A labeled peptide is a peptide carrying an extra feature that makes it easy to detect, measure or pull out of a mixture. In plain terms, the label is a handle. Stable-isotope labels make it heavier for mass spectrometry, fluorescent labels make it glow in an assay, and a biotin tag lets streptavidin capture it.
- A labeled peptide carries an added feature that makes it easier to detect, measure or capture.
- Stable-isotope labels change mass, not chemistry, and are read by mass spectrometry.
- SILAC (Ong et al., 2002) used deuterated leucine and reported complete incorporation after five cell doublings.
- Dye and quencher pairs, such as EDANS and DABCYL, give a signal when a protease cuts the peptide between them.
- Biotin tags bind streptavidin, allowing labeled peptides to be captured from complex mixtures.
What a label does
A researcher has a tube holding thousands of different peptides and cares about exactly one of them. How does she find it, count it, or fish it out? She gives it a tag in advance. That tag is the label.
A labeled peptide is an ordinary peptide sequence with one addition that makes it stand out to a particular instrument or reagent. The peptide part does the biology or chemistry being studied. The label is there for the scientist, so that the peptide can be seen, weighed or grabbed without confusing it with everything else in the sample.
Three families of label cover most lab work. Stable isotopes change a peptide's mass without changing its chemistry. Fluorescent groups make it give off light. Biotin gives it a handle that binds tightly to a protein called streptavidin. Each one answers a different question, and each needs a different instrument to read it.
Labels ride on the functional groups already present in a peptide, or they are built into the amino acids themselves during synthesis. Where the label sits matters, because a tag placed on a residue the experiment depends on can change the result.
- Stable isotope: heavier mass
- Fluorescent dye: gives off light
- Dye and quencher: signal on cleavage
- Biotin: capture with streptavidin
What is a stable-isotope label?
An isotope label swaps some atoms for heavier, non-radioactive versions of the same element, such as deuterium for hydrogen. The peptide's chemistry barely changes, but its mass goes up by a fixed and known amount. A mass spectrometer sees the light and heavy versions as two separate peaks, while a chromatography column treats them almost the same.
That pairing is the basis of quantitative proteomics. In 2002 Ong and colleagues described SILAC, stable isotope labeling by amino acids in cell culture. Cells were grown in media lacking a standard essential amino acid and supplemented with a labeled form, in their case deuterated leucine (Leu-d3). They reported complete incorporation after five cell doublings, and every leucine-containing peptide then carried either all normal leucine or all Leu-d3. Mixing labeled and unlabeled samples let them compare protein levels peak by peak.
Synthetic labeled peptides do a related job. Gerber and colleagues in 2003 described AQUA, in which peptides are synthesized with incorporated stable isotopes to act as internal standards that mimic the native peptides formed when proteins are digested. A known amount of the heavy standard is added to a sample, and the ratio of light to heavy signal in a tandem mass spectrometer gives an absolute quantity. They also made standards carrying phosphorylation, methylation and acetylation, chemically identical to natural modifications.
For the arithmetic behind those peaks, see our guide to the m/z ratio.
How do fluorescent labels work?
A fluorescent label is a dye group attached to the peptide that absorbs light at one wavelength and emits it at another. Plate readers, microscopes and flow instruments can pick up that signal at very low levels, which makes fluorescent peptides common in assays that need a continuous readout.
The classic design pairs two groups on one peptide. Matayoshi and colleagues, writing in Science in 1990, built a substrate for the HIV-1 protease with the quencher DABCYL at one end and the fluorescent group EDANS at the other, joined by the sequence Ser-Gln-Asn-Tyr-Pro-Ile-Val-Gln taken from a natural processing site. While the peptide is intact, the quencher sits close enough to absorb the fluorescent group's energy through resonance energy transfer, and the signal stays low. When the protease cuts the chain, the two separate and the fluorescence rises. That gave a continuous assay of enzyme activity in a test tube.
A 1996 review of intramolecular fluorescence energy transfer substrates made the general point: because donor and acceptor can be placed around any bond in the chain, the design lets a researcher target cleavage at a chosen point and compare enzymes of different specificity.
Fluorescent labels are bulky compared with an amino acid, so they are usually placed at the ends or on a side chain away from the region being studied.
Biotin labels
Biotin is a small molecule that binds very tightly to the avidin family of proteins, which includes streptavidin. A 2020 paper on biotinylated peptides describes it as a versatile molecular handle, prized for its small size and high affinity, that allows enrichment and purity not easily reached with other reagents.
