BAPTA Maleimide is an excellent building block to introduce BAPTA calcium chelator into a biological molecule that contains a thiol group such as antibodies, proteins, peptides and thiol-modified oligos. BAPTA (1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid) is a common calcium-specific chelator. The presence of four carboxylic acid functional groups makes BAPTA possibly binding two calcium ions. The extensive flexibility of the carboxylate ligands is critical to the coordination of calcium and other metal ions. BAPTA is commonly used to chelate Ca2+, similarly to EGTA.