Molecule guides
Research Peptides: What They Are and How to Choose
How to choose a research peptide: sequence and modifications, HPLC purity vs peptide content, TFA vs acetate salt, species, solubility and storage.
Research peptides are synthetic amino-acid chains supplied as lyophilised powder for laboratory use: receptor ligands, assay calibrators, immunogens, blocking peptides and protease substrates. They are the largest product type in the research catalog and the reference point for every other type — labeled peptides are these molecules with a tag, antibodies are raised against them, and assay kits are calibrated with them. Choosing one correctly comes down to five specifications: sequence and modifications, purity, peptide content, salt form, and species.
How a research peptide is made, and why it matters
Almost everything in this catalog is produced by solid-phase peptide synthesis: the chain is assembled one residue at a time on a resin, with side chains held under protecting groups, then cleaved, deprotected and purified by preparative HPLC. Three consequences follow directly. Synthesis efficiency falls with length, so peptides beyond roughly 50 residues become expensive and harder to purify — longer sequences are often recombinant instead. Cleavage uses trifluoroacetic acid, so the product arrives as a TFA salt unless exchanged. And the failure products are deletion sequences differing by one residue, which is why purity is quoted by HPLC and identity confirmed separately by mass spectrometry.
The five specifications to check before ordering
Purity versus content: the distinction that changes your numbers
These are separate figures and confusing them is the most common quantitation error in peptide work. Purity — from the HPLC chromatogram — is the proportion of peptide-related material corresponding to the target sequence. Peptide content — from amino-acid analysis or nitrogen determination — is the proportion of the vial's total mass that is peptide at all. A vial labelled 1 mg at 98% purity and 80% content holds roughly 0.8 mg of peptide, not 0.98 mg. Reconstituting on gross mass therefore overstates concentration by about 20%, which propagates through every derived value. For qualitative work this rarely matters; for a standard curve or a reported potency it is decisive. HPLC purity explained and the COA guide cover both figures on a real certificate.
Modifications, and why the catalog lists them as distinct products
A modification is not a variant of the same item — it is a different molecule with a different molecular weight and often different pharmacology. C-terminal amidation reproduces the natural processed form of most neuropeptides and is required for full receptor activity in families such as NPY, CGRP and substance P. N-terminal acetylation blocks aminopeptidase attack and removes the positive charge at that end. Disulfide bridges define the fold of defensins, somatostatin and endothelin, and a linear reduced version behaves differently. Pyroglutamate formation, phosphorylation, biotinylation and D-amino-acid substitution all likewise create separate catalog entries. Reading a peptide sequence decodes the notation these are written in.
Full-length, fragment or analog
Most family pages offer all three, and they answer different questions. The full-length native peptide is the right choice for standard curves, quantitation and physiological agonist work. A fragment is usually a receptor-subtype tool: CGRP(8–37) is an antagonist, PYY(3–36) is Y2-selective, AgRP(83–132) is the active core. Fragments are not cheaper substitutes for the parent. An analog — D-amino-acid substituted, stapled, PEGylated or otherwise stabilised — trades native identity for stability or selectivity, and is appropriate when a peptide must survive a plasma incubation or a longer in-vivo protocol. Browse the type page for what exists in a given family, and see the peptide libraries type when you need a systematic set rather than individual sequences.
Reconstitution, solubility and storage
Solubility is set by sequence, not by wishing. Highly charged hydrophilic peptides dissolve directly in water or buffer; basic peptides go into dilute acetic acid, acidic ones into dilute ammonium hydroxide; hydrophobic sequences generally need DMSO or acetonitrile first, then dilution into aqueous buffer. Check the isoelectric point — solubility is at its worst near it. Practical steps are in peptide solubility and the reconstitution guide; the molarity arithmetic is in molecular weight and moles.
Storage is simpler. Sealed lyophilised vials keep for years at −20 °C or below, protected from light and moisture — always let a vial reach room temperature before opening, or condensation will draw water into the powder. Reconstituted peptide is far less stable: aliquot into single-use volumes immediately and freeze, avoiding repeated freeze–thaw. Methionine, cysteine and tryptophan residues oxidise in solution, so minimise headspace and light exposure. See the storage guide, the aliquoting guide and common reconstitution mistakes.
Where research peptides fit against the other product types
Use a peptide standard when you need to apply a known quantity, calibrate an assay, or raise or absorb an antibody. Add a labeled peptide when detection requires a tag — binding assays, imaging, protease substrates. Order an antibody when you need to detect the endogenous molecule in tissue or on a blot. Order an assay kit when the question is how much endogenous peptide is present in a sample. Start from the catalog hub, or from a topic such as neuropeptides, diabetes or cancer. All catalog peptides are supplied for in-vitro and approved animal research only.
Frequently Asked Questions
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Questions
Is 95% purity good enough?
For most cell assays, immunogens and qualitative work, yes. For quantitative standards, receptor pharmacology and anything where a reported concentration matters, choose ≥98%. The remaining few percent are usually deletion sequences differing by one residue, which can retain partial activity — that is why higher purity matters more for potency measurements than for detection.
Why does my vial weigh more than the stated peptide amount?
Because the stated amount is peptide, and the vial also contains counter-ion salt and residual water — typically 10–30% of gross mass. That is the peptide content figure on the certificate of analysis. Reconstituting on gross vial mass overstates your concentration accordingly; use the content value for quantitative work.
When do I need acetate salt instead of TFA?
For cell culture, antimicrobial assays and animal protocols. Residual trifluoroacetate is cytotoxic and antimicrobial at the micromolar range these assays use, confounding viability and MIC readouts. Salt exchange to acetate is a standard request. For binding assays and analytical standards, TFA salt is generally acceptable.
How do I know the peptide is actually the sequence I ordered?
Mass spectrometry on the certificate of analysis. The observed mass should match the theoretical monoisotopic or average mass for the sequence including its modifications, within instrument tolerance. HPLC tells you how much of the material is one species; only MS tells you which species. See mass spectrometry and peptide identity .
Can I re-freeze a reconstituted peptide?
You can, but each cycle costs you material through aggregation and degradation, and the loss is sequence-dependent and rarely measured. Aliquoting into single-use volumes at first reconstitution removes the question entirely and also limits contamination from repeated septum entry. That is the standard laboratory practice.
Are research peptides sterile?
No, unless a specific item states otherwise. Lyophilised research peptides are not sterile-filtered, not endotoxin-tested and not manufactured under pharmaceutical conditions. Protocols requiring sterility need filtration at point of use, with the peptide loss that entails. They are supplied for in-vitro and approved animal research only.