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 K1 for all other anions studied. Relative stabilities of 1:1 complexes in DMSO are AcO− F− Cl−, Br−, HSO4−, H2PO4−, but in CHCl3 they follow a different order: Cl− HSO4−>F− AcO−>Br−>H2PO4−. The binding mode of anions to thiostrepton is discussed on the basis of solvent effects on the complexation selectivity.