Fluorescent molecular sensing of amino acids bearing an aromatic residue

Abstract

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 zinc(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 π-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.