Catalogs are organised by what buyers come looking for, not by chemical class. The practical result is that a peptide catalog contains a number of things that are not peptides, and several specification fields that are routine for a peptide are meaningless for them.
Knowing which is which prevents a specific mistake: expecting a sequence, a net peptide content figure or an amino-acid analysis on something that has no amino acids in it.
[NAD+](/p/nad) is a dinucleotide
Nicotinamide adenine dinucleotide is a coenzyme built from nicotinamide and adenine, each attached to a ribose sugar, joined through a pair of phosphate groups. There is no peptide bond anywhere in it and no amino acids.
It is manufactured by enzymatic or fermentation routes rather than by solid-phase synthesis, and the relevant analytical questions are those of a small organic molecule: identity by mass and by spectroscopy, purity by HPLC against related substances such as the reduced form, and water content. Sequence, net peptide content and amino acid analysis do not apply.
[5-Amino-1MQ](/p/5-amino-1mq) and L-carnitine are small molecules
5-Amino-1MQ is a quinolinium compound, studied in the literature as an inhibitor of nicotinamide N-methyltransferase. It is a single small organic molecule with a ring system, not a chain of residues.
L-carnitine is a quaternary ammonium compound. It is biosynthesised in the body from lysine and methionine, which is presumably why it drifts into amino-acid company, but the finished molecule is not a peptide and contains no peptide bond.
For both, the documentation that matters is a small-molecule specification: identity, assay by HPLC or titration, related substances, residual solvents, water and, where relevant for carnitine, enantiomeric purity, since the L and D forms are distinct.
[Glutathione](/p/glutathione) is a peptide, and an unusual one
This one deserves care, because the easy version is wrong in both directions. Glutathione is a tripeptide: glutamate, cysteine, glycine. So it is a peptide.
What makes it atypical is the linkage. The bond between glutamate and cysteine is formed through the side-chain carboxyl of glutamate rather than its alpha-carboxyl, making it a gamma-peptide bond. Standard peptide chemistry and most proteases work on alpha linkages, which is why that bond resists cleavage by the enzymes that would otherwise degrade a tripeptide quickly.
Two practical consequences. Commercial glutathione is generally produced by fermentation rather than by solid-phase synthesis. And the free cysteine thiol makes it readily oxidisable: glutathione converts to its oxidised disulfide dimer on exposure to air, so the ratio of reduced to oxidised form is a real specification and a lot that has oxidised is no longer what the label says.
HCG is a glycoprotein
Covered in detail elsewhere, but it belongs on this list: a two-subunit, heavily glycosylated protein produced recombinantly or by extraction, quantified in international units of activity rather than in milligrams. It has amino acids, so it is on the protein side of the line rather than outside it entirely, but it is not a synthetic peptide and its documentation looks nothing like one.
Lab supplies are not compounds at all
Bacteriostatic water, reconstitution solution and kits are consumables. Their specification is about what they contain and what they are free of — sterility, preservative concentration, container integrity — rather than purity of an active substance. A certificate for bacteriostatic water that reports an HPLC purity figure has misunderstood the product.
The practical rule
Before reading a certificate, establish what kind of molecule the item is, because that determines which fields should be present.
- Synthetic peptide: sequence, observed mass, HPLC purity, net peptide content, residual solvents, counterion.
- Small molecule or nucleotide: identity, assay, related substances, residual solvents, water, enantiomeric purity where relevant.
- Glycoprotein: potency in activity units with the assay named, production route, and identity by appropriate methods rather than a single exact mass.
- Consumable: sterility, composition, container integrity.
A certificate carrying net peptide content on a dinucleotide is not being thorough. It is being copied from a template, and that is worth noticing, because it suggests the document was assembled rather than measured.
Why the distinction changes the certificate
A small molecule, a glycoprotein and a short synthetic peptide are characterised by different methods, and a certificate format designed for one answers the wrong questions for another. Purity by reversed-phase HPLC means something different for a small molecule than for a peptide, and net peptide content is meaningless for something that is not a peptide at all.
So the first question about any catalogue item is what class of material it is, because that determines which fields on the certificate carry weight and which are being reported out of habit.
What to check for a non-peptide item
- That the purity method suits the material rather than being inherited from the peptide template.
- That identity is established by a method appropriate to the molecule.
- That a content figure, where present, is defined in terms that make sense for the material.
- That the certificate does not report fields that cannot apply, which is a sign of a template rather than a measurement.
Why catalogues mix these at all
Because buyers in this space order across the categories, and a supplier serving them stocks what is asked for. That is a commercial reason rather than a scientific one, and it means the heading above a listing carries no promise about what class of molecule sits under it.
A practical closing test. Read the certificate before the listing description, and ask whether each reported field could meaningfully apply to the molecule named. A field that cannot apply is a template artefact, and it tells you how the document was produced.
What the molecules in a catalogue actually are
Reading a listing starts with establishing what class of molecule sits behind the name, because that determines which parts of a certificate carry weight.
- Peptide or protein covers where the line sits and why it moves.
- Reading peptide nomenclature covers sequences, fragments and modifications.
- Molecular weight covers checking a certificate mass against the formula.
- TFA and acetate salt forms covers the mass in the vial that is not peptide.
- Why some peptides resist dissolving covers reading solubility off a sequence.
The compound classes
Catalogue headings group compounds by the system they are studied in, which hides how differently they behave as materials. Each of these covers what the class is and what it is not.
- GLP-1 and incretin peptides and semaglutide, tirzepatide and retatrutide compared.
- Growth hormone secretagogues, covering two different receptor routes under one heading.
- Melanocortin peptides, where selectivity is the whole story.
- Mitochondrial peptides, a group defined by origin rather than function.
- Bioregulator peptides, and how to read a contested literature.
- Gonadotropic and reproductive peptides, four molecules with four characterisation problems.
- Semax and Selank, BPC-157, TB-500 and thymosin beta-4 and GHK-Cu individually.
How it is supplied
Format changes what a certificate has to report and what the handling has to protect against, independently of which molecule is inside.
- Blends and single vials covers what a combination costs in documentation.
- Nasal sprays and liquid formats covers what changes when the water is already there.
- Dissolvable strips covers why uniformity replaces fill accuracy.