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.
Anions can bind to the NH groups of cyclopeptides by hydrogen bond formation. Cyclic hexapeptides with 3-aminobenzoic acid subunits and glutamic acid or proline residues adopt conformations with converging NH groups upon complexation of sulfonate that are optimal for anion binding. Hence, the optimal preorganization for anion-binding of a cyclopeptide that prefers such a conformation also in the absence of the guest should cause high affinity. The characterization of the binding properties of a cyclic hexapeptide with L-proline und 6-aminopicolinic acid subunits confirmed this assumption.1
The converging arrangement of the three NH groups of this cyclopeptide is clearly visible in the crystal structure.
[For an interactive version of the crystal structure click here]
This peptide binds anions such as iodide or phenylsulfonate in DMSO in the form of 1:1 complexes.2 Interestingly, complexation of halides or sulfate is even possible in highly competitive aqueous solvent mixtures, for example in 80% water/methanol. Under these conditions, sandwich-type 2:1 complexes are, however, formed in which the anion is positioned between two interdigitating cyclopeptide rings binding to six NH groups simultaneously. This arrangement was also detected in the crystalline iodide complex. This cyclopeptide represents one of the few neutral systems that allows the complexation of anions in aqueous media.
Formation of the 2:1 complex in aqueous solvent mixtures is a cooperative process since the formation of the 2:1 complex from the 1:1 complex is associated with a significantly larger equilibrium constant than the formation of the 1:1 complex from the components.3
The 2:1 complexes could be converted into 1:1 complexes by covalently linking two cyclopeptide rings together which improved complexation efficiency significantly.4
Water-soluble bis(cyclopeptides) were synthesized recently, which bind anions in water.5 Their anion affinity in water and organic solvents could be correlated with the properties of the anions and the solvent.
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