Forming an amide bond between a carboxylic acid and an amine is one of the most common reactions in chemistry, from peptide synthesis to drug discovery and bioconjugation. Three reagents come up again and again: N-hydroxysuccinimide (NHS) and its esters, 1-hydroxybenzotriazole (HOBt), and the base N-methylmorpholine (NMM). This guide explains what each one does and how to buy and handle them.
Why carboxylic acids need activating
A carboxylic acid and an amine mostly just form a salt at room temperature. To make an amide, the acid must be turned into a more reactive “activated” form. Coupling reagents such as carbodiimides (DCC, DIC, EDC) do that, and additives like NHS and HOBt make the activated intermediate more useful.
N-Hydroxysuccinimide and NHS esters
N-Hydroxysuccinimide (NHS, German: N-Hydroxysuccinimid) reacts with activated acids to form NHS esters. These are stable enough to isolate, store and sell, yet react cleanly with primary amines. That makes them the workhorse of bioconjugation: fluorescent dyes, biotin labels and crosslinkers are commonly supplied as NHS esters ready to attach to the lysine side chains of proteins and antibodies. A water-soluble version, sulfo-NHS, is used when reactions run in buffer.
Practical points: NHS esters slowly hydrolyse in water and moist air, so store them dry and cold, let the container warm to room temperature before opening, and avoid amine-containing buffers such as Tris during labelling.
1-Hydroxybenzotriazole (HOBt)
HOBt is a classic additive in carbodiimide couplings, especially in peptide synthesis. It forms an active ester that couples efficiently while reducing racemisation, the loss of stereochemical purity at the amino acid, and suppresses unwanted side products.
Safety note: anhydrous HOBt is classed as an explosive hazard. It is therefore usually sold as the hydrate or in a desensitised form, and shipping restrictions apply. Many labs have moved to alternatives such as OxymaPure or uronium/phosphonium reagents (HATU, HBTU, PyBOP), partly for this reason. Check the supplier’s SDS and transport classification before ordering.
N-Methylmorpholine (NMM)
N-Methylmorpholine is a tertiary amine base used to neutralise acids generated during coupling and to keep the amine partner reactive. It is milder than triethylamine or DIPEA, which can help limit racemisation in sensitive peptide couplings. NMM is a flammable, corrosive liquid: store it in a flammables cabinet and handle it in a fume cupboard.
Quick comparison
| Reagent | Role | Typical use | Handling |
|---|---|---|---|
| NHS / NHS esters | Additive; active ester | Bioconjugation, labelling | Keep dry and cold |
| HOBt (hydrate) | Additive; limits racemisation | Peptide coupling | Explosive when anhydrous; check transport rules |
| NMM | Base | Peptide and amide coupling | Flammable, corrosive |
Buying and storing coupling reagents
- Search by CAS number to get the right form (for example HOBt hydrate versus anhydrous).
- Choose the grade your method needs; peptide synthesis grade reagents are tested for relevant impurities. See reagent grades.
- Buy small packs of moisture-sensitive reagents and date them on opening.
- Carbodiimides such as DCC are potent skin sensitisers: wear suitable gloves and avoid dust.
Frequently asked questions
What is an NHS ester used for?
Mainly for attaching labels, dyes or linkers to amine groups on proteins and other molecules.
Why is HOBt sold as a hydrate?
Because the dry, anhydrous form has explosive properties; the hydrate is much safer to store and transport.
Is NMM the same as NMP?
No. NMM is N-methylmorpholine, a base. NMP is N-methyl-2-pyrrolidone, a solvent.
