Enantioselective fluorescence quenching by a chiral copper(II) complex in ligand exchange equilibria

Abstract

The first case of enantioselective fluorescence quenching of dansyl(DNS)-amino acids (2) by a chiral complex, bis-(L-phenylalanylamidato)copper(II)(1) in aqueous solution is reported. The nature of the quenching, whether static or dynamic, has been investigated by time-resolved fluorescence techniques. The fluorescence lifetimes of the unquenched D- and L-DNS-phenylalanine were found to be the same as the quenched ones: 90% of the total fluorescence intensity for all samples was due to a 8.9 ns component. Thus, the decrease in the fluorescence intensities was ascribed to ground state interactions between the dansyl moiety and the copper complex (static quenching). From the Stern–Volmer plot it was possible to calculate the formation constants of the ternary diastereometric complexes formed according to a ligand exchange reaction. The difference between the logarithms of the stability constants of these complexes accounts for the enantio-selectivity observed in aqueous solutions. The present results are relevant also for the understanding of the mechanism of chiral recognition in HPLC using chiral copper(II) complexes as additives to the eluent (Chiral Ligand Exchange Chromatography), and must be taken into account for correct quantitative determination.