TCO-PEG4-biotin a PEG derivative containing TCO and biotin moieties. This reagent can be used to derivatize carboxyl groups or activated esters (e.g. NHS esters) with the TCO moiety through the formation of a stable amide bond in the presence of activators (e.g. EDC, or DCC). The hydrophilic PEG spacer improves the water solubility of compounds. The biotin group can react with strepavadin and avadin groups.