Peptide Coupling Reagents Explained: How Amino Acids Are Joined

Published October 2026 · Patriot Labs Research Team · ~21 min read

The short answer

A peptide coupling reagent is a chemical that activates the carboxylic acid of one amino acid so it joins the amine of the next, forming the amide bond that links them. Left alone, an acid and an amine do not combine into an amide at room temperature; the reagent converts the acid into a reactive species — usually an active ester — that the amine attacks readily. The main families are carbodiimides such as DIC and DCC, aminium/uronium salts such as HBTU and HATU, and phosphonium salts such as PyBOP, very often paired with an additive such as HOBt, HOAt or Oxyma that improves yield and protects stereochemistry.^[1][2]

The reason the field has so many named reagents — a “letter soup” of acronyms, as one landmark review put it — is that two things have to be balanced at once.^[1] The coupling has to go fast and to completion, because an incomplete coupling leaves a deletion impurity behind, and it has to proceed without racemizing the amino acid, because a flipped stereocentre produces a diastereomer that is hard to remove. Different reagents strike that balance differently, and the history of the field is largely the history of making couplings cleaner, faster and safer.

The rest of this guide takes the families in turn, explains the chemistry each one uses to activate the acid, and then covers the practical questions — which base to use, how reagents are chosen, and why several of the most common ones carry real safety hazards. For where this step sits in the larger process, see What Is Solid-Phase Peptide Synthesis; for the bond being formed, see What Is a Peptide Bond.

Why the amide bond needs help

An amide bond is thermodynamically stable but kinetically reluctant to form, which is exactly why it needs a reagent. Mixing a carboxylic acid with an amine does not give an amide; it gives a salt, because the acid simply hands its proton to the more basic amine and the two ions sit together without reacting.^[2][3]

To make the bond form, the carboxyl group has to be turned into something more electrophilic — a carbonyl carrying a good leaving group, so that the amine's lone pair can attack it and expel the leaving group. Driving the reaction by brute force, such as high heat to boil off water, is possible but destroys the delicate side chains and stereocentres of a real peptide. The alternative, developed over seventy years of peptide chemistry, is to activate the acid chemically under mild conditions. That is the single job of a coupling reagent: it takes the carboxyl group and converts it, in one or two steps, into an activated carbonyl — an anhydride, an O-acylisourea or, most usefully, a stable but still reactive active ester — which the incoming amine can then attack.^[1][2][3]

This matters most in solid-phase peptide synthesis, the method introduced by Merrifield in 1963, where the growing chain is anchored to a resin bead and each new residue is coupled on in turn.^[23] Every residue added is one coupling event, so a 30-residue peptide means roughly 30 activations, and the efficiency and cleanliness of the coupling reagent compounds across the whole synthesis. A reagent that couples at 99.5 percent per step still loses roughly a seventh of the chains over 30 couplings, which is why so much effort has gone into reagents that push each step as close to completion as possible.

The families of coupling reagent

Coupling reagents sort into a small number of families defined by how they activate the acid and what they leave behind. Knowing the family tells you most of what you need to know about a given acronym, because members of a family share a mechanism and a byproduct.

The three workhorse families are the carbodiimides, the aminium/uronium salts and the phosphonium salts.^[1][2] Carbodiimides activate the acid directly into an O-acylisourea and are almost always run with a separate additive. Aminium/uronium salts and phosphonium salts are “onium” reagents that already contain the additive's anion built in, so they deliver an active ester in one reagent.^[16] A practical quirk of the naming: the first letter of an onium reagent's acronym usually marks its counter-anion — an H for hexafluorophosphate, a T for tetrafluoroborate — so HBTU and TBTU are the same cation with different salts.^[2] Beyond these three sit the additives themselves (HOBt, HOAt, Oxyma), which are not stand-alone reagents but are so central that they are covered in their own section, and a handful of other activators such as propylphosphonic anhydride (T3P) and the acyl halides.

Carbodiimides: DCC, DIC and EDC

Carbodiimides are the oldest and conceptually simplest coupling reagents: a molecule with the R–N=C=N–R core that soaks up the elements of water as the amide bond forms. They are the reagents on which modern peptide coupling was founded, and they remain in daily use.

The breakthrough came in 1955, when John Sheehan and George Hess reported dicyclohexylcarbodiimide (DCC) as a new way of forming peptide bonds under mild conditions, a result that helped open the door to practical peptide synthesis.^[4] DCC works by letting the carboxylate add across one of its C=N double bonds, producing a reactive O-acylisourea in which the carboxyl group now carries a good leaving group; the amine then attacks and the carbodiimide departs as a urea.^[2][5][6] DCC's drawback is that byproduct: it leaves dicyclohexylurea (DCU), which is nearly insoluble in most solvents and in water, so most of it can be filtered off but the last traces are awkward to remove from a product.

Diisopropylcarbodiimide (DIC) was adopted largely to fix the byproduct problem. DIC is a liquid, which makes it easy to dispense, and its urea byproduct is soluble in the organic solvents used in synthesis, so it washes away cleanly — a decisive advantage on resin, where everything has to be rinsed off the beads.^[2] For that reason DIC, usually with an additive, is the default carbodiimide of modern solid-phase work. The third member, water-soluble EDC (also called EDAC), carries a tertiary-amine tail that makes both the reagent and its urea byproduct water-soluble; Sheehan and Cruickshank described its preparation in 1968, and it is the carbodiimide of choice for couplings and bioconjugations run in or near water.^[7]

What all three share is that, used by themselves, they are prone to two side reactions that waste material and scramble stereochemistry. That is why a carbodiimide is almost never used alone in peptide work — it is paired with an additive, for reasons the next section makes concrete.

The O-acylisourea problem and racemization

The reactive O-acylisourea that a carbodiimide forms is a double-edged intermediate: it is reactive enough to couple, but also reactive enough to go wrong in two specific ways. Understanding those two failure modes is the key to understanding why every other reagent class exists.

The first failure is rearrangement. If the amine does not reach the O-acylisourea quickly, the intermediate can rearrange internally into an N-acylurea, a stable and unreactive compound in which the acyl group has migrated onto nitrogen.^[2][5] That amino acid is now dead weight — it cannot couple and it contaminates the product. Low-polarity solvents such as dichloromethane slow this rearrangement, but they do not stop it.^[5]

The second failure is racemization. When the carboxyl is activated, the proton on the neighbouring alpha-carbon becomes acidic enough to be removed, and the activated acid can cyclise into a five-membered ring called an oxazolone (an azlactone). The oxazolone's stereocentre scrambles freely, so when it finally reacts it delivers a mixture of the original L-amino acid and its mirror-image D-epimer.^[1][2] A single flipped residue creates a diastereomeric impurity that is chemically very similar to the target and therefore hard to separate, a problem covered in Peptide Impurities Explained and D-Amino Acids in Peptides.

Both failures have the same fix: convert the fleeting, over-reactive O-acylisourea into a calmer, well-behaved active ester before anything goes wrong. That is the job of an additive, and it is also the logic built into the onium reagents, which deliver the active ester directly. Everything that follows is a variation on that one idea.

Additives: HOBt, HOAt and Oxyma

An additive is a small molecule that steps in between activation and coupling, trapping the activated acid as a stable active ester that still reacts with amines but no longer rearranges or racemizes. It is the single most important supporting character in peptide coupling.

The classic additive is 1-hydroxybenzotriazole (HOBt), introduced as a carbodiimide additive by Wolfgang König and Rolf Geiger in 1970.^[8] HOBt intercepts the O-acylisourea to form an HOBt active ester (an OBt ester), which suppresses both the N-acylurea rearrangement and the oxazolone racemization while remaining reactive enough to acylate the incoming amine.^[1][2] Because the HOBt is regenerated as the reaction proceeds, a catalytic-to-stoichiometric amount transforms the quality of a carbodiimide coupling.

In 1993 Louis Carpino reported a more powerful relative, 1-hydroxy-7-azabenzotriazole (HOAt), which adds a nitrogen atom at the 7-position of the ring.^[9] That extra nitrogen sits next to the reactive oxygen and is thought to assist the incoming amine through a hydrogen-bonded, seven-membered cyclic transition state — a “neighbouring-group effect” that makes HOAt esters couple faster and racemize less than the corresponding HOBt esters.^[9][10] Carpino and co-workers later confirmed by comparing ring isomers that the 7-aza position is the one that matters.^[10]

The newest of the common additives is Oxyma (ethyl cyano(hydroxyimino)acetate, sold as Oxyma Pure), introduced by Subirós-Funosas, El-Faham, Albericio and co-workers in 2009 specifically as an efficient racemization suppressor with a lower risk of explosion than the benzotriazoles HOBt and HOAt.^[18] Oxyma is not a benzotriazole at all, which is the point: it sidesteps that class's explosive hazard while matching much of its coupling performance, and it has become a mainstay of greener solid-phase protocols. Each of these additives also exists built into a stand-alone onium reagent, which is where the story goes next.

