Labeled peptides are synthetic peptides carrying a reporter group — a fluorophore, a biotin tag or a radioisotope — so the molecule can be detected, tracked or captured. They occupy the labeled peptides section of the research catalog and exist for one reason: most peptides have no intrinsic signal, so binding, trafficking, capture and cleavage experiments need a tag that reports where the peptide is or whether it has been cut. The label is a specification in its own right, and choosing it badly wastes more experiments than choosing the wrong sequence.
The three label chemistries, and what each is for
| Label type | Typical examples | Primary use | Main limitation |
|---|---|---|---|
| Fluorescent | FAM/FITC, TAMRA, Cy3/Cy5, rhodamine, Dabcyl/Edans FRET pairs | Imaging, uptake and localisation, fluorescence polarisation, fluorogenic protease substrates | Photobleaching; bulky dyes can perturb receptor binding; pH-sensitive emission for fluorescein |
| Biotin | Biotin, biotin with aminohexanoic acid spacer | Streptavidin capture, pull-downs, bead-based assays, ELISA detection | Endogenous biotin in some samples; steric hindrance without a spacer |
| Radioisotope | I-125 (Bolton-Hunter or direct iodination), tritium | Competition binding, receptor autoradiography, classical RIA | 60-day I-125 half-life, licensing, waste handling, decay-driven scheduling |
Definitions for each are in the glossary: fluorescent labels, biotin label and I-125 radiolabel.
Where the label sits, and why that decides the experiment
Position is as important as chemistry. An N-terminal label leaves the C-terminus free, which matters when C-terminal amidation is required for receptor activity — true for most neuropeptides. A C-terminal label does the reverse. Internal labelling on a lysine side chain preserves both termini but may sit in the binding epitope. For a peptide whose activity depends on a specific end, label the other one, and confirm with a functional assay that the labeled version retains activity — a labeled ligand that no longer binds is a common and expensive discovery made late.
Spacers exist for the same reason. Aminohexanoic acid (Ahx) or short PEG linkers hold the tag away from the peptide, reducing steric interference with both receptor binding and streptavidin capture. Biotin without a spacer is frequently inaccessible when the peptide is bound to a surface or a large partner.
Fluorogenic substrates: a special case
Protease assays use a distinct design. The peptide carries a fluorophore and a quencher flanking the cleavage site; while intact, energy transfer keeps the signal low, and cleavage separates the pair so fluorescence rises. Here the label pair defines the assay: the quencher must overlap the fluorophore's emission, and the excitation and emission wavelengths must match your plate reader's filter set. Ordering a Cy5-based substrate for an instrument configured for fluorescein produces no data at all. These substrates dominate the cathepsin and matrix-protease sections and appear throughout the cancer topic.
Choosing between labels for a binding study
The classical competition binding experiment pairs an unlabeled peptide with a labeled tracer. Which tracer depends on the receptor and the readout. I-125 remains the most sensitive option and is essentially unmatched for receptor autoradiography on tissue sections, which is why it persists in cardiovascular and neuropeptide pharmacology despite the handling burden. Fluorescent tracers support live-cell imaging and fluorescence polarisation, and carry no licensing requirement, but are less sensitive and can be perturbed by the dye's bulk. Biotinylated peptides are the choice when the goal is capture rather than quantitative affinity — pull-downs, bead-based multiplex assays and ELISA detection, usually alongside magnetic beads. Our labeled peptides guide and the FAM, biotin and I-125 explainer compare them in more depth.
Specifications to check before ordering
Beyond the usual sequence, purity and species checks that apply to every research peptide, labeled items add four. Label identity and position — stated explicitly, including which residue carries an internal tag. Spectral properties for fluorescent labels: excitation and emission maxima, and the extinction coefficient needed to calculate concentration by absorbance. Degree of labelling, where relevant: a preparation should be singly labeled, and a mixture of mono- and di-labeled species complicates quantitation. Specific activity for radiolabeled items, together with the reference date — an I-125 tracer is at half strength roughly 60 days after calibration, so both the value and the date are needed to plan the experiment.
Storage and handling
Labeled peptides are less robust than their unlabeled parents. Protect from light at all times — fluorophores photobleach in ambient laboratory lighting, so use amber vials or foil, and work under reduced light when aliquoting. Aliquot on first reconstitution; repeated freeze–thaw degrades both peptide and dye. Watch the pH for fluorescein-based labels, whose emission drops sharply below pH 7 — a buffer change can look like a biological effect. Radiolabeled peptides require an appropriate licence, dedicated handling space, shielding and a documented waste route, and decay imposes a hard scheduling constraint on ordering. General practice for the peptide itself is unchanged: sealed at −20 °C or below, room temperature before opening, single-use aliquots (storage guide, aliquoting guide).
Where labeled peptides fit against the other product types
Order an unlabeled peptide standard when you apply a known quantity or build a standard curve; add a labeled version when the peptide itself must be detected. Use an antibody when the endogenous molecule must be detected in tissue, and an assay kit when the question is concentration in a sample. Labeled antibodies apply the same tag chemistries to immunoglobulins. Start from the catalog hub or a family page such as GLP-1 or NPY to see which labeled variants exist. All catalog items are supplied for in-vitro and approved animal research only.