Overview

In biochemistry, a peptide is a molecule formed when individual amino acids are linked together by successive peptide bonds. Peptides range from dipeptides (two residues) to oligopeptides and short polypeptides; when long enough and folded into a stable functional unit they are generally called proteins. The term covers natural products, laboratory syntheses, and engineered compounds used in research and medicine.

Structure and properties

A peptide backbone consists of repeating units connected by an amide linkage formed in a condensation reaction that releases water. Each residue presents a side chain that determines chemical properties such as charge, polarity and steric bulk. Peptides have distinct ends: an N-terminus (free amino group) and a C-terminus (free carboxyl group). Depending on sequence and length they may adopt secondary motifs such as helices or beta turns, but many short peptides remain largely unstructured in solution.

Types and common examples

  • Oligopeptides: short chains of roughly 2–20 residues, often used in laboratory studies.
  • Polypeptides: longer chains that can fold into functional domains; multiple polypeptides assemble into many proteins.
  • Cyclic and modified peptides: side‑chain or terminal links and chemical modifications alter stability and activity.

Biological roles and applications

Peptides act as hormones, neurotransmitters, signalling ligands, antimicrobials and enzyme substrates. Examples include regulatory peptides that control metabolism, short antimicrobial peptides that disrupt microbial membranes, and peptide epitopes used in vaccines. In medicine and biotechnology, synthetic peptides serve as research tools, diagnostics, therapeutic candidates and building blocks for biomaterials.

Synthesis, analysis and production

Peptides are made biologically by ribosomal translation of mRNA or nonribosomally by specialized enzymes; chemically they are commonly assembled by stepwise solid‑phase peptide synthesis. Analytical methods such as high‑performance liquid chromatography (HPLC) and mass spectrometry are used to purify and verify sequence and mass. For practical guidance and protocols see external resources: general chemistry overviews (peptide chemistry), polymer concepts (polymers), laboratory guides (experimental methods), and bond descriptions (amide bond chemistry).

History, distinctions and notable facts

Recognition that proteins are composed of amino‑acid chains grew over the 19th and 20th centuries as methods for hydrolysis and sequencing advanced. In practice, the boundary between a peptide and a protein is not strictly defined and is often based on length and function rather than a fixed cutoff. Modified and synthetic peptides called peptidomimetics are an active area of drug development because they can preserve biological activity while improving stability and delivery. For additional reading on modern classifications and examples, consult specialized summaries and reviews (amino acids, peptide bonds, proteins, amide chemistry).