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Using dynamic combinatorial disulfide chemistry we have developed a new generation of neutral synthetic receptors for anions, based on a macrobicyclic peptide structure. These receptors show an exceptional affinity and selectivity for sulfate ions in aqueous solution [log K(a) = 8.67 in 41 mol% (67 volume%) acetonitrile in water]. The high affinity depends on a delicate balance between rigidity and flexibility in the structure of the receptor.

Cyclopeptides are an important class of substances in nature, and their physiological effects are frequently based on the tendency to form bioactive conformations. Therefore the investigation of their structure yields an understanding of their functionalities. Mass-selective combined IR/UV spectroscopy in molecular beam experiments represents an ideal tool for structural analyses on isolated molecules in the gas phase, such as the investigated cyclo[L-Tyr(Me)-D-Pro](2) peptide and its complexes with water. Using the chosen spectroscopic method in combination with DFT calculations, an assignment of a structure with two intramolecular hydrogen bonds for the naked cyclopeptide is possible. For the monohydrated cluster two isomers have to be discussed: in one of them the water molecule is simply attached to the assigned monomer structure as hydrogen donor, whereas the second isomer can be characterized by a water molecule that is inserted into one of the intramolecular hydrogen bonds.

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Fluorescence of a bis(cyclopeptide) in which two cyclohexapeptide moieties with alternating L-proline and 6-aminopicolinic acid subunits are attached to a 4,4'-bis(dimethylamino)biphenyl linker is quenched in 1:1 (v/v) water/methanol in the presence of sulfate. Of eight other anions tested, none produced a similar effect. This bis(cyclopeptide) allows the qualitative and quantitative detection of sulfate even in the presence of an excess of chloride anions. Calculations provided insight into the causes of fluorescence quenching and anion selectivity.