Used in bioconjugation and pharmaceutical research to introduce a carboxylic acid functional group at the end of a long, hydrophilic PEG spacer. The Boc-protected amine allows for selective deprotection and further coupling in multi-step syntheses. Commonly employed in the development of peptide-based drugs, antibody-drug conjugates (ADCs), and PEGylated molecules to improve solubility, reduce immunogenicity, and extend circulation half-life. The five-unit PEG chain provides flexibility and optimal spacing between functional moieties in complex molecular architectures.