N-[6-(Biotinamido)hexyl]-3’-(2’-pyridyldithio)-propionamide (Biotin-HPDP) is a sulfhydryl reactive biotinylation reagent with a long chain, cleavable spacer arm. Biotin-HPDP biotinylation occurs between pH 6-9 through its pyridyldithiol group reacting with sulfhydryls to form a stable disulfide bond. The 29.2 angstrom spacer arm prevents steric hindrance and may also be cleaved at the disulfide bond using TCEP-HCl, DTT or ß-Mercaptoethanol. Biotin-HPDP must be dissolved in an organic solvent, such as DMSO or DMF, then added to an aqueous biotinylation reaction. The resulting biotinylated protein can then be detected when used in conjunction with streptavidin or Immobilized Streptavidin.