Fmoc-aminooxy-PEG4-acid (CAS No.: 1895922-70-9) is an organic compound containing a fluorenylmethyloxycarbonyl (Fmoc) protected aminoxy group, a tetraethylene glycol (PEG4) spacer arm, and a terminal carboxyl group. Its molecular formula is C₂₆H₃₃NO₉, with a molecular weight of 503.54. In its structure, the Fmoc group provides reversible amino protection under basic conditions, the PEG4 chain imparts molecular flexibility and water solubility, and the terminal carboxyl group can participate in condensation reactions or conjugate with biomolecules. The production process employs solid-phase synthesis technology, constructing peptide chains stepwise using the Fmoc protection strategy, followed by deprotection and purification to obtain a high-purity product. This compound is widely used in PROTAC molecule design, peptide-drug conjugate (PDC) development, and bioconjugate chemistry as a flexible linker to optimize molecular conformation and pharmacokinetic properties.