Hg2+ and Cd2+ binding of a bioinspired hexapeptide with two cysteine units constructed as a minimalistic metal ion sensing fluorescent probe. Issue 23 (21st May 2019) - Szekeres, Levente I.
Abstract : Efficient and selective Hg 2+ binding of a hexapeptide containing two cysteine residues is demonstrated both in solution and immobilized forms. Abstract : Hg 2+ and Cd 2+ complexation of a short hexapeptide, Ac-DCSSCY-NH2 (DY), was studied by pH-potentiometry, UV and NMR spectroscopy and fluorimetry in aqueous solutions and the Hg 2+ -binding ability of the ligand was also described in an immobilized form, where the peptides were anchored to a hydrophilic resin. Hg 2+ was demonstrated to form a 1 : 1 complex with the ligand even at pH = 2.0 while Cd 2+ coordination by the peptide takes place only above pH ∼ 3.5. Both metal ions form bis-ligand complexes by the coordination of four Cys-thiolates at ligand excess above pH ∼ 5.5 (Cd 2+ ) and 7.0 (Hg 2+ ). Fluorescence studies demonstrated a Hg 2+ induced concentration-dependent quenching of the Tyr fluorescence until a 1 : 1 Hg 2+ : DY ratio. The fluorescence emission intensity decreases linearly with the increasing Hg 2+ concentration in a range of over two orders of magnitude. The fact that this occurs even in the presence of 1.0 eq. of Cd 2+ per ligand reflects a complete displacement of the latter metal ion by Hg 2+ from its peptide-bound form. The immobilized peptide was also shown to bind Hg 2+ very efficiently even from samples at pH = 2.0. However, the existence of lower affinity binding sites was also demonstrated by binding of more than 1.0 eq. of Hg 2+ per immobilized DY molecule under Hg 2+ -excess conditions. Experiments performed with a mixture of four metal ions, Hg 2+, Cd 2+, Zn 2+ and Ni 2+, indicate that this molecular probe may potentially be used in Hg 2+ -sensing systems under acidic conditions for the measurement of μM range concentrations.