What Are Peptides?
The structural definition of a peptide, how the chains are assembled, and how chain length separates a peptide from a protein.
5 min read
A chain of amino acids
A peptide is a chain of amino acids joined end to end. Each amino acid carries an amine group at one end and a carboxyl group at the other. A peptide bond forms when the carboxyl group of one residue joins the amine group of the next, releasing a molecule of water. Repeat that reaction and the result is a chain with a repeating backbone and a distinct side chain projecting from each position.
The units in the chain are called residues, because each has lost part of its structure to the bond that holds it in place. A chain of fifteen amino acids is a pentadecapeptide; a chain of three is a tripeptide. The names describe length alone.
Because the chain has a direction, chemists write the sequence from the free amine end (the N-terminus) to the free carboxyl end (the C-terminus). Two peptides with the same residues in reverse order are different molecules.
Where peptides end and proteins begin
The boundary between a peptide and a protein is a naming convention. Chains of up to roughly fifty residues are generally called peptides, and longer chains are generally called proteins. The convention is useful because it tracks a difference in behaviour: shorter chains tend to be structurally simpler and are usually assembled synthetically, while longer chains fold into complex three-dimensional structures and are typically produced biologically.
For analytical work, this means peptides are small enough to characterise in detail. A synthetic peptide has one expected sequence and one expected mass, and an analyst can check both directly.
How synthetic peptides are assembled
Most research peptides are made by solid-phase peptide synthesis, a method in which the chain is built one residue at a time while anchored to an insoluble resin support. Because the growing chain stays fixed to the resin, excess reagents and by-products can be washed away after every step without losing the product.
Each cycle follows the same pattern: the protecting group on the end of the chain is removed, the next amino acid is coupled, and the support is washed. The cycle repeats for every position in the sequence. When the chain is complete it is cleaved from the resin and the remaining side-chain protecting groups are removed.
That repetition makes purification necessary. If a single coupling step is incomplete, some fraction of chains will be missing that residue, and synthesis continues on those shortened chains as though nothing happened. The result is a crude product containing the intended sequence alongside closely related sequences that must be separated from it. These related sequences make up most of the impurities that a purity figure accounts for.
Why they are studied
Peptides sit between small molecules and proteins. They are large enough to carry specific structural information in their sequence, and small enough to be synthesised to a defined composition and characterised with routine analytical methods. That combination makes them useful subjects in biochemical and analytical laboratory research.
This article describes peptides as a class of molecules. It does not discuss any particular compound's effects, and nothing here is a recommendation regarding use.
This article is general reference material describing methods and records. It does not describe any particular material and is not medical advice.
