The synthesis of six small peptide anion receptors based on thiourea and squaramide recognition moieties is
described. These new receptors bind to tetrahedral sulfate anions with remarkable affinity and selectivity in aqueous solution as shown by NMR spectroscopy. Molecular modelling suggests that selectivity is mediated by a hydrogen bond
network incorporating the amide backbone protons in a manner similar to that found in the sulfate-binding protein.