peptide bond
/ PEP-tide /
A peptide bond is the link that joins one amino acid to the next, the stitch that sews a string of amino acids into a protein. Despite the special name, it is nothing more than an amide bond — the very C-N linkage you studied between a carboxylic acid and an amine — formed here between the -COOH of one amino acid and the -NH2 of the next. Two amino acids linked make a dipeptide; many make a polypeptide; a folded polypeptide is a protein.
The formation is plain to picture. The carboxyl group of amino acid one and the amino group of amino acid two come together; the C-OH of the acid and an H of the amine leave as a molecule of water, and a new C-N bond ties the two together: -CO-NH-. This is a condensation (water is lost), and it is exactly amide formation. The resulting peptide bond has a quietly important feature you already know about amides: resonance. The lone pair on the nitrogen delocalizes onto the carbonyl oxygen, giving the C-N bond partial double-bond character. That means the six atoms of the peptide group (the carbonyl C and O, the N and its H, and the two flanking carbons) are locked flat and rigid, and rotation about the C-N bond is restricted — a structural fact that shapes how the whole protein folds.
Peptide bonds matter because the protein is the workhorse molecule of life, and its entire backbone is a chain of these amide links. The sequence of amino acids strung along that backbone (the primary structure) dictates how the chain coils into helices and sheets (secondary), folds into a 3-D shape (tertiary), and assembles with other chains (quaternary). The peptide bond's rigidity and its hydrogen-bonding carbonyl and N-H are exactly what make those folded shapes possible. So the most important polymer in biology is, at its joints, just amide chemistry repeated.
Glycine plus alanine condense to the dipeptide Gly-Ala: glycine's -COOH and alanine's -NH2 join as -CO-NH-, expelling one water. Resonance flattens that amide group and stops free rotation about the C-N bond, which is why protein backbones fold into regular shapes rather than flopping randomly.
Two amino acids condense into an amide; resonance makes the peptide bond flat and rigid.
A peptide bond is simply an amide; it does not need exotic 'biological' chemistry to break — acid, base, or enzymes hydrolyze it back to amino acids, the same as any amide hydrolysis. Its famous rigidity comes from resonance, not from a real, separate double bond.