Aminium and uronium salts: HBTU and HATU

Aminium/uronium salts are one-pot reagents that carry the additive's anion already attached, so a single reagent plus a base delivers the active ester directly. They are the fast, convenient workhorses of modern peptide synthesis, and HBTU and HATU are the two most recognisable names in the whole field.

HBTU was reported by Dourtoglou and co-workers in 1978 and carries the HOBt anion; on activation it hands the acid an OBt active ester, releasing tetramethylurea, and it is prized for mild, low-racemization couplings.^[13][14] HATU, reported by Carpino in 1993, is its HOAt-based counterpart: it delivers the more reactive OAt ester and benefits from the same 7-aza neighbouring-group effect that makes HOAt couplings fast and clean, so HATU is generally the stronger reagent for sterically hindered or otherwise difficult couplings.^[1][9] The pair differ by a single ring nitrogen, and that difference is the whole reason both exist side by side: HBTU for routine work, HATU when a coupling is stubborn.

There is a well-known naming twist worth knowing. These reagents were originally drawn and named as uronium salts, with the central carbon bonded to the oxygen of the benzotriazole. X-ray crystallography later showed that the commercial materials actually crystallize as the guanidinium isomer, with the carbon bonded to a ring nitrogen instead — which is why the more precise term is “aminium/uronium” salt.^[11][12] In 2002 Carpino and a large international group showed that the genuine O-uronium forms could be isolated and are in fact the more reactive species, titling their paper “finally the true uronium salts.”^[11] For practical purposes the commercial guanidinium reagents still couple extremely well, and the acronyms have stuck. The family extends to the tetrafluoroborate salts TBTU and TATU and the chloro-substituted HCTU, which are variations on the same theme.^[1][16]

Phosphonium salts: BOP, PyBOP and PyAOP

Phosphonium salts activate the acid through a phosphorus centre rather than a carbon one, and they share the onium advantage of delivering a benzotriazolyl active ester in a single reagent. Their defining practical trait is that, unlike the aminium/uronium salts, they do not cap the amine, which makes them valuable for demanding couplings and for cyclizations.

The original phosphonium reagent was BOP, often called Castro's reagent, which carries the HOBt anion and activates the acid efficiently.^[2][19] Its drawback is its byproduct: BOP releases hexamethylphosphoramide (HMPA), a compound of significant toxicological concern, as a stoichiometric waste product.^[19] That single liability drove the development of a direct replacement. In 1990 Coste, Le-Nguyen and Castro reported PyBOP, in which the three dimethylamino groups of BOP are replaced by pyrrolidino rings, explicitly as a “new peptide coupling reagent devoid of toxic by-product” — same chemistry, no HMPA.^[15] The HOAt-based analogue PyAOP followed, applying the neighbouring-group advantage of HOAt to the phosphonium family for use in solid-phase synthesis.^[17][19]

Because phosphonium reagents are not themselves attacked by the amine, they avoid a side reaction that troubles the aminium/uronium salts: when base or stoichiometry is poorly controlled, an aminium/uronium reagent can react with the peptide's own N-terminal amine to form an unreactive guanidino cap, quietly terminating that chain.^[2] Phosphonium reagents sidestep that, which is one reason they are favoured for segment condensations and head-to-tail cyclizations where the free amine is precious.

Newer and greener reagents: COMU and T3P

Two more recent reagents are worth knowing because they address the practical shortcomings — safety and workup — of the classic onium salts rather than their coupling power. Both reflect a broader push toward safer, cleaner peptide chemistry.

COMU is an aminium/uronium reagent built on the Oxyma anion rather than a benzotriazole, with a morpholino group on the central carbon. Because it is not benzotriazole-based, it is marketed as a safer and more effective replacement for benzotriazole-based uronium coupling reagents, and it carries two useful features: it is highly soluble, and it changes colour as it is consumed, giving a visible readout of whether a coupling is still running.^[1] It pairs the Oxyma racemization-suppression story with the one-pot convenience of HBTU and HATU.

