N-terminal

The N-terminal end (or N-terminus) of a peptide is the polypeptide chain end bearing a free amine group (-NH₂), as opposed to the C-terminus bearing the carboxyl group. By convention, the sequence is always written from N-terminus (left) to C-terminus (right), reproducing ribosomal biosynthesis order where the initiator methionine always occupies the N-terminal position.

The N-terminus is the zone most exposed to enzymatic degradation in physiological conditions. Many aminopeptidases, leucine aminopeptidase (LAP), pyroglutamate aminopeptidase and especially dipeptidyl peptidase 4 (DPP-4) cleave the first amino acids from the N-terminal end. DPP-4 in particular, by cutting after a Pro or Ala at position 2, inactivates within minutes many major regulatory peptides: native GLP-1, GIP, substance P, NPY, VIP, GH-releasing hormone.

This vulnerability explains why N-terminal protection is a central strategy in designing stable therapeutic analogs. Common approaches include: amino acid substitution at position 2 (Ala → Aib in semaglutide), N-terminal acylation (acetylation, spontaneous pyroglutamatation), addition of non-standard amino acids (D-isomers, β-amino acids), grafting of bulky groups (PEG, lipids).

In mass spectrometry, the N-terminus is an essential analytical landmark. Edman sequencing methods (sequential chemical degradation from the N-terminal end) historically enabled determination of thousands of peptide sequences. Today, MS/MS fragmentation exploits b ions (N-terminal side cleavages) and y ions (C-terminal side cleavages) to reconstruct the sequence. Detailed knowledge of N-terminal architecture thus remains central in modern peptide research.

The N-terminal end is the last residue added during SPPS and the one generally functionalised to improve pharmacological properties. Most common N-terminal modifications in peptide research include: acetylation (-Ac) protecting against aminopeptidases (thymosin β-4, Ac-SDKP); pyroglutamate (pGlu) spontaneously formed from N-terminal Gln or Glu; myristoylation or palmitoylation (Matrixyl, dermatological palmitoyl-peptides); long fatty-acid acylation like the C18 chain of semaglutide or C20 of tirzepatide/cagrilintide, conferring albumin binding and extended half-life; PEGylation or FITC conjugation for imaging tools.

N-terminal analysis by Edman degradation (PITC → PTH-amino acid) remains the reference for sequencing a short peptide from the free amine, with picomolar sensitivity and ~1 residue per 45-min cycle. If the N-terminal end is blocked (acetylated, formylated, pGlu), Edman is inoperative and de novo sequencing by LC-MS/MS with CID or HCD fragmentation of precursor ions is required. N-terminal signature is also captured by MALDI-TOF mass spectrometry: a characteristic b1 or a1 ion peak reveals the first residue identity.