Glossary
Biotin Label
Biotin tags give a peptide a streptavidin handle of femtomolar strength. Where they are used in assays, and the linker, tissue biotin and recovery caveats.
A biotin tag is biotin (vitamin B7) chemically coupled to a peptide, most often on the amino terminus or a lysine side chain. What makes it useful is its binding to streptavidin: the dissociation constant lies in the femtomolar range, making it one of the tightest non-covalent interactions known in biology, and the complex survives detergent, salty buffers and mild denaturing conditions.
Applications and practical limits
Biotin-tagged peptides are the go-to reagent for pull-down experiments and target identification using streptavidin magnetic beads, for immobilisation on streptavidin-coated SPR sensor chips or ELISA wells, and in signal-amplified detection in which a streptavidin-HRP conjugate takes the place of a labelled secondary antibody.
Three limitations come up repeatedly. The first is spacer length: biotin’s valeric acid tail is brief and sits deep inside the binding cleft, which is why an Ahx or PEG linker normally separates tag and sequence. The second is endogenous biotin: liver, kidney, brain and similar tissues carry free biotin at levels that compete, so an avidin/biotin blocking step becomes necessary. The third is elution: the bond is so tight that intact complexes can generally only be recovered by boiling in SDS, by long incubation with excess free biotin, or through a cleavable linker planned into the design from the outset.
Related terms and material
See also purity, the FAM/FITC label and the I-125 radiolabel entry. Matching catalogue sections cover tagged peptides and tagged antibodies; our guide to peptide labels goes deeper.