OPSS-PEG-NHS is a linear heterobifunctional PEG reagent with an OPSS and a NHS ester. It is a useful crosslinking reagent with a PEG spacer. OPSS stands for ortho-pyridyl disulfide or ortho pyridyldithio and reacts with thiol, SH, sulfhydryl or mercapto to form a S-S bond. OPSS is sometimes called PDP, pyridyl dithio propionate. NHS ester is an activated carboxylic acid and can react with amine.