How to improve guanidinium cations for oxoanion binding in aqueous solution?: The design of artificial peptide receptors

Guanidinium cations are a prominent recognition motif for oxoanion binding both in Nature and in abiotic systems. However, simple ion pairing based on alkyl guanidinium cations is not strong enough to achieve an efficient complexation in aqueous solvents. Nature uses the less polar microenvironment of a protein to shield the ion pair from the solvent thereby increasing complex stability. For artificial supramolecular systems other ways to improve the binding affinity of guanidium cations have to be found. We will describe herein our use of modified acylguanidinium cations with additional H-bond donor sites to achieve oxoanion binding in aqueous solvents. The thermodynamic characterization of such systems is described as well as some applications from the field of bioorganic chemistry (e.g. artificial receptors for anionic biomolecules).