Biotin-X free acid (6-((Biotinoyl)amino)hexanoic acid) can be easily converted to the mixed anhydride form, which is more reactive than the succinimidyl ester and thus can be used to react with aromatic amines or sterically hindered amines.    White solid soluble in DMSO and moderately soluble in DMF  Store at 4°C  C16H27N3O4S  MW: 357.47  [72040-64-3]