding or slow one route of degradation in the published research describes the molecule’s chemistry; it is not a statement that any research peptide is safe, effective, or suitable for any purpose.
• Whether a given peptide is amidated depends on the specific peptide. The examples here are drawn from the scientific literature on natural hormones; they do not indicate the modification state of any particular product, which is defined by that product’s own specification and certificate of analysis.
• Nothing here describes where Patriot Labs’ peptides are synthesized. Patriot Labs is a first-responder-owned, US-based company, and the only quality facts used here are that every lot is tested by an independent US laboratory by HPLC and mass spectrometry, with the COA available on the product page.

Frequently asked questions

What is amidation in peptides?

Amidation is a modification of a peptide's C-terminus in which the terminal carboxylic acid group (–COOH) is converted into a primary amide (–CONH₂). It is one of the most common modifications found in natural peptide hormones and is routinely built into synthetic research peptides. The change removes the negatively charged carboxylate and replaces it with a neutral amide, which alters the peptide's charge, its shape and how resistant it is to certain enzymes.

What is C-terminal amidation?

C-terminal amidation refers specifically to the amide group at the very end of the chain, the carboxy terminus. Every peptide has an N-terminus (a free amine) and a C-terminus; in an unmodified synthetic peptide the C-terminus is a free carboxylic acid, while in an amidated peptide it is a carboxamide. The term “amidation” in peptide chemistry almost always means this C-terminal carboxamide, as distinct from the amide bonds that link residues together along the backbone.

What enzyme amidates peptides, and how?

In animals the only known enzyme is peptidylglycine α-amidating monooxygenase (PAM), a bifunctional enzyme with two catalytic parts. The first, PHM (peptidylglycine α-hydroxylating monooxygenase), hydroxylates the α-carbon of a C-terminal glycine using copper, vitamin C (ascorbate) and molecular oxygen. The second, PAL (peptidyl-α-hydroxyglycine α-amidating lyase), splits the hydroxylated intermediate to release the amidated peptide and glyoxylate. The amide nitrogen comes from that C-terminal glycine.

Why are peptides amidated?

In the body, amidation is often required for a peptide to be recognised by its receptor; many peptide hormones are inactive in their free-acid form. The amide removes the C-terminal negative charge, lets the end of the chain mimic an internal peptide bond rather than a charged carboxylate, and can help stabilise a C-terminal helix. It also makes the peptide harder for carboxypeptidases to trim from the C-terminal end.

Does amidation make a peptide more stable?

Against one specific route, yes. Carboxypeptidases recognise and remove residues from a free C-terminal carboxylate, and replacing that carboxylate with an amide removes the feature they depend on, so amidated peptides resist carboxypeptidase cleavage and are reported to degrade more slowly than their free-acid analogues in laboratory and animal studies. It does not protect the rest of the chain: endopeptidases and aminopeptidases act elsewhere, so amidation is one factor in stability, not a general shield.

What is the difference between an amidated peptide and a free-acid peptide?

They differ at a single atom at the C-terminus: a free-acid peptide ends in –COOH, which carries a negative charge at physiological pH, while an amidated peptide ends in –CONH₂, which is neutral. That difference changes the molecule's net charge, its behaviour in chromatography, and often its biological activity. The amide form is also about one dalton lighter, which is how the two are told apart by mass spectrometry.

What resin is used to make a C-terminal peptide amide?

In Fmoc solid-phase peptide synthesis, the C-terminus is decided by the resin at the very first step. Amide-forming supports such as Rink amide (Rink amide AM and Rink amide MBHA), PAL and Ramage resins release a primary amide when the finished peptide is cleaved with trifluoroacetic acid, whereas Wang and 2-chlorotrityl resins release a free carboxylic acid. To make an amidated peptide, the chemist starts on an amide resin rather than converting the acid afterwards.

How do you detect amidation, and what is the mass change?

Amidation is read directly from the intact mass. Replacing the C-terminal hydroxyl (–OH) with an amino group (–NH₂) lowers the monoisotopic mass by about 0.984 daltons relative to the free acid, so an amidated peptide is roughly one dalton lighter than its free-acid counterpart, a shift that high-resolution mass spectrometry resolves cleanly. A certificate of analysis for a peptide amide should report a measured mass matching the amidated form, and the peptide usually shifts slightly in HPLC because it is less polar than the acid.

Which peptides are amidated? Is oxytocin amidated?

