A peptide bond is a covalent bond that joins two amino acids. It forms when the carboxyl group of one amino acid reacts with the amine group of another, releasing a single water molecule in what chemists call a condensation reaction, producing a CO–NH linkage.

Correct alignment of the two amino acids is essential for the bond to form. The simplest result is a dipeptide, a peptide made of just two amino acids.

By convention, chains of 50 amino acids or fewer are called peptides, chains of 50 to 100 are polypeptides, and chains longer than 100 are proteins. Smaller chains of roughly 10 to 20 amino acids are often called oligopeptides.

Peptide bonds can also be broken. Hydrolysis, a reaction with water, slowly cleaves the bond, which is why peptides, polypeptides, and proteins can degrade under certain conditions.

Peptide bonds have several distinctive structural characteristics that shape how peptides behave:

• They are rigid and planar, as demonstrated by X-ray diffraction studies.
• The amide nitrogen can delocalize electrons into the carbonyl oxygen, creating partial double-bond character that limits rotation around the bond.
• They favor a trans configuration between the carbonyl oxygen and the amide hydrogen, which reduces steric clashes.
• While the single bonds adjacent to the peptide bond allow some rotation, resonance stabilizes the structure and restricts full rotation.

These properties are far from academic trivia. They directly determine how peptides fold and how they interact within larger proteins, which in turn governs their biological function.