What is Peptide bond?

A peptide bond forms when the carboxyl group of one amino acid condenses with the amine group of the next, releasing water. The resulting –CO–NH– link is an amide, and a linear chain of n residues contains n − 1 of them: three in a tetrapeptide, fourteen in the 15-residue BPC-157.

The mass bookkeeping

Each bond formed removes one H2O, 18.02 Da on average masses or 18.0106 Da monoisotopic. Two glycines (75.07 g/mol each) therefore make glycylglycine at 75.07 + 75.07 − 18.02 = 132.12 g/mol. The same subtraction, applied n − 1 times, is how sequence-based molecular weight calculators work, including the peptide molecular weight calculator. It also runs in reverse: hydrolysis of one bond adds 18.01 Da to the combined fragments, a shift mass spectrometry picks up when a peptide starts to break down.

Structure: flat and stiff

The nitrogen's lone pair is shared with the neighbouring carbonyl, giving the C–N bond partial double-bond character. Three consequences follow:

• Planarity. The atoms around each peptide bond lie in one plane and rotation about C–N is restricted, so backbone flexibility comes from the bonds on either side of it.
• Trans preference. Almost all peptide bonds adopt the trans arrangement. The exception is the bond preceding proline, where cis and trans are closer in energy and both forms occur; slow interconversion can broaden the HPLC peaks of proline-rich peptides.
• UV absorbance. The bond absorbs strongly below about 230 nm, which is why HPLC detects peptides at 214–220 nm regardless of their side chains.

Stability: favourable to break, slow to break

Hydrolysis of a peptide bond is thermodynamically favoured, but uncatalysed it is very slow at neutral pH and room temperature, which is why peptides last in solution at all. Breaking every bond on purpose, for amino acid analysis, typically takes 6 M HCl at about 110 °C for around 24 hours. Two factors speed it up:

• Sequence. Asp–Pro bonds are unusually acid-labile, and aspartic acid residues generally are hot spots for backbone reactions.
• Enzymes. Proteases cut specific bonds quickly. In human plasma in vitro, GLP-1(7-36)amide lost its N-terminal dipeptide to become GLP-1(9-36)amide with a half-life of about 20 minutes at 37 °C, and inhibitors of dipeptidyl peptidase-IV prevented the cleavage completely ^[1]. One bond, in other words, set the peptide's half-life in that system.

Not every amide is a peptide bond

A C-terminal amide (–CONH2), as in ipamorelin, is an amide that links to no further residue. Side-chain amides join groups outside the backbone: semaglutide, for example, is derivatised at lysine 26, where its fatty-acid side chain is attached ^[2]. Calculators and sequence notations treat these as modifications, not extra peptide bonds; the sequence-reading guide shows how they are written, and solid-phase peptide synthesis explains how peptide bonds are formed one coupling at a time.