DBCO-PEG4-Maleimide allows for simple combination of a DBCO group and thiol-containing molecules, such as cysteine-containing peptides. The maleimide group reacts with sulfhydryl groups at pH 6.5 – 7.5 to form stable thioether bonds. Its long length enhances accessibility of the azide-reactive DBCO and reactive thiols, which may be buried. The polyethylene glycol (PEG) spacer arm is hydrophilic and improves solubility in aqueous buffers.