Molecule guides
Labelled Peptides (Fluorescent, Biotin, I-125): What They Are and How to Pick One
The tag is a specification in its own right. Which chemistry suits which experiment, where to attach it, and the checks that stop a labelled ligand failing late.
A labelled peptide is a synthetic sequence that carries a reporter group — a fluorophore, a biotin tag or a radioisotope — allowing the molecule to be followed, detected or pulled out of a mixture. These items make up the labelled peptides section of the research catalog, and they exist because peptides generally produce no signal of their own: experiments on binding, trafficking, capture or cleavage need something that reports where the peptide has gone or whether it has been cut. The tag counts as a specification in itself, and picking the wrong one ruins more experiments than picking the wrong sequence does.
Three tag chemistries and their uses
The glossary defines each of them: fluorescent labels, the biotin label and the I-125 radiolabel.
Attachment site, and why it settles the experiment
Where the tag goes matters as much as what it is. Putting it on the N-terminus leaves the C-terminus unmodified, which is essential whenever C-terminal amidation is needed for receptor activity — the case for most neuropeptides. A C-terminal tag has the opposite effect. Labelling internally on a lysine side chain spares both ends but risks landing inside the binding epitope. So label whichever terminus the peptide's activity does not depend on, and then verify in a functional assay that the tagged version still works — discovering late that a labelled ligand no longer binds is a common and costly outcome.
Spacers serve the same purpose. A short PEG linker or aminohexanoic acid (Ahx) keeps the tag at a distance from the peptide, cutting steric interference with receptor binding and with streptavidin capture alike. Biotin attached with no spacer often cannot be reached once the peptide is bound to a surface or a bulky partner.
A special case: fluorogenic substrates
Protease assays rely on a different arrangement. Fluorophore and quencher sit on either side of the cleavage site, so that energy transfer keeps signal low while the peptide is intact and fluorescence climbs once cleavage pulls the pair apart. In this design the label pair is the assay: the quencher has to overlap the fluorophore's emission, and the excitation and emission wavelengths must suit the filter set in your plate reader. Order a Cy5-based substrate for an instrument set up for fluorescein and you will collect no data whatsoever. Substrates of this kind fill the cathepsin and matrix-protease sections and recur throughout the cancer topic.
Picking a tag for a binding study
A classical competition binding experiment combines unlabelled peptide with a labelled tracer, and the right tracer depends on both the receptor and the readout. I-125 is still the most sensitive choice and has essentially no rival for receptor autoradiography on tissue sections, which is why it endures in cardiovascular and neuropeptide pharmacology in spite of the handling overheads. Fluorescent tracers enable live-cell imaging and fluorescence polarisation and need no licence, though they are less sensitive and the dye's bulk can perturb binding. Biotinylated peptides suit capture rather than quantitative affinity work — pull-downs, bead-based multiplex assays and ELISA detection, typically together with magnetic beads. Our labelled peptides guide and the FAM, biotin and I-125 explainer compare them in greater detail.
Specifications worth checking before you order
On top of the sequence, purity and species checks that apply to any research peptide, labelled items bring four more. The tag's identity and position should be stated outright, including which residue bears an internal label. For fluorescent tags you need spectral data: excitation and emission maxima plus the extinction coefficient required to determine concentration by absorbance. Degree of labelling matters where applicable, since a preparation ought to be singly labelled and a blend of mono- and di-labelled species muddies quantitation. Radiolabelled items need a specific activity together with its reference date — an I-125 tracer has lost half its strength around 60 days after calibration, so planning an experiment requires both the figure and the date.
Storage and handling
Tagged peptides are more fragile than their unlabelled counterparts. Keep them away from light at all times, since fluorophores photobleach under ordinary laboratory lighting: use foil or amber vials and dim the lights when aliquoting. Aliquot at first reconstitution, because repeated freeze–thaw damages the dye as well as the peptide. Mind the pH with fluorescein-based tags, whose emission falls steeply below pH 7 — a change of buffer can easily masquerade as a biological effect. Radiolabelled peptides require a suitable licence, dedicated space, shielding and a documented waste route, and their decay places a firm limit on when they can be ordered and used. For the peptide itself nothing else changes: sealed at −20 °C or lower, warmed to room temperature before opening, and split into single-use aliquots (storage guide, aliquoting guide).
How labelled peptides relate to the other product types
Choose an unlabelled peptide standard when you are applying a known quantity or constructing a standard curve, and add a labelled version when the peptide itself has to be detected. Reach for an antibody when the endogenous molecule needs detecting in tissue, and for an assay kit when the question is a concentration in a sample. Labelled antibodies apply these same tag chemistries to immunoglobulins instead. Begin at the catalog hub, or at a family page such as GLP-1 or NPY, to see which labelled variants are available. Every catalog item is supplied for in-vitro use and approved animal research only.
Questions
Could the tag alter how my peptide behaves?
It may well, and the safe assumption is that it does until you have checked. Fluorescent dyes are bulky, sometimes rivalling a short peptide in mass, and a tag close to the binding epitope will weaken or destroy affinity. Attach it to the terminus that activity does not require, include a spacer, and test the labelled peptide functionally before depending on it.
Are FAM and FITC interchangeable?
They are related but distinct. FITC, fluorescein isothiocyanate, attaches through a thiourea bond that can prove unstable, whereas 5(6)-FAM is carboxyfluorescein, attached as an amide and more robust. Their spectra are nearly the same. That stability difference is why FAM is usually preferred for peptide conjugates, and catalog entries state which was used.
How should the concentration of a fluorescently labelled peptide be determined?
Measure absorbance at the dye's maximum and apply the quoted extinction coefficient, rather than working from vial mass, which includes salt, water and the dye. If you also read aromatic absorbance at 280 nm, correct for what the dye contributes there. The certificate should give both the extinction coefficient and the degree of labelling.
For how long is an I-125 labelled peptide usable?
In practice, from a few weeks up to a couple of months after the reference date. With a half-life around 60 days, I-125 loses half its specific activity over that span while radiolytic damage builds up within the peptide. Schedule binding experiments near the calibration date and compute activity from that date, not from when the vial arrived.
Is a spacer necessary on a biotinylated peptide?
For capture work, generally yes. The biotin-binding site in streptavidin is recessed, so a biotin attached directly to the backbone is frequently out of reach once the peptide is also engaged with a receptor or a surface. An aminohexanoic acid or short PEG spacer fixes this, though for straightforward detection on a blot it matters less.
Will a fluorogenic protease substrate work on any plate reader?
Only where the filters correspond. The fluorophore-quencher combination fixes the excitation and emission wavelengths, so a substrate built around a red-shifted dye registers nothing on an instrument configured for fluorescein. Compare the stated spectra with the filter sets your instrument offers before ordering rather than afterwards.