What are peptides?

An amino acid carries an amino group (−NH₂) and a carboxylic acid group (−COOH) on the same carbon atom, the alpha carbon, together with a side chain. The side chain is what makes glycine different from lysine: in glycine it is a single hydrogen atom, in lysine a chain of four carbons ending in a second amino group.

Proteins in living things are built from 20 standard amino acids. Each has a three-letter and a one-letter code, so glycine is Gly or G and lysine is Lys or K. Amino acid codes gives all twenty with their formulas and masses.

All of them except glycine exist in two mirror-image forms, L and D. Proteins use the L form. Synthetic peptides sometimes include a D amino acid on purpose, and the name then says so.

When the acid group of one amino acid reacts with the amino group of the next, the two join and give off one molecule of water. The new link is an amide bond; between two amino acids it is called a peptide bond. Repeat the reaction and the chain grows by one unit, and one water, each time.

Two properties of the bond matter in practice:

• It is flat and stiff. The bond has partial double-bond character, so the atoms around it lie in one plane and do not rotate freely. That stiffness is part of why chains fold the way they do.
• Water can break it. The reverse reaction, hydrolysis, splits the bond and returns the two amino acids. In neutral water at room temperature it is slow; acids, bases, heat and enzymes speed it up. This is one reason peptides keep far longer as dry powders than in solution.

Because each link costs a water molecule, the units inside a chain are called residues, and a residue weighs one water less than the free amino acid. Masses on a certificate are built from residue masses.

A peptide chain has two different ends. One keeps a free amino group: the N-terminus. The other keeps a free acid group: the C-terminus. Sequences are always written from the N-terminus on the left to the C-terminus on the right.

So Gly-His-Lys (GHK in one-letter code) and Lys-His-Gly (KHG) are different molecules, with the same atoms and the same mass. GHK is the peptide in copper tripeptide-1, a common cosmetic ingredient.

The number of possible sequences grows quickly. With 20 amino acids there are 400 possible dipeptides, 8,000 tripeptides and 3.2 million pentapeptides.

Insulin sits on the border: 51 amino acids in two chains, held together by disulfide bridges between cysteines, and it is called both a peptide and a protein. The EDA's peptide guide, following the US Food and Drug Administration's wording, defines synthetic peptides as alpha amino acid polymers of 40 or fewer amino acids, and asks for an extra identity test, peptide mapping, for registered peptide medicines longer than about 20 amino acids.

In living cells. Cells make peptides and proteins on ribosomes, reading the sequence from a gene, and they also cut larger proteins into peptides. Digestion does the same to the protein in food: it is broken into peptides and then into free amino acids.

By chemical synthesis. Most synthetic peptides are made by solid-phase peptide synthesis, a method developed by Bruce Merrifield, first published in 1963, for which he received the Nobel Prize in Chemistry in 1984. The chain is anchored by its C-terminal end to tiny resin beads and built one amino acid at a time, each arriving with its reactive groups protected. At the end the peptide is cut from the resin, freed of its protecting groups, purified by HPLC and freeze-dried. Purification leaves counter-ions bound to the peptide, which is why a peptide powder is never 100% peptide by weight. Peptide purity explained covers what that means for a certificate.

By recombinant DNA technology. Engineered cells are given a gene and produce the peptide or protein. The EDA's peptide guide is written for synthetic copies of medicines first made this way.

Peptides are grouped in several ways, and a single peptide fits more than one group.

• By length: dipeptides, tripeptides, oligopeptides, polypeptides.
• By origin: natural (made by living things), synthetic (made by chemistry), recombinant (made by engineered cells).
• By shape: linear chains, and cyclic peptides closed into a ring, end to end or through side chains. Two cysteines can also be joined by a disulfide bridge.
• By modification: an acetyl group on the N-terminus, an amide at the C-terminus, a fatty acid chain such as palmitoyl, or a metal held by the peptide, as copper is in copper tripeptide-1.
• By what they are sold as: a registered medicine, a cosmetic ingredient, a food ingredient such as collagen peptides, or a research material for laboratories.

The last grouping is the one that decides the rules. In Egypt each of the four is regulated differently, and the same molecule can be lawful in one category and not in another. Are peptides legal in Egypt? sets out the categories.

Suppliers write modified peptides in a shorthand: