Fmoc-9-amino-4,7-dioxanonanoic acid is a specialized non-natural amino acid derivative. Its structure features a polyethylene glycol (PEG)-like spacer arm (4,7-dioxanonanoic acid) terminated with an amino group, which is protected by a 9-fluorenylmethoxycarbonyl (Fmoc) group. This design incorporates both a reactive handle and a hydrophilic linker into a single building block. This compound is primarily used in solid-phase peptide synthesis (SPPS) as a key building block. The PEG-based spacer introduces flexibility and hydrophilicity into the peptide chain, which is crucial for improving the solubility, bioavailability, and pharmacokinetic properties of synthetic peptides and peptide-drug conjugates. It is widely employed to create linkers between functional domains or to attach payloads in the development of therapeutic peptides and antibody-drug conjugates (ADCs). As a protected amino acid derivative with a unique bifunctional design, it also serves as a valuable organic synthesis building block for constructing molecules requiring hydrophilic spacers. It is commercially available from major suppliers of peptide synthesis reagents and fine chemicals for research and development purposes.