Fluorescent molecular sensing of amino acids bearing an aromatic residue

The tetra-amino tripodal ligand, 3, containing two anthracene subunits, has been prepared through a multi-step synthesis and its solution properties have been studied by potentiometric and spectrofluorimetric techniques. The zine(II) complex, [Zn-II(3)](2+), which displays the typical emission of anthracene derivatives. interacts with natural amino acids, showing a particular affinity towards phenylalanine and tryptophan. This selective behaviour of the complex derives from its ability to establish two kinds of interaction: (i) a metal-ligand interaction between the zinc(ii) ion and the amino acid's carboxylate group; (ii) a pi -stacking interaction involving the aromatic moieties positioned on the complex and on the amino acid, inducing an extra stability of the adducts with tryptophan and phenylalanine (7-8 kJ mol(-1)). The formation of the 1 : 1 adduct with tryptophan is signalled by a strong fluorescence quenching while no effect on the emission intensity has been observed in all other cases.