Used in bioconjugation reactions, particularly in click chemistry applications. The propargyl group enables copper-catalyzed azide-alkyne cycloaddition (CuAAC), allowing efficient labeling or attachment to azide-containing biomolecules such as peptides, proteins, or nucleic acids. The PEG4 spacer improves solubility and provides flexibility, enhancing binding accessibility and reducing steric hindrance in complex biological systems. Commonly applied in the development of antibody-drug conjugates (ADCs), diagnostic probes, and targeted therapeutics. The aldehyde (Ald) group allows site-specific conjugation to aminooxy or hydrazide moieties, enabling orthogonal coupling strategies in multi-step labeling protocols.