Molecule guides
Choosing a Research Peptide: Five Specifications That Decide the Order
Sequence, purity, content, salt form and species. Get these right and the rest of the catalogue follows; get content wrong and every number shifts.
Research peptides are synthetic chains of amino acids, delivered freeze-dried for laboratory work. In practice they play many roles: ligands for receptor studies, calibrators for assays, immunogens for antibody production, blocking reagents and protease substrates. This is the biggest product group in the research catalogue, and every other group leans on it — labelled peptides are these same sequences with a tag added, antibodies are generated against them, and assay kit standards are made from them. Choosing well means checking five things: the sequence with its modifications, HPLC purity, net peptide content, the counter-ion, and which species the sequence comes from.
Manufacture, and why it shapes the product
Almost every sequence in this section is made by solid-phase synthesis: residues are coupled one at a time to a resin-bound chain with their side chains protected, after which the peptide is cleaved off, deprotected and purified on preparative HPLC. Three practical consequences follow. Coupling yield falls with every added residue, so anything much longer than 50 residues becomes expensive and hard to purify, and longer molecules are often expressed recombinantly instead. Trifluoroacetic acid is used for cleavage, so the product comes as a TFA salt unless a salt exchange is performed. And the typical impurities are deletion sequences lacking one residue — the reason purity is measured by HPLC but identity has to be confirmed independently by mass spectrometry.
Purity and content are different numbers
Confusing purity with content is the most frequent quantitation mistake in peptide labs. Purity comes from the HPLC trace and tells you what fraction of the peptide-related material is the correct sequence. Content comes from amino acid analysis or nitrogen measurement and tells you what fraction of everything in the vial is peptide in the first place. Take a vial sold as 1 mg with 98% purity and 80% content: it holds roughly 0.8 mg of peptide, not 0.98 mg. Dissolve it as if the gross mass were all peptide and your concentration is about 20% too high — an error that carries through to every value calculated from it. Qualitative experiments seldom notice; standard curves and potency figures do. For a worked example of both numbers on a real certificate, see HPLC purity explained and the COA guide.
Why modified versions are separate catalogue entries
Adding a modification creates a new molecule, not a flavour of the old one — the molecular weight changes and the pharmacology often does too. C-terminal amidation mirrors the mature processed form of most neuropeptides, and families such as NPY, CGRP and substance P need it for full activity at their receptors. N-terminal acetylation removes the positive charge at that end and protects against aminopeptidases. In defensins, somatostatin and endothelin the disulfide bonds define the three-dimensional fold, so a reduced, linear copy is not equivalent. The same logic gives pyroglutamate, phosphorylated, biotinylated and D-amino-acid versions their own catalogue entries. How these changes are written is explained in reading a peptide sequence.
Choosing between full-length, fragment and analogue
Many families are offered as native peptide, fragments and analogues, and each serves a different purpose. For calibration curves, quantitation and experiments that need the physiological agonist, use the full-length native form. Fragments are mostly tools for receptor subtypes — CGRP(8–37) blocks its receptor, PYY(3–36) prefers Y2, and the AgRP(83–132) fragment is its functional core — and should not be seen as cheaper substitutes for the whole molecule. Analogues (D-substituted, stapled, PEGylated or stabilised in other ways) give up native identity in return for stability or selectivity, which pays off when the peptide has to survive incubation in plasma or an extended in-vivo protocol. The type page shows what each family includes; for a systematic series rather than single sequences, turn to the peptide libraries type.
Solubility, reconstitution and storage
Solubility is fixed by the sequence, whatever the protocol hopes. Highly charged hydrophilic peptides go straight into water or buffer. For a basic sequence try weak acetic acid; for an acidic one, weak ammonium hydroxide; a hydrophobic one generally needs a little DMSO or acetonitrile first, with the aqueous buffer added afterwards. Knowing the pI helps, because a peptide is least soluble near its isoelectric point. For bench procedure, read the reconstitution guide and the page on peptide solubility; the molarity arithmetic lives on the molecular weight and moles page.
Storage is simpler. Kept sealed, cold (−20 °C or lower), dark and dry, freeze-dried vials remain stable for years; just let each vial warm to room temperature before you open it, otherwise condensation will soak into the powder. Solutions are far less stable, so divide them into single-use portions straight away and freeze those, rather than thawing and refreezing a stock. Dissolved methionine, cysteine and tryptophan residues oxidise readily, so keep vials full and out of the light. Our guides on aliquoting and storage, and the list of common reconstitution mistakes, go further.
How peptides relate to the other product types
In short: a peptide standard is the tool for dosing a defined amount, for assay calibration, and for raising or pre-absorbing antibodies. Pick a labelled peptide when detection depends on a tag — binding studies, imaging, protease substrates. An antibody locates the native peptide on a blot or in a tissue section, and an assay kit measures how much endogenous peptide a sample contains. Browse from the catalogue hub or from a research area — cancer, diabetes or neuropeptides, for example. Every peptide in the catalogue is sold for in-vitro work and approved animal studies only.
Questions
Will 95% purity do?
For most cell-based assays, immunisations and qualitative work, yes. Once you are building a quantitative standard, doing receptor pharmacology or reporting a concentration, ask for ≥98%. The impurities that make up the last few percent are mainly single-residue deletion sequences, which may still be partly active — so purity matters more when you measure potency than when you only need detection.
Why is the vial heavier than the peptide quantity on the label?
The label quantity is the peptide itself; the vial additionally contains counter-ion and residual moisture, typically 10-30% of the gross weight. The peptide content value on the certificate captures exactly this. Dissolving against the gross weight therefore gives an inflated concentration, and quantitative experiments should be calculated from the content figure.
When is acetate salt needed rather than TFA?
Cell culture, antimicrobial testing and animal work are where it matters. Residual trifluoroacetate is toxic to cells and has antimicrobial activity at the micromolar levels these assays reach, skewing viability readouts and MIC values. Exchange to the acetate salt is a standard option. Binding assays and analytical standards generally tolerate the TFA form.
How can I be sure the peptide is the sequence ordered?
Check the mass spectrometry value on the certificate. The measured mass should agree, within the instrument's tolerance, with the calculated monoisotopic or average mass of the sequence including any modifications. HPLC only tells you how homogeneous the material is; MS is what tells you what it actually is. More in mass spectrometry and peptide identity.
Can reconstituted peptide be refrozen?
Technically yes, but each freeze–thaw costs some peptide through aggregation and breakdown, by an amount that varies with sequence and is rarely quantified. Splitting the solution into single-use portions at the first reconstitution avoids the question entirely and reduces contamination from repeatedly piercing the septum — which is why labs do it routinely.
Do research peptides arrive sterile?
No, unless a specific product page states otherwise. These freeze-dried research peptides are not sterile-filtered, not tested for endotoxin and not manufactured under pharmaceutical GMP. If your protocol requires sterility, filter at the point of use and accept some peptide loss on the membrane. They are sold for in-vitro work and approved animal studies only.