Anion Capture

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Science  27 Jul 2001:
Vol. 293, Issue 5530, pp. 575
DOI: 10.1126/science.293.5530.575d

Anions in biological systems often are captured and bound by hydrogen bonds alone. In contrast, artificial systems for anion complexation in water generally require stronger forces, such as electrostatic interactions, to overcome the high solvation energy of many anions.

Kubik et al. describe a ligand (made from proline and 6-aminopicolinic acid) that works in spite of the absence of such strong interactions. Their system resembles a molecular capsule, with two identical cyclic hexapeptides assembling to form a cavity that holds the anion and shields it from solvent. Complexes of halides and sulfate were detected, and a crystal structure of the complex with iodide showed that there are no direct interactions between the peptides. Instead, the assemblage is stabilized entirely by hydrogen-bonding interactions between the anion and the N-H groups of the ligands. This system may serve as a model for the recognition of anions by natural receptors and as a complement to the established recognition of cations by the other portion of the peptide bond, the carbonyl group. — JU

Angew. Chem. Int. Ed.40, 2648 (2001).

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