SPDP-PEG4-acid is a PEG derivative contianing SPDP crosslinker and carboxylic acid moieties. SPDP is reactive toward amine and thiol groups and contains a cleavable disulfide bond. Its membrane permeability allows for intracellular crosslinking reactions. The terminal caboxylic acid can react with primary amines in the presence of activators such as EDC and DCC to form stable amide bonds. The hydrophilic properties of the PEG linkers increase compound solubility in aqueous media. The water solubility properties of PEG linkers is enhanced by longer PEG chains.