Preparation and characterization of poly(ethylene glycol) vinyl sulfone

Synthesis of the vinyl sulfone and chloroethyl sulfone derivatives of poly(ethylene glycol) (PEG) is described. The chloroethyl sulfone (CES-PEG) is rapidly converted to the vinyl sulfone (VS-PEG) in the presence of base but is stable in water at neutral pH. Reactions with small molecules such as beta-mercaptoethanol and N-alpha-acetyllysine show that the vinyl sulfone derivative is highly selective for reaction with sulfhydryl groups relative to reaction with amino groups. Also, VS-PEG is stable in water. These properties indicate that VS-PEG should be useful for selective attachment of PEG to protein cysteine groups. This hypothesis was verified by reacting VS-PEG with cysteine groups of reduced ribonuclease (RNase); the reaction is rapid and selective at pH 7-9. Reaction at lysine sites of unreduced RNase occurs slowly at pH 9.3 and is essentially complete after 100 h. Amino acid residues other than lysine and cysteine are not reactive toward VS-PEG The covalent linkage between VS-PEG and lysine or cysteine groups is shown to be stable.