T3P (propylphosphonic anhydride, also called propanephosphonic acid anhydride) works on a different principle: it is a cyclic anhydride that activates the acid dehydratively, and its byproducts are water-soluble phosphonates that rinse away easily. In comparative work it has shown high yield and low epimerization, and it is valued for its mild, low-toxicity profile, which makes it attractive on scale.^[2] Also in the toolkit, though less common in routine chain assembly, are the pre-formed acyl halides — acid fluorides in particular — which are among the most reactive activated species and are reserved for especially hindered couplings.^[1]

The role of base and solvent

A coupling reagent rarely works alone: it needs a base to free the carboxylic acid and a solvent to carry the reaction, and both choices affect how clean the coupling is. These are the dials a chemist turns once the reagent is chosen.

Most onium couplings use a non-nucleophilic tertiary-amine base, usually N,N-diisopropylethylamine (Hünig's base, DIPEA), and sometimes triethylamine or 2,4,6-collidine.^[1][2] The base deprotonates the acid so that the carboxylate can attack the reagent; without it the activation stalls. But base is a balancing act, because the same deprotonation that enables coupling also feeds the oxazolone pathway that causes racemization, so excess base and long pre-activation times tend to increase epimerization. This is one reason a hindered or milder base such as collidine is sometimes chosen for sensitive residues, and why many protocols add the amine promptly rather than letting the activated acid sit.

Solvent matters too. Polar aprotic solvents such as dimethylformamide (DMF) and N-methylpyrrolidone are standard because they dissolve both the reagents and the resin-bound peptide, though the drive toward greener synthesis has pushed interest in alternatives such as 2-methyltetrahydrofuran, ethyl acetate and dimethyl carbonate, often paired with Oxyma-based reagents.^[18] For the carbodiimides specifically, a low-polarity solvent can help suppress the N-acylurea rearrangement, as noted earlier.^[5]

How a reagent is matched to the job

There is no single best coupling reagent; there is a reagent that fits a given coupling, and the choice follows a few consistent considerations. The table below summarises how the common reagents tend to be positioned, drawing on the standard reviews of the field.^[1][2]

In broad strokes, routine couplings on a straightforward sequence are handled well by DIC with an additive, or by HBTU; difficult couplings — sterically hindered residues, aggregation-prone sequences, segment joins — call for the more powerful HATU or the HOAt-based phosphonium PyAOP; cyclizations and couplings where the free amine must be protected favour phosphonium reagents; and where safety and workup dominate the decision, Oxyma-based reagents such as COMU, or T3P on scale, come to the fore. The additive is chosen alongside the reagent: HOAt and Oxyma for the hardest couplings, HOBt for everyday ones.

Safety: sensitizers and the explosive additives

Coupling reagents are not benign laboratory chemicals, and two hazard classes in particular shape how they are handled: skin sensitization and explosivity. These are properties of the reagents as chemicals and are the reason they are shipped, stored and weighed with care.

Several coupling reagents are potent skin sensitizers. DCC is classed as a strong sensitizer, and the aminium/uronium reagents are moderate sensitizers; repeated workplace exposure to HATU, HBTU and HCTU has been documented to induce allergic reactions, including a case series describing anaphylaxis after repeated exposure.^[21][22] This is why these reagents are handled with gloves in well-ventilated spaces and treated as agents that can provoke sensitization over time.

The explosivity concern centres on the benzotriazoles. Anhydrous HOBt is a recognised explosive — it has been reclassified for transport as a Class 1 explosive (UN0508), which restricts how it can be shipped — and it is normally supplied as a water-wetted or hydrated form precisely to tame that hazard.^[24] A 2018 thermal-stability study using differential scanning calorimetry found that several benzotriazole-based coupling reagents, including HBTU, HATU and PyBOP, decompose with enough energy to be rated potentially explosive, information that matters for anyone handling them at scale.^[20] This hazard is a large part of why Oxyma and its derivatives were developed as non-benzotriazole alternatives.^[18] None of this bears on any finished research compound; it concerns the manufacturing reagents, which do not remain in a purified peptide.

