polypeptides

Polypeptides are linear polymers composed of amino acids linked by peptide bonds. The monomer units are α-amino acids, typically the standard 20 in biology. Dipeptides and oligopeptides are short chains; longer chains are polypeptides. They have directionality, with an N-terminal amino group and a C-terminal carboxyl group. Peptide bonds form by condensation and have partial double-bond character, giving planarity and restricting rotation; the trans configuration predominates.

The primary structure is the linear sequence of amino acids. Secondary structure includes α-helices and β-sheets stabilized by hydrogen bonds, while tertiary structure refers to the overall three-dimensional fold. Some polypeptides associate into quaternary structures with multiple chains. The sequence and chemistry of the residues determine folding, stability, and function.

In cells, polypeptides are synthesized by ribosomes during translation from mRNA. Many proteins undergo post-translational modifications, such as glycosylation or phosphorylation, and may be proteolytically processed to become mature. Covalent disulfide bonds and other cross-links can stabilize structure.

Polypeptides perform diverse roles, including enzymes, hormones, structural components, and antibodies. Examples include insulin, collagen, actin, and immunoglobulins. Function depends on precise sequence and folded structure, as well as interactions with other molecules.

Polypeptides can be produced biologically or synthetically. Solid-phase peptide synthesis enables incorporation of non-natural amino acids and modifications, supporting research and therapeutics. Synthetic polypeptides are studied for structure–function relationships and as drugs or biomaterials.