C-Terminal Amidation And What The -NH2 On A Label Means

Every linear peptide has two ends. The N-terminus carries a free amine unless something has been put on it, and the C-terminus carries a carboxylic acid unless it has been amidated. Written sequences record the second case with a trailing -NH2, so a sequence ending in Arg-NH2 has an amidated arginine at the C-terminus while the same sequence ending in Arg has a free acid there.

Chemically the difference sits on a single carbon. In the free acid the terminal carbonyl carries a hydroxyl group. In the amide it carries an amino group instead. Nothing else about the sequence changes, which is why the two forms are so easy to confuse on paper and so hard to separate in practice.

The consequence that matters most is electrostatic. A free C-terminal carboxyl has a pKa in the region of 3.1 to 3.7 and is therefore deprotonated and negatively charged at neutral pH. A carboxamide has no ionisable proton in that range at all. Amidation removes one negative charge from the molecule, raising net charge by one unit and shifting the isoelectric point upward.

Where amidation comes from in synthesis

In solid-phase synthesis the C-terminal chemistry is determined by the linker on the resin, because the peptide is built from the C-terminus toward the N-terminus and the last thing to happen is cleavage from that linker. A resin whose linker releases a carboxamide gives an amidated peptide. A resin whose linker releases a carboxylic acid gives the free acid. The decision is made before the first amino acid is loaded.

Nature reaches the same endpoint by a different route. Peptidylglycine alpha-amidating monooxygenase acts on a precursor that carries an extra glycine past the intended terminus, cleaving the glycine nitrogen-carbon bond and leaving the amide behind. This is why so many endogenous signalling peptides are amidated: the amide is the mature form and the free acid is the precursor that was never finished.

The 0.98 Da problem

Swapping hydroxyl for amino removes one oxygen at 15.9949 Da and adds one nitrogen at 14.0031 Da plus one hydrogen at 1.0078 Da. The net change is a loss of 0.9840 Da on the monoisotopic scale. That is the single most consequential number in this subject, because 0.98 Da is close enough to 1 Da to sit inside the isotope envelope of the molecule itself.

On a low-resolution average-mass readout a 1 Da offset is indistinguishable from a first-isotope peak, so the acid and the amide of the same sequence can be read as one another. Resolving them takes accurate mass, and the distinction between an averaged and a monoisotopic figure is the thing that decides whether the comparison is meaningful at all.

The failure mode: incomplete amidation that co-elutes

The failure worth naming is a batch that carries the free acid alongside the amide. It arises from an amide linker that was partially hydrolysed before or during cleavage, or from a cleavage cocktail held too long with too much water present. What comes out is one sequence in two terminal forms.

How it shows up: the acid form is more polar and slightly more retained or slightly less retained depending on the mobile phase, but in reverse-phase separation of a peptide of any real length the two forms differ by a fraction of a minute at best and frequently not at all. The chromatogram shows one peak, or a shoulder that integration folds into the main peak. Area percent reported from that trace counts the free acid as product, because area percent counts what elutes under the peak and not what it is.

What the -NH2 changes and what it does not

• Net charge rises by one unit at neutral pH, because one carboxylate is gone. Isoelectric point moves upward accordingly.
• Molecular weight falls by 0.984 Da monoisotopic, so a calculated mass has to be built with the correct terminus or it will be off by that amount.
• The terminal carboxylate recognised by carboxypeptidases is absent, which is a structural fact about the molecule rather than a claim about anything it does.
• Reverse-phase retention shifts slightly, usually later for the amide in acidic mobile phases, but not reliably enough to use as an identity test.
• The rest of the sequence is untouched. Amidation says nothing about side-chain modifications, disulfide pairing or stereochemistry.

Because the terminus feeds into the calculated mass, it also feeds into any figure derived from that mass. Converting a fill mass into moles uses the molecular weight of the exact form in the vial, and a molecular weight computed for the free acid of an amidated sequence carries that 0.98 Da error into everything downstream.

How a label should read

A complete written specification for a terminus names it explicitly. Ending a sequence in -NH2 or writing H-...-NH2 states the amide. Writing -OH or leaving the terminus bare conventionally states the acid, though the convention is weaker and worth spelling out rather than implying. Where a label gives only a one-letter sequence with no terminal annotation, the terminus is genuinely unspecified by that label.

The N-terminus is annotated separately and independently, and acetylation there follows its own conventions. A sequence can carry both, one, or neither.

Where our own specification stops

We state the terminus as part of the sequence specification for an item, and identity is supported by mass spectrometry consistent with that stated form. A method quantifying the proportion of free acid in an amidated lot is not among the specifications Aurum publishes, and neither is a percentage for amidation completeness. A purity figure by chromatographic area cannot carry that information, and reporting one as though it did would overstate what the method measured.

Stated plainly: the -NH2 on our labels records the form specified and synthesised, verified by mass, and not a measured amidation yield. Anyone who needs a quantified acid-form fraction needs an accurate-mass or ion-mobility method run for that purpose, which is a separate piece of analysis from the one behind our published figures.

Common questions

Is the amide or the acid the correct form of a given sequence?

Whichever the reference sequence specifies. For many endogenous peptides the mature form is amidated, so literature sequences written without a terminus can be ambiguous rather than free acid by default.

Can a free acid be amidated after synthesis?

Solution-phase amidation of a finished peptide is possible in principle but competes with side-chain carboxylates on aspartate and glutamate, so the terminus is chosen at the resin instead.

Does amidation change how a peptide behaves on HPLC?

Slightly. Removing a charge makes the molecule marginally less polar, which tends to increase retention in acidic reverse-phase conditions. The shift is small and sequence dependent.

Why does a mass spectrum sometimes show both forms?

Because both are present. A peak 0.98 Da above the expected amide mass, above what the isotope pattern accounts for, is the signature of free acid in the material.

Does amidation affect the salt form?

Indirectly. One fewer acidic group changes the counterion stoichiometry the molecule can support, which is part of why salt form is specified separately from sequence.