On that last point: a reputable supplier verifies that the reagents and their byproducts have been cleared from the final material. Patriot Labs is a first-responder-owned, US-based company, and every batch it sells is tested by an independent US laboratory by HPLC for purity and mass spectrometry for identity, with the certificate of analysis published on the product page — the analytical checks described in Understanding Peptide COA Testing.

What this guide does not establish

This guide is an educational account of the chemistry of peptide coupling reagents. Several limits are worth stating plainly.

• It describes synthesis chemistry, not any product. Which reagent was used to make a given peptide is a process detail; it cannot be inferred from a compound's name, and the finished peptide does not contain the coupling reagent.
• It is not a laboratory protocol. The mechanisms, equivalents and conditions are described to explain how the chemistry works, not as instructions for carrying out a coupling; reagent handling demands primary literature, safety data sheets and trained supervision.
• Hazard statements are about the reagents. The sensitization and explosivity points concern the coupling reagents as chemicals, not any research compound offered for sale and not any form of use in a person or animal.
• Nothing here describes where any peptide is synthesised. The chemistry is general to the field; it is not a claim about the origin or manufacturing of any specific material.
• Nothing here concerns use in humans. All of the chemistry described happens in a reaction vessel, not a body, and this guide does not describe how to use any product.

Frequently asked questions

What is a peptide coupling reagent?

A peptide coupling reagent is a chemical that activates the carboxylic acid of one amino acid so it will join to the amine of the next, forming the amide (peptide) bond that links them. On its own, a carboxylic acid and an amine do not combine into an amide at room temperature; the coupling reagent converts the acid into a reactive species, usually an active ester, that the amine attacks readily. The main families are carbodiimides such as DIC and DCC, aminium/uronium salts such as HBTU and HATU, and phosphonium salts such as PyBOP, often used with an additive such as HOBt, HOAt or Oxyma.

What does a coupling reagent do in peptide synthesis?

It makes the carboxyl group electrophilic enough to react. The reagent turns the slow, high-energy direct condensation of an acid and an amine into a fast two-step sequence: first it activates the acid into an O-acylisourea or an active ester, releasing a byproduct such as a urea or a phosphine oxide; then the free amine of the next residue attacks that activated carbonyl and displaces the leaving group to form the new peptide bond. In solid-phase synthesis this cycle is repeated once for every residue added.

What is the difference between HATU and HBTU?

HATU and HBTU are closely related aminium/uronium coupling reagents that differ by one atom. HBTU is built on 1-hydroxybenzotriazole (HOBt) and generates an OBt active ester; HATU is built on 1-hydroxy-7-azabenzotriazole (HOAt), which carries an extra nitrogen in the ring and generates an OAt ester. That ring nitrogen sets up a neighbouring-group effect that is thought to speed the reaction and reduce racemization, which is why HATU is generally the more powerful of the two for difficult couplings; HBTU is milder and widely used for routine ones.

Why is HOBt added to a carbodiimide coupling?

HOBt (1-hydroxybenzotriazole) is added to intercept the reactive O-acylisourea that a carbodiimide forms, converting it into a more controlled HOBt active ester. This does two things: it prevents the O-acylisourea from rearranging into the unreactive N-acylurea that wastes the amino acid, and it suppresses racemization by shutting down the oxazolone pathway that scrambles the stereocentre. König and Geiger introduced HOBt as a carbodiimide additive in 1970, and the strategy remains standard.

Are HATU and HBTU uronium or guanidinium salts?

Both were originally drawn and named as uronium salts (the O-form), but X-ray crystallography later showed that the commercial reagents actually crystallize as the guanidinium isomer (the N-form), in which the central carbon is bonded to the benzotriazole nitrogen rather than its oxygen. Carpino and co-workers documented this in 2002 and showed that the true O-uronium forms can be made separately and are more reactive. In practice the names HATU and HBTU are kept for the commercial guanidinium reagents, and they work well regardless of the naming history.

Why are peptide coupling reagents explosive?

The explosive concern comes mainly from the benzotriazole additives and the salts made from them. Anhydrous HOBt is a recognised explosive and has been reclassified for transport as a Class 1 hazardous material, and differential scanning calorimetry has shown that benzotriazole-based reagents such as HBTU, HATU and PyBOP decompose with enough energy to be rated potentially explosive. HOBt is normally supplied as a water-wetted or hydrated form to reduce the hazard, and non-benzotriazole additives such as Oxyma were developed partly to lower this risk.

