Example 1: PEG-SS (Succinimidyl Succinate) pegylates amine groups under mild conditions by forming amide bond. The ester linkage in the backbone is susceptible to hydrolytic cleavage.

Example 2: PEG-SG (Succinimidyl Glutarate) pegylates amine groups of the target compound. The PEG-SG may be more resistant to hydrolysis than the PEG-SS because of an additional CH2 chain.

Example 3: PEG-NPC (p-nitrophenyl carbonate) is also reactive toward amines, and forms a stable urethane linkage.

Example 4: PEG-isocyanate is also a useful derivative for pegylation of amine groups resulting in a stable urethane linkage.

Example 5: PEG-aldehyde may pegylate amines through sodium cyanoborohydride reduction. The reaction pH may be important for target selectivity, and N-terminal amine pegylation may be at around pH 5.