Definition
A peptide bond is the amide bond formed when the carboxyl group of one amino acid condenses with the amino group of another, releasing water. Repeated peptide bonds form the backbone of every peptide and protein. Resonance gives the bond partial double-bond character, so it is planar and rigid and almost always adopts the trans configuration; bonds preceding proline are the notable exception, where the cis form occurs more often. The peptide bond absorbs ultraviolet light strongly around 214 nm, which is why HPLC purity is measured near that wavelength. Peptide bonds are thermodynamically unstable in water but hydrolyse very slowly without enzymes, acids or bases, so proteases largely determine a peptide's lifetime in biological systems.