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Capped Ends: C-Terminal Amidation and N-Terminal Acetylation in Peptides
A textbook peptide has a free amino group at one end and a free carboxylic acid at the other. Many real ones do not. Nature and chemists both cap the ends, and a look at the sequence notation (an NH2 at the end or an Ac at the start) tells you it has happened.
Key takeaways
- C-terminal amidation replaces the acid with an amide and removes a negative charge.
- N-terminal acetylation caps the amine and removes a positive charge.
- Both reduce attack by exopeptidases and can improve receptor binding.
- The mass changes are small enough that high-resolution MS is needed to tell them apart from other changes.
C-terminal amidation
In an amidated peptide the final carboxyl group (COOH) is replaced with a carboxamide (CONH2). The peptide loses one negative charge at neutral pH. Nature does this with an enzyme called peptidylglycine alpha-amidating monooxygenase, which converts a glycine at the end of a precursor into the amide on the previous residue. Oxytocin is the textbook example: it ends in glycinamide. Many neuropeptides and hormones, including alpha-MSH, are amidated. In synthesis, chemists get an amide by choosing an amide-forming resin.
The mass difference between the acid and the amide is small: the amide is 0.984 Da lighter. That is the same size as the shift of deamidation but opposite in direction, a reminder that mass alone must be read with the sequence in hand.
N-terminal acetylation
An acetyl group (CH3CO) attached to the N-terminal amine adds 42.011 Da and removes the positive charge. Alpha-MSH is both N-acetylated and C-amidated in its natural form, which is part of how the body protects it. Acetylation blocks aminopeptidases from chewing in from the front, and it can mimic the way the peptide sits in a larger protein, where the end is not free.
Why designers use caps
- Protease resistance. Free termini are the entry points for exopeptidases.
- Charge balance. Capping changes the net charge, which alters solubility, as in isoelectric point and solubility.
- Receptor fit. Many receptors bind amidated forms better, because the natural ligand is amidated.
- Mimicking a protein segment. A peptide cut from the middle of a protein has no natural free ends, so caps reproduce that.
How to tell if a peptide is capped
- Look at the notation: Ac- at the start and -NH2 at the end are standard.
- Compare the measured mass with the calculation for the free acid and the amide.
- Use MS/MS to confirm the terminal fragments (see MS/MS sequencing).
- Expect a retention shift on HPLC, because the capped form is less charged and typically more hydrophobic.
Why it matters when buying
Two products with the same amino acid order but one amidated and one free acid are different compounds with different charge, stability and potency. The name on the label should state which, or the report should show the molecular weight that makes it clear. Our post on how peptides are engineered for longer life explains more of these edits.
The bottom line
The ends of a peptide are not an afterthought. A cap can be the difference between a molecule that lasts minutes and one that lasts hours, and between a molecule that fits a receptor and one that does not.
Related reading
- How Researchers Make Peptides Last Longer: D-Amino Acids, Acylation and More
- N-Terminal Chemistry: Diketopiperazines and Pyroglutamate Formation
- Oxytocin Research: What the Current Studies Show
- Browse the full Research Library
For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.