A biotin label is a molecule of biotin (vitamin B7) conjugated to a peptide, normally at the N-terminus or on a lysine side chain. Its value is the streptavidin interaction: a dissociation constant in the femtomolar range, one of the strongest non-covalent bindings in biology, stable to detergents, high salt and moderate denaturants.
How it is used, and what constrains it
Biotinylated peptides are the standard tool for pull-downs and target identification on streptavidin magnetic beads, for immobilisation on streptavidin-coated ELISA plates and SPR chips, and for amplified detection where a streptavidin-HRP conjugate replaces a labelled secondary antibody.
Three practical constraints recur. First, spacer length: biotin's valeric acid tail is short and the binding pocket is deep, so an aminohexanoic acid or PEG linker is normally inserted to keep the peptide accessible. Second, endogenous biotin: tissues such as liver, kidney and brain contain enough free biotin to compete, requiring an avidin/biotin blocking step. Third, elution: the interaction is so strong that recovering intact complexes generally requires boiling in SDS, excess free biotin over long incubation, or a cleavable linker designed in from the start.
Related terms and material
FAM/FITC label · I-125 radiolabel · purity. Reference material: labeled peptides, labeled antibodies. Further reading: labeled peptides guide.