Biotin-PEG-Alk (MW 3400), a biotinylated polyethylene glycol (PEG) compound with an alkyne group, serves as a versatile click chemistry reagent. The biotin moiety acts as an enzyme cofactor desirable for protein labeling, PEG imparts hydrophilicity and aqueous solubility with minimal toxicity, and the alkyne (a type of acyclic hydrocarbon) facilitates efficient target binding by reacting with azides via copper-catalyzed azide-alkyne cycloaddition (CuAAC).