Reference
What is a peptide?
A short chain of amino acids joined by peptide bonds. Everything else — how they are named, how they are made, why they arrive as powder — follows from that.
Amino acids and the peptide bond
Amino acids are small molecules that each carry an amine group at one end and a carboxylic acid group at the other, with a side chain that distinguishes one from another. Twenty of them are the standard building blocks of biological proteins.
When the acid group of one amino acid joins to the amine group of the next, a molecule of water is released and the link that remains is a peptide bond. A chain of amino acids joined this way is a peptide. Each amino acid in the chain is called a residue, because part of it was lost forming the bonds.
A chain has direction. The end with a free amine group is the N-terminus, the end with a free acid group is the C-terminus, and sequences are written N-terminus first by convention.
Peptide or protein?
The difference is length, and the boundary is a convention rather than a law of nature. Chains up to roughly fifty residues are generally called peptides; longer chains that fold into a stable three-dimensional shape are called proteins. Some terms are narrower still — a dipeptide is two residues, a tripeptide three, an oligopeptide a few, a polypeptide many.
How the names work
Peptide naming is inconsistent, which is why it looks arbitrary. Broadly there are three patterns:
- Sequence letters. Some names are literally the sequence. GHK is glycine–histidine–lysine, three residues, named after them. GHK-Cu is the same tripeptide complexed with a copper ion.
- Origin. BPC-157 is a fifteen-residue partial sequence of a larger protective protein found in gastric juice, from which it takes the “body protection compound” name.
- Laboratory codes. Many names are simply a project designation that stuck — a letter prefix and a number, carrying no descriptive meaning at all.
Abbreviations then accumulate in use: retatrutide becomes “reta”, tirzepatide “tirz”. These are informal and carry no technical meaning.
Where some of the known sequences came from
GHK was first described in the early 1970s by the biochemist Loren Pickart, identified as a tripeptide present in human plasma; its copper-bound form, GHK-Cu, is what is usually meant by the name today. BPC-157 was described considerably later, derived as a stable partial sequence of the gastric protein rather than isolated whole.
Others are entirely synthetic constructions with no natural counterpart — designed molecules, not discovered ones. Whether a sequence occurs in nature says nothing about anything beyond its origin, and we make no claim here about what any of them do.
How synthetic peptides are made
Almost all of them by solid-phase peptide synthesis, the method developed by Robert Bruce Merrifield, for which he received the Nobel Prize in Chemistry in 1984. The first amino acid is anchored to an insoluble resin bead; each subsequent residue is coupled on in turn, with the growing chain held in place by the bead so that excess reagents can simply be washed away between steps. When the sequence is complete the chain is cleaved from the resin and purified.
Because each coupling step is imperfect, a synthesis produces the target sequence alongside related impurities — chains missing a residue, chains with side reactions. Purification removes most of these, and the residue is what an HPLC purity figure is measuring.
Why they arrive as powder
Peptides in solution are considerably less stable than peptides dry, so they are supplied freeze-dried — lyophilised — as a sealed vial of solid.
