A Galα1-3lactose (α-Gal epitope) ligand equipped with a terminal carboxylic acid through a hydrophilic PEG4 spacer. The carboxylic acid can be activated to NHS, sulfo-NHS, or pentafluorophenyl esters and reacted with amine-functionalized platforms or lysine residues on proteins and peptides to form amide bonds. This compound presents a terminal Galα1-3 linkage corresponding to the α-Gal epitope (Galα1-3Galβ1-4GlcNAc / Galα1-3lactose), the target of abundant naturally occurring human anti-Gal antibodies and central to xenotransplantation and anti-Gal immunology research.