The pivotal role of copper(II) in the enantiorecognition of tryptophan and histidine by gold nanoparticles

Stereoselective amino acid analysis has increasingly moved into the scope of interest of the scientific community. In this work, we report a study on the chiral recognition of D, L-Trp and D, L-His using L -Cys-capped gold nanoparticles (AuNPs) and copper(II) ion. In the L-Cys-capped AuNPs, the thiol group of the amino acid interacts with AuNPs through the formation of Au-S bond, whereas the alpha-amino and alpha-carboxyl groups of the surface-confined cysteine can coordinate the copper(II) ion, which in turn, binds the L- or D-amino acid present in solution forming diastereoisomeric complexes. The resulting systems have been characterized by UV-Vis spectra and dynamic light scattering measurements, obtaining different results for L- and D-Trp, as well as for L- and D-His. The knowledge of the solution equilibria of the investigated systems allowed us to accurately calculate in advance the concentrations of the species present in solution and to optimize the system performances, highlighting the pivotal role of copper(II) ion in the enantio-discrimination processes.