Peptide Bond

A peptide bond is the covalent amide linkage that joins amino acid residues in peptides and proteins. It connects the carbonyl carbon derived from the carboxyl group of one amino acid to the nitrogen derived from the amino group of the next. Repeated peptide bonds form the continuous backbone of a polypeptide chain, with amino acid side chains projecting from this backbone .¹

The peptide bond has partial double-bond character because electrons are delocalized across the carbonyl group and the carbon-nitrogen bond. As a result, rotation around the peptide carbon-nitrogen bond is restricted. The atoms surrounding the bond therefore form a nearly planar unit called the peptide plane. Most peptide bonds in proteins adopt the trans configuration, although cis peptide bonds also occur, particularly when proline is involved .²

Although the peptide bond itself is relatively rigid, a polypeptide chain can still change shape through rotation around neighboring bonds on either side of each amino acid's alpha carbon. These rotations help determine how the protein backbone folds into structures such as alpha helices and beta sheets. Carbonyl and amide groups in the peptide backbone can also participate in hydrogen bonding, an important contributor to protein secondary structure .^1,2

Peptide bonds should not be confused with interactions between amino acid side chains. Peptide bonds covalently connect amino acid residues to form the polypeptide backbone, whereas noncovalent interactions and other covalent links such as disulfide bonds can help stabilize higher levels of protein structure .¹