Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors

4 years ago

Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors

Stefan Kubik

Anion receptors often rely on coordinative or multiple ionic interactions to be active in water. In the absence of such strong interactions, anion binding in water can also be efficient, however, as demonstrated by a number of anion receptors developed in recent years. The cyclopeptide-derived receptors comprising an alternating sequence of l-proline and 6-aminopicolinic acid subunits are an example. These cyclopeptides are neutral and, at first sight, can only engage in hydrogen-bond formation with an anionic substrate. Nevertheless, they even interact with strongly solvated sulfate anions in water. The intrinsic anion affinity of these cyclopeptides can be related to structural aspects of their highly preorganized concave binding site, which comprises a wall of hydrophobic proline units arranged around the peptide NH groups at the cavity base. When anions are incorporated into this cavity they can engage in hydrogen-bonding interactions to the NH groups, and complex formation also benefits from cavity dehydration. Formation of 1:1 complexes, in which an anion binds to a single cyclopeptide ring, is associated with only small stability constants, however, whereas significantly more stable complexes are formed if the anion is buried between two cyclopeptide molecules.

Publisher URL: http://dx.doi.org/10.1021/acs.accounts.7b00458

DOI: 10.1021/acs.accounts.7b00458