OPSS-PEG-COOH is a linear heterobifunctional PEG reagent with an OPSS and a carboxyl group. It is a useful crosslinking reagent with a PEG spacer. OPSS stands for orthopyridyl 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. The COOH can react with amine or hydroxyl under standard acid/amine or acid/alcohol coupling conditions.