Labeled peptides are ordinary sequences with a reporter group attached, and the label you choose is decided by the detection instrument you already own — fluorescence for imaging and plate-based binding, biotin for capture and enzymatic amplification, radioiodine for the highest sensitivity in receptor and immunoassay work. Each label adds mass, may add a spacer, and can perturb binding if attached at the wrong end. This guide covers what each label family is for, the mass each one adds so you can check an identity result, the practical constraints of each, and a decay calculation for radiolabeled material. Research use only.
Labeled peptides: the three label families at a glance
| Label | Mass added (Da) | Detected by | Typical application | Main constraint |
|---|---|---|---|---|
| 5(6)-FAM / FITC | ~358 | Fluorescence, ex ~495 nm / em ~520 nm | Imaging, fluorescence polarisation binding, FRET substrates | Photobleaching; fluorescence drops sharply below pH 7 |
| TAMRA | ~412 | Fluorescence, ex ~546 nm / em ~579 nm | Two-colour work with labeled antibodies, FRET acceptor | More hydrophobic; can alter peptide solubility |
| Cy3 / Cy5 | ~500–600 | Fluorescence, red and far-red | Low-autofluorescence imaging, single-molecule work | Cost; substantial mass relative to a short peptide |
| Biotin | 226.3 (+113.2 per Ahx spacer) | Streptavidin conjugates — HRP, fluorophore, bead | Pull-down, plate immobilisation, amplified detection | Endogenous biotin background; steric hindrance without a spacer |
| I-125 | ~126 per iodine, on Tyr or His | Gamma counter | RIA, receptor saturation and competition binding | 59.4-day half-life; licensing and disposal requirements |
| Dabcyl / quencher | ~250 | Non-fluorescent acceptor | Paired with FAM in protease cleavage substrates | Only meaningful as half of a pair |
Fluorescent labels
Fluorescein derivatives are the default because the excitation and emission wavelengths match the filter sets on essentially every plate reader, microscope and flow cytometer built in the last thirty years. Two applications dominate:
- Fluorescence polarisation binding assays. A small labeled peptide tumbles fast and depolarises emitted light; bound to a large receptor or antibody it tumbles slowly and polarisation rises. The readout is homogeneous — no separation step — which makes it the workhorse for competition binding.
- FRET protease substrates. A donor fluorophore and a quencher flank a cleavage site. Intact, emission is quenched; cleaved, the fragments separate and signal rises. The design requires that both label positions be tolerated by the enzyme.
Two practical constraints are worth stating plainly. Fluorescein's emission falls off steeply below pH 7 because the phenolate form is the fluorescent one, so a labeled peptide in an acidic buffer can appear absent when it is merely dark. And fluorophores photobleach — keep stocks in the dark, minimise exposure during reads, and include a same-day reference well.
Biotin labels
Biotin's value is the affinity of the biotin–streptavidin interaction, with a dissociation constant around 10⁻¹⁴ M — effectively irreversible under normal assay conditions. That lets a biotinylated peptide be immobilised on a streptavidin-coated plate or bead and washed aggressively without loss, and it lets detection be amplified through a streptavidin–enzyme conjugate — the same chemistry underlying the sandwich formats in the assay kits range.
The recurring design question is the spacer. Biotin attached directly to a short peptide sits close enough to the sequence that streptavidin's binding pocket and the peptide's own binding partner compete sterically. Inserting an aminohexanoic acid (Ahx) spacer — 113.2 Da per unit, sometimes two — moves biotin roughly 10–20 Å away and typically restores both interactions. If a biotinylated peptide captures poorly, the spacer is the first thing to check.
Worked example: verifying a labeled peptide by mass
- Start from the unlabeled mass. Semax is Met-Glu-His-Phe-Pro-Gly-Pro at 813.93 Da.
- Add a 5-FAM label. Carboxyfluorescein couples through an amide bond with loss of water, adding roughly 358 Da: 813.93 + 358 = ≈1,172 Da.
- Add biotin with one Ahx spacer instead. 813.93 + 226.3 + 113.2 = ≈1,153 Da.
- Check against the certificate. An observed mass matching the unlabeled value means the coupling failed; a value 358 Da higher than expected on a singly labeled peptide means a second label went on. The method for reading these results is in mass spectrometry and peptide identity.
- Recompute molarity. The label is part of the molecule, so molar calculations must use the labeled mass — 1,172 rather than 814 is a 44% difference. See molecular weight, moles and molarity.
Radiolabels
Iodine-125 remains the most sensitive routinely available label for peptide work. It is introduced onto tyrosine or histidine residues, detected in a gamma counter with no substrate or amplification step, and reaches a theoretical maximum specific activity of about 2,176 Ci/mmol for a single iodine per molecule. That sensitivity is why radioligand binding remains the reference method against which fluorescence-based binding assays are validated, and why radioimmunoassay persists in neuropeptide work, much of it catalogued under neuropeptide research, where an ELISA lacks the required detection limit.
Worked example: decay and usable window
- Half-life. I-125 decays with a half-life of 59.4 days.
- After 30 days. Remaining activity = 0.5 raised to the power (30 ÷ 59.4) = 0.704. A 100 µCi preparation is now 70.4 µCi.
- After 120 days. 0.5 raised to (120 ÷ 59.4) = 0.247 — about a quarter of the original activity, and the practical end of most binding-assay windows.
- Consequence for design. Counting statistics degrade continuously, so specific activity must be recalculated at every use rather than taken from the shipping certificate, and long time-course studies need to account for the change between the first and last plate.
Radiolabeled peptides also carry licensing, storage, monitoring and disposal obligations that fluorescent and biotinylated alternatives do not, and radiolysis slowly damages the peptide itself. For many binding questions a fluorescence-polarisation format is now the pragmatic choice; for the lowest detection limits, radioiodine still wins.
Where to attach the label
- Label the end that is not the pharmacophore. Many signalling peptides bind through the C-terminus — particularly amidated ones — so N-terminal labelling is the usual default. Where the N-terminus matters, as in peptides recognised by aminopeptidase-sensitive receptors, the choice reverses.
- Side-chain labelling through a lysine ε-amine is possible but gives a mixture unless there is exactly one lysine, and mixtures behave inconsistently.
- Always compare against the unlabeled parent in the same assay. A label that shifts affinity ten-fold has invalidated the experiment quietly.
- Expect lower purity specifications. Labelling adds a coupling and a second purification, and labeled peptides are commonly released at slightly lower purity than their unlabeled counterparts.
Storage and handling
- Store lyophilized, at −20 °C or below, in the dark. Fluorophores are light-sensitive as solids as well as in solution.
- Protect fluorescent stocks from repeated warming; aliquot on first reconstitution.
- Keep fluorescein-labeled peptides at or above pH 7 for reads, and record the buffer pH alongside the signal.
- Handle radiolabeled material only under the applicable licence, with survey monitoring and dedicated waste routes.
The labeled range and its catalogue structure are described in labeled peptides: what they are and how to choose, browsable under labeled peptides in the research catalog.