A primer on the chemistry, written for someone who is technically literate but not a peptide chemist.
In short
- A peptide is a chain of amino acids joined by amide bonds — conventionally under about fifty residues, above which the molecule is called a protein.
- Peptides occupy a middle ground between small molecules and biologics: large enough to bind a receptor with high selectivity, small enough to synthesise chemically.
- Their central liability is stability. Peptidases in plasma and tissue cleave them in minutes, and most of the medicinal chemistry in the field exists to slow that down.
- Muttenthaler and colleagues counted a sharp rise in approved peptide drugs across the 2010s, with metabolic disease and oncology accounting for most of it.
The bond, and the number fifty
A peptide bond is an amide: the carboxyl carbon of one amino acid joined to the nitrogen of the next, with a water molecule released. Chain two amino acids that way and you have a dipeptide; chain twenty and you have a twenty-residue peptide. The boundary between peptide and protein is a convention, not a chemical fact — most writers put it somewhere around fifty residues, where chains reliably begin to fold into stable tertiary structure.
Because the bond is planar and its rotation restricted, a peptide chain has a limited conformational repertoire. That matters practically: short peptides are often flexible in solution and only adopt a defined shape on binding, while cyclised or constrained ones hold a shape in advance. Constraint is one of the field's main design levers.
Everything in the catalog on this site sits inside that definition, and the range within it is wide. Some entries are three residues long. Some are forty-four. Some are not peptides at all but cofactors and solvents that peptide work depends on.
Why the middle ground is interesting
Small molecules are cheap to make, orally available, and promiscuous — a few hundred daltons of scaffold rarely distinguishes closely related receptor subtypes. Antibodies are exquisitely selective and stable but enormous, expensive, and confined to extracellular targets. Peptides sit between the two.
A peptide is large enough to present multiple contact points to a receptor, which is what buys subtype selectivity. It is small enough to be built on a synthesiser rather than grown in a cell line, which is what makes bespoke analogues tractable. And because most peptides in this space are analogues of endogenous ligands, the receptor they engage is usually already characterised.
Muttenthaler, King, Adams and Alewood surveyed the field in Nature Reviews Drug Discovery in 2021 and described exactly this: a class that spent decades as a niche became a substantial fraction of new approvals, driven less by novel chemistry than by solutions to delivery and half-life.
The stability problem, and what is done about it
An unmodified linear peptide in plasma has a half-life measured in minutes. Endo- and exopeptidases cleave it; the kidney clears the fragments. For a research compound this is not incidental — it determines what an experiment can even measure, and it is the reason two analogues of the same parent sequence can behave completely differently in the same assay.
The standard interventions are well catalogued: substitute a D-amino acid at a cleavage site so the protease no longer recognises it; replace a labile residue with a non-natural one; cyclise the backbone; cap the termini by acetylation or amidation; or attach something bulky — a fatty acid, a polymer, an albumin-binding group — that slows renal filtration and keeps the molecule in circulation.
That last strategy is visible directly in this catalog. Several entries exist as paired variants where one carries a half-life-extending modification and the other does not, and the published pharmacokinetics of the two differ by orders of magnitude. Wang and colleagues review the toolkit in Signal Transduction and Targeted Therapy.
What the word does not tell you
Calling something a peptide says almost nothing about what it does. Insulin is a peptide. So is a growth-hormone secretagogue, a melanocortin receptor agonist, an antimicrobial defensin, and a tripeptide antioxidant cofactor. The shared chemistry does not imply a shared pharmacology, a shared safety profile, or a shared level of evidence.
That is worth stating clearly, because the word gets used commercially as though it were a category of effect. It is not. It is a category of molecule. The evidence base behind any individual compound has to be read on its own, which is what the rest of this library is for.
It is also worth stating that a substantial number of compounds studied in this space have never been through a controlled human trial of any kind. Where that is the case, the articles here say so.
References
- Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in peptide drug discovery. Nature Reviews Drug Discovery. 2021;20(4):309–325.
- Wang L, Wang N, Zhang W, et al.. Therapeutic peptides: current applications and future directions. Signal Transduction and Targeted Therapy. 2022;7:48.
What this article is, and is not
This is a summary of published research, written for qualified professionals evaluating compounds for laboratory work. Every compound discussed is supplied by strictly for in-vitro and laboratory research use. None is a drug, a dietary supplement, or a cosmetic; none is intended for human or veterinary use; and nothing above is medical advice, a treatment recommendation, or a claim that any compound produces any effect in a person. We publish no dosing or administration guidance of any kind.
