Anion recognition by thiostrepton

A bicyclic polypeptide antibiotic thiostrepton forms both 1:1 and 1:2 complexes with anions (as tetrabutylammonium salts) in organic solvents with K2 >K1 for F- and K2<>Cl-, Br-, HSO4-, H2PO4-, but in CHCl3 they follow a different order: Cl- approximatelyHSO4- >F- approximately AcO- > Br > H2PO4-. The binding mode of anions to thiostrepton is discussed on the basis of solvent effects on the complexation selectivity.