Biotin-HPDP is a thiol-specific biotinylating reagent that forms a reversible disulfide bond with free cysteine residues on proteins or peptides. It is widely used to selectively tag and detect sulfhydryl groups, especially in biochemical assays. In the Biotin Switch assay, Biotin-HPDP labels newly exposed cysteine thiols after reduction of S-nitrosylated or S-sulfhydrated proteins, enabling detection and purification via streptavidin-based methods.