Mal-PEG6-PFP is a PEG derivative with maleimide and PFP moieties. Maleimides are thiol reactive between pH 6.5 and 7.5, forming thiolester bonds. The PFP is amine reactive and forms stable amide bonds. PFP was also found to be less susceptible toward hydrolysis than other amine reactive groups. The hydrophilic PEG linker increases the water solubility of a compound in aqueous environments. Increasing the number of units in the PEG chain ehnahces their solubility properties.