Peptide Bond
The covalent chemical bond formed between the carboxyl group of one amino acid and the amine group of another, releasing a water molecule in the process.
Peptide bonds are the backbone of all peptide and protein structures — they are what joins amino acids into chains. The bond is relatively stable but can be cleaved by proteases (enzymes that digest proteins), which is why most research peptides are degraded in the gastrointestinal tract and must be injected rather than taken orally.
The geometry of the peptide bond is planar and partially double-bond in character, which constrains the angles of the protein backbone and contributes to the secondary structure of the resulting chain (alpha helices, beta sheets, etc.). Researchers developing stable peptide analogs often modify or protect specific peptide bonds — for example, using N-methylation or D-amino acid substitutions — to resist enzymatic cleavage and extend half-life.