Used in bioconjugation processes, particularly in the synthesis of peptide and protein derivatives. The amino-PEG4-CH2 chain with a protected carboxylic acid group allows for selective coupling reactions in complex molecular assemblies. The PEG4 spacer enhances water solubility, reduces protein aggregation, and provides flexibility in linker design. Commonly applied in the development of antibody-drug conjugates (ADCs) and targeted therapeutics where controlled linker stability and solubility are critical. The tert-butyl ester protection enables stepwise synthesis, offering compatibility with solid-phase peptide synthesis and orthogonal deprotection strategies. Also utilized in creating spacers to reduce steric hindrance in labeled biomolecules for imaging and diagnostic applications.