Peptides and proteins are made of the same building blocks: amino acids linked by peptide bonds. The differences are in size, structure and how they are produced.
Size
By convention, chains under about 50 amino acids are peptides and longer chains are proteins. The line is blurred. Insulin, at 51 residues in two chains, is often called both.
Structure
Proteins usually fold into stable three-dimensional shapes, with secondary structure (helices and sheets) packed into a tertiary fold. Many also assemble into multi-chain complexes. Short peptides are often flexible in solution. Some are held in a fixed shape by a ring (cyclisation) or a disulfide bridge, as in oxytocin.
Production
Peptides are usually made by chemical synthesis, which allows non-standard and D-amino acids, modified ends and attached side chains. Proteins are usually made by recombinant expression in cells, because chemical synthesis becomes impractical at that length.
Analysis
Peptide identity is routinely confirmed by mass spectrometry, and purity by reversed-phase HPLC. Protein analysis often adds methods that check folding, aggregation and activity.
Quick quiz
For laboratory research use only. This article describes chemistry and handling. It does not describe human use.