In practice, a biotin-labeled peptide can be pulled out of a complex mixture with streptavidin-coated beads. A 1996 study coupled that capture step directly to MALDI-TOF mass spectrometry: biotinylated peptides were bound on magnetic streptavidin beads, released into the matrix solution and measured, with detection limits of less than 100 femtomoles. The authors pointed to peptide and protein interaction studies as the main use.
Biotin also works the other way round. Proximity-labeling methods such as APEX and BioID attach biotin to proteins near a protein of interest inside a sample, and the 2020 paper notes that these newer methods need the exact biotinylated residue identified. Its authors describe signature fragment ions that biotinylated peptides produce in tandem mass spectrometry, which helps locate the tag.
Biotin tags are usually joined through an amine, such as the N-terminus or a lysine side chain, often with a spacer so the bulky streptavidin can reach it.
Choosing a label
| Label | What it changes | Read by | Typical lab use |
|---|---|---|---|
| Stable isotope | Mass only | Mass spectrometry | Relative or absolute quantification (SILAC, AQUA-type standards) |
| Fluorescent dye or dye-quencher pair | Light absorbed and emitted | Fluorometer, plate reader, microscope | Continuous enzyme assays, binding and localization studies |
| Biotin | Adds a capture handle | Streptavidin beads or surfaces, then any detector | Enrichment, pull-down, interaction studies |
The question to ask first is what you need to measure. If the answer is how much, an isotope-labeled standard read by mass spectrometry is the usual route. If the answer is when, as in a cleavage reaction you want to follow minute by minute, a fluorescent pair fits. If the answer is which molecules bind to the peptide, a biotin tag lets you pull out the complex. Two labels can share one peptide when an experiment needs both capture and detection.
Plan a control alongside the labeled peptide. The same sequence without its tag shows whether the label itself changes the behavior being measured, whether that is binding, cleavage or how the peptide runs on a column. If the labeled and unlabeled forms behave alike in the parts of the experiment that do not depend on the tag, the label can be trusted to report without interfering. If they differ, the label may need to move to another residue or sit behind a longer spacer. Isotope labels rarely raise this problem, since the heavy and light forms are chemically the same peptide, while bulky dyes and biotin are the tags most worth checking.
Checking a labeled peptide
A label adds a new thing to verify. Identity testing has to confirm the peptide and the label together: the observed mass should match the sequence plus the tag, and for isotope labels, the heavy form should sit at the expected offset from the light one. The same LC-MS identity testing used for unlabeled peptides applies, with the theoretical mass rebuilt to include the label.
Purity matters in a specific way here. Unlabeled or partly labeled material in a heavy standard can blur the very ratio the standard is meant to measure, and a free dye in a fluorescent substrate raises the background. A certificate for a labeled peptide should state where the label sits, which form it is, and how identity and purity were measured.
Our catalog peptides are supplied unlabeled. Results for our own lots appear only once the certificate for that lot is published on the certificate index.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
Common questions
What is a heavy-labeled peptide?
Are isotope-labeled peptides radioactive?
How does a fluorescent protease substrate work?
Why attach biotin to a peptide?
More science guides
Sources
- Ong SE et al., Stable isotope labeling by amino acids in cell culture, SILAC, Mol Cell Proteomics 2002 (Europe PMC record). Leu-d3 labeling, complete incorporation after five doublings, relative quantification by mass spectrometry.
- Gerber SA et al., Absolute quantification of proteins and phosphoproteins from cell lysates by tandem MS, PNAS 2003 (Europe PMC record). AQUA: synthetic stable-isotope peptides as internal standards, including modified forms.
- Matayoshi ED et al., Novel fluorogenic substrates for assaying retroviral proteases by resonance energy transfer, Science 1990 (Europe PMC record). DABCYL and EDANS on a peptide from an HIV-1 protease processing site; continuous assay.
- Gershkovich AA, Kholodovych VV, Fluorogenic substrates for proteases based on intramolecular fluorescence energy transfer (IFETS), J Biochem Biophys Methods 1996 (Europe PMC record). Donor and acceptor placement lets cleavage be targeted at any point in the chain.
- Girault S et al., Coupling of MALDI-TOF mass analysis to the separation of biotinylated peptides by magnetic streptavidin beads, Anal Chem 1996 (Europe PMC record). Streptavidin bead capture of biotinylated peptides with detection below 100 fmol.
- Renuse S et al., Signature Fragment Ions of Biotinylated Peptides, J Am Soc Mass Spectrom 2020 (Europe PMC record). Biotin as a small, high-affinity handle; APEX and BioID; locating biotinylation sites by LC-MS/MS.
