The main research area of the group is supramolecular chemistry. An important research topic is the development of synthetic anion receptors, most of which are based on cyclic peptides and pseudopeptides. Of particular interest is the recognition of anions in water and the understanding of the underlying principles. In addition to macrocyclic receptors, gold nanoparticles and polymeric materials have recently been used for anion binding. A completely different research topic, but also rooted in supramolecular chemistry, is the development of compounds that rapidly convert neurotoxic organophosphates into non-toxic metabolites under physiological conditions, allowing their use to treat poisoning with these nerve agents.
While cation affinity of the cyclopeptides is due to the aromatic subunits and the carbonyl groups along the macrocyclic cavity, anions can bind to the cyclopeptide NH groups. Some cyclopeptides bind both components of an ion-pair simultaneously. A glutamic acid containing cyclic hexapeptide, for example, interacts with sulfonates or phosphonates by adopting a conformation with all six NH groups converging toward the center of the macrocyclic cavity. Complexation of the anions increases cation affinity because it improves preorganization of the peptide for cation binding and allows the included cation inside the cyclopeptide cavity to interact with the amino acid residues and the anion.1
A similar behavior was observed for the proline-containing cyclopeptide. In this case, however, even weakly coordinating anions can be bound because, in contract to the glutamic acid containing derivative, the NH groups of this peptide are not involved in intramolecular hydrogen bonds in the absence of guests.2 The simultaneous interaction with both components of the ion pair is clearly visible in the crystal structure of the N-methylquinuclidinium iodide complex.
[For an interactive version of the crystal structure click here]
Yet, a specific complexation of the anion is not required for an anion effect on cation complexation. Also anions that are not bound by the cyclopeptide but are closely associated to the cation in organic solvents can influence cation affinity. Investigations carried out in collaboration with the group of Jerôme Lacour showed, for example, that the stability of the complexes of a cyclopeptide with a chiral quaternary ammonium ion does not only depend on the absolute configuration of the cation but also on that of the chiral TRISPHAT counterion, which does not specifically bind to the cyclopeptide.3
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