Is Oxyma safer than HOBt?

Oxyma (ethyl cyano(hydroxyimino)acetate) was introduced in 2009 as an additive with a lower risk of explosion than the benzotriazoles HOBt and HOAt, while still suppressing racemization effectively. It is not a benzotriazole, so it avoids that class's explosive classification, and it is often cited as part of greener solid-phase synthesis. It is not hazard-free, and its uronium-type derivative COMU is marketed as a more soluble, safer replacement for benzotriazole-based coupling salts, but on the specific question of benzotriazole explosivity Oxyma is the lower-risk choice.

What base is used with HATU?

HATU and the other aminium/uronium reagents are used with a non-nucleophilic tertiary amine base, most commonly N,N-diisopropylethylamine (Hünig's base, DIPEA), and sometimes triethylamine or 2,4,6-collidine. The base deprotonates the carboxylic acid so it can attack the reagent, and keeping the base choice and stoichiometry controlled matters because excess base or long activation times can increase racemization and, with aminium/uronium reagents, can cap the amine as an unreactive guanidino by-product.

References

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2. Montalbetti, C. A. G. N., & Falque, V. (2005). Amide bond formation and peptide coupling. Tetrahedron, 61(46), 10827–10852. doi:10.1016/j.tet.2005.08.031
3. Valeur, E., & Bradley, M. (2009). Amide bond formation: beyond the myth of coupling reagents. Chemical Society Reviews, 38(2), 606–631. doi:10.1039/B701677H
4. Sheehan, J. C., & Hess, G. P. (1955). A New Method of Forming Peptide Bonds. Journal of the American Chemical Society, 77(4), 1067–1068. doi:10.1021/ja01609a099
5. Williams, A., & Ibrahim, I. T. (1981). Carbodiimide Chemistry: Recent Advances. Chemical Reviews, 81(6), 589–636. doi:10.1021/cr00046a004
6. Kurzer, F., & Douraghi-Zadeh, K. (1967). Advances in the Chemistry of Carbodiimides. Chemical Reviews, 67(2), 107–152. doi:10.1021/cr60246a001
7. Sheehan, J. C., & Cruickshank, P. A. (1968). 1-Ethyl-3-(3-Dimethylamino)propylcarbodiimide Hydrochloride and Methiodide. Organic Syntheses, 48, 83. doi:10.15227/orgsyn.048.0083
8. König, W., & Geiger, R. (1970). Eine neue Methode zur Synthese von Peptiden: Aktivierung der Carboxylgruppe mit Dicyclohexylcarbodiimid unter Zusatz von 1-Hydroxy-benzotriazolen. Chemische Berichte, 103(3), 788–798. doi:10.1002/cber.19701030319
9. Carpino, L. A. (1993). 1-Hydroxy-7-azabenzotriazole. An Efficient Peptide Coupling Additive. Journal of the American Chemical Society, 115(10), 4397–4398. doi:10.1021/ja00063a082
10. Carpino, L. A., Imazumi, H., Foxman, B. M., Vela, M. J., Henklein, P., El-Faham, A., Klose, J., & Bienert, M. (2000). Comparison of the Effects of 5- and 6-HOAt on Model Peptide Coupling Reactions Relative to the Cases for the 4- and 7-Isomers. Organic Letters, 2(15), 2253–2256. doi:10.1021/ol006013z
11. Carpino, L. A., Imazumi, H., El-Faham, A., Ferrer, F. J., Zhang, C., Lee, Y., Foxman, B. M., Henklein, P., Hanay, C., Mügge, C., Wenschuh, H., Klose, J., Beyermann, M., & Bienert, M. (2002). The Uronium/Guanidinium Peptide Coupling Reagents: Finally the True Uronium Salts. Angewandte Chemie International Edition, 41(3), 441–445. doi:10.1002/1521-3773(20020201)41:3<441::AID-ANIE441>3.0.CO;2-N
12. Abdelmoty, I., Albericio, F., Carpino, L. A., Foxman, B. M., & Kates, S. A. (1994). Structural studies of reagents for peptide bond formation: Crystal and molecular structures of HBTU and HATU. Letters in Peptide Science, 1(2), 57–67. doi:10.1007/BF00126274
