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What is a peptide?
What peptides are, how they differ from proteins, how their sequences are written, and how research peptides are made.
Short chains of amino acids
A peptide is a short chain of amino acids joined by peptide bonds, the link that forms between one amino acid's carboxyl group and the next one's amino group. There are 20 standard amino acids, and the order they appear in, the sequence, decides what the peptide is.
Short peptides are named by length: a dipeptide has two amino acids, a tripeptide three, and so on. GHK, for example, is a tripeptide; Semax and Selank are heptapeptides (seven); BPC-157 has fifteen.
Peptides and proteins
There's no hard cut-off, but chains of up to about 50 amino acids are usually called peptides, and longer ones proteins. Proteins also tend to fold into stable three-dimensional shapes, while most short peptides are more flexible.
Thymosin beta-4, at 43 amino acids, sits near the boundary and is often called a small protein or a polypeptide.
How sequences are written
Sequences are written from the N-terminus (the free amino group) to the C-terminus (the free carboxyl group), using either three-letter codes (Gly-His-Lys) or one-letter codes (GHK).
Extra notes show changes to the basic chain: "-NHâ‚‚" means the C-terminus is amidated, "D-" marks an amino acid in its mirror-image form, and names such as Aib refer to non-standard amino acids used to make a peptide more stable.
How research peptides are made
Most research peptides are made by solid-phase peptide synthesis, a method developed by Bruce Merrifield in the 1960s and recognised with the 1984 Nobel Prize in Chemistry. The chain is built one amino acid at a time on a solid resin, then cut free, purified and freeze-dried into a powder.
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