Used in bioconjugation processes to link biomolecules such as peptides, proteins, or antibodies with other functional groups, especially in the development of antibody-drug conjugates (ADCs) and targeted therapeutics. The aldehyde group (Ald) enables selective reaction with amines or hydrazides under mild conditions, forming stable conjugates. The PEG4 spacer improves solubility, reduces aggregation, and enhances the pharmacokinetic properties of the final construct. The carboxylic acid terminus allows for further activation and coupling to amines via standard amide bond formation. Commonly applied in pharmaceutical research, diagnostic probes, and PEGylation strategies where controlled, site-specific modification is required.