What a peptide is, in plain English
No chemistry background needed. What these molecules are, how they differ from proteins, and why sequence matters.
A peptide is a short chain of amino acids joined end to end. That is the whole definition. Everything else is detail about how long the chain is, what order the links come in, and what that particular order does.
Amino acids, links and chains
There are twenty or so amino acids that living things commonly use. Think of them as beads of different shapes. Join two together and you have a dipeptide. Join a handful and you have a peptide. Join many dozens or hundreds and, by convention, people start calling it a protein instead. The line between “long peptide” and “small protein” is a matter of custom rather than a hard rule — somewhere around fifty residues.
The bond that holds the beads together is called a peptide bond, which is where the name comes from.
Why the order matters so much
The sequence — which amino acid sits at position one, two, three and so on — determines the shape the chain folds into, and shape determines what the molecule can interact with. Swap two residues and you may have something with entirely different properties, or nothing of interest at all.
This is why a peptide is always identified by its sequence rather than by a trade name. When you see a sequence written out as Lys-Pro-Val, that is the actual identity of the material. Names like KPV are shorthand for it.
Where research peptides come from
Some are fragments of naturally occurring proteins, copied because that particular region is the interesting one. Some are analogues — a natural sequence with a deliberate modification to make it more stable or easier to study. Some are entirely synthetic constructs designed for a specific experiment.
Almost none of what is used in research is extracted from a biological source any more. It is built from scratch, one amino acid at a time, in a process covered in How peptides are made.
Why they are difficult to work with
Peptides are comparatively fragile. They are sensitive to heat, moisture, light and repeated temperature swings, and they degrade in solution far faster than in dry form. That is why they are supplied as a dry powder, and why storage conditions matter more than people expect.