Selective sensing of sulfate in aqueous solution using a fluorescent bis(cyclopeptide)

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.