C-terminal amidation converts the free carboxylic acid at the end of a peptide chain into a primary amide, written as –CONH2 or as a trailing "-NH2" in a sequence line. It removes one negative charge and lowers the molecule's mass by about 0.98 Da relative to the free-acid form. It mirrors a genuine post-translational modification, produced in cells by peptidylglycine alpha-amidating monooxygenase acting on a C-terminal glycine.
Where amidated peptides appear
Many native signalling peptides are amidated and lose most receptor activity without it — oxytocin, gastrin, calcitonin, PACAP and the growth hormone-releasing peptides among them. Research analogs follow: Ipamorelin, GHRP-2 and GHRP-6 are supplied as C-terminal amides, and products such as N-Acetyl Semax Amidate carry amidation together with N-terminal acetylation. Catalog names usually flag it explicitly with "amide" or "amidate".
Why it matters
Two practical consequences. Amidation blocks carboxypeptidase trimming, so amidated analogs generally show longer in vitro half-lives than their free acids. And it changes the target number on identity testing: a mass spectrometry result must be compared with the theoretical molecular weight of the amidated form, or the lot will read 1 Da low. If a supplier's sequence line and its MS figure disagree on amidation, treat identity as unconfirmed on the certificate of analysis.