13. Dourtoglou, V., Ziegler, J.-C., & Gross, B. (1978). L'hexafluorophosphate de O-benzotriazolyl-N,N-tetramethyluronium: un réactif de couplage peptidique nouveau et efficace. Tetrahedron Letters, 19(15), 1269–1272. doi:10.1016/0040-4039(78)80103-8
14. Knorr, R., Trzeciak, A., Bannwarth, W., & Gillessen, D. (1989). New coupling reagents in peptide chemistry. Tetrahedron Letters, 30(15), 1927–1930. doi:10.1016/S0040-4039(00)99616-3
15. Coste, J., Le-Nguyen, D., & Castro, B. (1990). PyBOP: A new peptide coupling reagent devoid of toxic by-product. Tetrahedron Letters, 31(2), 205–208. doi:10.1016/S0040-4039(00)94371-5
16. Albericio, F., Bofill, J. M., El-Faham, A., & Kates, S. A. (1998). Use of Onium Salt-Based Coupling Reagents in Peptide Synthesis. Journal of Organic Chemistry, 63(26), 9678–9683. doi:10.1021/jo980807y
17. Albericio, F., Cases, M., Alsina, J., Triolo, S. A., Carpino, L. A., & Kates, S. A. (1997). On the use of PyAOP, a phosphonium salt derived from HOAt, in solid-phase peptide synthesis. Tetrahedron Letters, 38(27), 4853–4856. doi:10.1016/S0040-4039(97)01011-3
18. Subirós-Funosas, R., Prohens, R., Barbas, R., El-Faham, A., & Albericio, F. (2009). Oxyma: An Efficient Additive for Peptide Synthesis to Replace the Benzotriazole-Based HOBt and HOAt with a Lower Risk of Explosion. Chemistry – A European Journal, 15(37), 9394–9403. doi:10.1002/chem.200900614
19. Mansour, T. S., Bardhan, S., & Wan, Z.-K. (2010). Phosphonium- and Benzotriazolyloxy-Mediated Bond-Forming Reactions and Their Synthetic Applications. Synlett, 2010(8), 1143–1169. doi:10.1055/s-0029-1219820
20. Sperry, J. B., Minteer, C. J., Tao, J., Johnson, R., Duzguner, R., Hawksworth, M., Oke, S., Richardson, P. F., Barnhart, R., Bill, D. R., Giusto, R. A., & Weaver, J. D. (2018). Thermal Stability Assessment of Peptide Coupling Reagents Commonly Used in Pharmaceutical Manufacturing. Organic Process Research & Development, 22(9), 1262–1275. doi:10.1021/acs.oprd.8b00193
21. McKnelly, K. J., Sokol, W., & Nowick, J. S. (2020). Anaphylaxis Induced by Peptide Coupling Agents: Lessons Learned from Repeated Exposure to HATU, HBTU, and HCTU. Journal of Organic Chemistry, 85(3), 1764–1768. doi:10.1021/acs.joc.9b03280
22. Graham, J. C., Trejo-Martin, A., Chilton, M. L., Kostal, J., Bercu, J., Beutner, G. L., Bruen, U. S., Dolan, D. G., Gomez, S., Hillegass, J., Nicolette, J., & Schmitz, M. (2022). An Evaluation of the Occupational Health Hazards of Peptide Couplers. Chemical Research in Toxicology, 35(6), 1011–1022. doi:10.1021/acs.chemrestox.2c00031
23. 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
24. Hydroxybenzotriazole (reclassification of anhydrous HOBt as UN0508, Class 1.3C explosive; transport restrictions under 49 CFR). Wikipedia. https://en.wikipedia.org/wiki/Hydroxybenzotriazole

For where this step sits in the whole build, see What Is Solid-Phase Peptide Synthesis and How Peptides Are Made; for the protecting groups that work alongside these reagents, see What Is a Protecting Group and Fmoc vs Boc Peptide Synthesis; and for the bond being formed and the impurities a coupling can leave behind, see What Is a Peptide Bond and Peptide Impurities Explained.

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 the chemistry of peptide coupling reagents in general terms; it is not medical advice, is not a laboratory protocol, does not describe how to use any product, and makes no claim about the manufacturing location or process of any specific product.