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N-hydroxylsuccinimide (NHS) functionalized polyethylene glycol (PEG-NHS) is an amino (-NH₂) reactive PEG derivative that can be used to modify protein, peptide or any other surfaces with their available amino groups. NHS esters react with primary amine groups at pH 7~8.5 to form stable amide bonds.Compared to other PEG NHS ester derivatives, our succinimidyl carbonate (SC) functionalized mPEG-NHS offers superior reactivity and higher stability in aqueous solution. NHS esters react with deprotonated primary amines, therefore, the reaction requires neutral to basic pH values to proceed. Primary amines react with NHS esters by nucleophilic attack and NHS is released as a byproduct. Hydrolysis of the NHS-ester competes with the reaction in aqueous solution and increases with increasing pH.