Yes, oxytocin is amidated, ending in a glycinamide. Many well-studied peptide hormones share the feature, including vasopressin, gastrin, cholecystokinin, calcitonin, substance P, neuropeptide Y, α-MSH and calcitonin gene-related peptide. Estimates commonly cited in the literature are that roughly half of all known bioactive peptides carry a C-terminal amide, which is why amidation is such a frequently requested modification.

References

1. Bradbury, A. F., Finnie, M. D. A., & Smyth, D. G. (1982). Mechanism of C-terminal amide formation by pituitary enzymes. Nature, 298, 686–688. doi:10.1038/298686a0
2. Kumar, D., Mains, R. E., & Eipper, B. A. (2016). 60 YEARS OF POMC: From POMC and α-MSH to PAM, molecular oxygen, copper, and vitamin C. Journal of Molecular Endocrinology, 56(4), T63–T76. doi:10.1530/JME-15-0266
3. Merkler, D. J., Hawley, A. J., Eipper, B. A., & Mains, R. E. (2022). Peptidylglycine α-amidating monooxygenase as a therapeutic target or biomarker for human diseases. British Journal of Pharmacology, 179. doi:10.1111/bph.15815
4. Prigge, S. T., Mains, R. E., Eipper, B. A., & Amzel, L. M. (2000). New insights into copper monooxygenases and peptide amidation: structure, mechanism and function. Cellular and Molecular Life Sciences, 57. https://www.semanticscholar.org/paper/6a21b18761e85be0894e3a363597d5b3c6a1d706
5. Coassolo, L., Wiggenhorn, A., & Svensson, K. J. (2025). Understanding peptide hormones: from precursor proteins to bioactive molecules. Trends in Biochemical Sciences. doi:10.1016/j.tibs.2025.03.014; PMCID PMC12145250
6. Cawley, N. X., Li, Z., & Loh, Y. P. (2016). 60 YEARS OF POMC: Biosynthesis, trafficking, and secretion of pro-opiomelanocortin-derived peptides. Journal of Molecular Endocrinology, 56. doi:10.1530/JME-15-0323
7. Novel insights into peptide amidation and amidating activity in the human circulation (2021). Scientific Reports, 11. doi:10.1038/s41598-021-95305-y
8. Rink, H. (1987). A new acid-labile anchor group for the solid-phase synthesis of C-terminal peptide amides by the Fmoc method. Tetrahedron Letters, 28. https://www.sciencedirect.com/science/article/abs/pii/S0040403900960556
9. Merrifield, R. B. (1963). Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. Journal of the American Chemical Society, 85(14), 2149–2154. doi:10.1021/ja00897a025
10. Merck / Sigma-Aldrich. Linkers for Fmoc Solid-Phase Peptide Synthesis (technical article). https://www.sigmaaldrich.com/US/en/technical-documents/technical-article/chemistry-and-synthesis/peptide-synthesis/linkers-fmoc-spps
11. Grolimund, J. (2020, June 20). Amidation. Blog pi (Prot pi). https://www.protpi.ch/blog/bioinformatics/2020/06/amidation/
12. Biosynth. The Indispensable Role of Terminal Modifications in Peptides (N- and C-terminal modifications white paper). https://www.biosynth.com/
13. PeptideTrace. Peptide C-Terminal Modifications and Amidation: Structural Impact on Protease Resistance, Receptor Binding, and In Vivo Stability. https://www.peptidetrace.com/articles/peptide-c-terminal-amidation-structural-impact-protease-resistance-receptor-binding-stability
14. α-Amidated Peptides: Approaches for Analysis. In Methods in Molecular Biology. Springer. doi:10.1007/978-1-60327-084-7_5
15. ScienceDirect Topics. Peptidylglycine Alpha-Amidating Monooxygenase (overview). Elsevier. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/peptidylglycine-alpha-amidating-monooxygenase

For related chemistry, see What Is a Peptide Bond? for the backbone amide that links residues, What Is DAC in Peptides? and What Is Lipidation? for other ways peptides are modified, Peptide Half-Life Extension for the broader toolkit of stability modifications, and TFA vs Acetate in Peptides for another detail that distinguishes one vial from another.

All Patriot Labs products are sold strictly for in-vitro research and laboratory use only. Not for human or veterinary consumption. This guide is educational and describes amidation as a chemical and biological modification in general terms; it is not medical advice, does not describe how to use any product, and makes no claim about the composition or modification state of any specific product.