Peptide basics
The vocabulary underneath everything else. If a term on a certificate or a product page has ever gone past you, start here.
What is a peptide?
A peptide is a short chain of amino acids joined by peptide bonds. Chains are generally called peptides up to roughly 50 amino acids and proteins beyond that, though the boundary is a convention rather than a rule. Retatrutide, at 39 amino acids, sits firmly in peptide territory.
Full guide: What Is a Research Peptide? A Plain-English Guide →
What are amino acids?
Amino acids are the building blocks peptides and proteins are made from. Twenty standard amino acids appear in biological systems, each with a shared backbone and a distinct side chain. The order in which they are strung together determines the molecule's shape, and its shape determines what it binds to.
What is a peptide bond?
A peptide bond is the covalent link joining one amino acid to the next, formed when an amino group reacts with a carboxyl group and a water molecule is released. It is a strong, rigid bond, which is why a peptide's sequence stays intact under normal handling while its folded shape is far more fragile.
What is molecular weight?
Molecular weight is the mass of one molecule, reported in daltons (Da) or grams per mole. On a peptide certificate it matters because mass spectrometry confirms identity by measuring it: the observed mass should match the theoretical mass calculated from the sequence, within a small tolerance.
Full guide: What Does 99% Peptide Purity Mean? HPLC & Mass Spec Explained →
What is a receptor?
A receptor is a protein, usually sitting in a cell membrane, that a specific molecule binds to. Binding changes the receptor's shape, which triggers a signalling cascade inside the cell. Receptors are selective — the fit between molecule and receptor is what makes one compound active where another does nothing.
What is an agonist?
An agonist is a molecule that binds a receptor and activates it, producing the same kind of response the body's own signalling molecule would. Agonists vary in strength: a full agonist produces the maximum possible response, a partial agonist produces less even when every receptor is occupied.
What is the difference between an agonist and an antagonist?
Both bind the same receptor; the difference is what happens next. An agonist activates the receptor and produces a response. An antagonist occupies it without activating it, and by sitting there blocks the natural signalling molecule from binding. One switches the signal on, the other blocks the switch.
What does in vitro mean?
In vitro means "in glass" — research carried out in a controlled vessel such as a test tube, plate or dish, outside a living organism. It is the setting research-grade peptides are supplied for. Results in vitro do not automatically predict what happens in a living system.
What is the difference between in vitro and in vivo?
In vitro work happens outside a living organism, in controlled laboratory vessels. In vivo work happens inside one. In vitro isolates a single variable cleanly but strips away absorption, distribution, metabolism and clearance — which is exactly why a compound can behave one way in a dish and quite differently in an organism.