The synthesis and characterization of an active poly(ethylene glycol) (PEG) derivative with new properties has been afforded starting from a side reaction of N-hydroxysuccinimide (NHS) in presence of N,N-dicyclohexylcarbodiimide (DCC). In particular, a ring opening of NHS in presence of DCC forms a P-alanine active derivative, through Lossen rearrangement, which then reacts with PEG-NH2 yielding the active PEG derivative: PEG-NH-CO-beta Ala-NH-CO-NHS. The active group R-NH-CO-NHS showed lower reactivity towards amines and a higher stability in alkaline solution. This can be especially useful in polymer coupling to proteins, because it may lead to a better selectivity among all the amino groups present in a protein, thus yielding less heterogeneous PEG-protein conjugates mixture because only the most nucleophilic and solvent exposed amines can react. A comparison with one of the most used PEGylating agent, the PEG-succinimidyl carboxymethylate (PEG-O-CH2-CO-NHS), displayed the usefulness of this new PEG derivative for obtaining PEG-protein conjugates. Furthermore, it is of interest that the obtained protein-conjugates are slowly hydrolyzed in water releasing the free protein, therefore they can be considered protein prodrugs. (c) 2007 Elsevier Ltd. All rights reserved.