A new peptide-based fluorescent probe selective for zinc(ii) and copper(ii).

A novel metal ion-sensitive fluorescent peptidyl-probe has been designed based on the most common five-residue repeat in mammalian histidine rich glycoproteins (HRGs). A dansyl-amide moiety at the N-terminus and a tryptophan residue at the C-terminus of the peptide were added as they can act as a FRET (fluorescence resonance energy transfer) pair. The dansyl fluorophore was chosen also because it frequently shows strong CHEF (chelation enhanced fluorescence) and solvatochromic effects. The designed peptide, dansyl-HPHGHW-NH (dH3w), showed a selective fluorescence turn-on response to Zn in aqueous solutions at pH 7.0 when excited at both 295 nm and 340 nm, thus indicating that both FRET and CHEF or solvatochromic effects are active in the metal/peptide complex. Steady-state fluorescence and isothermal titration calorimetry (ITC) measurements demonstrated that two peptide molecules bind to one zinc ion with an association constant K = 5.7 × 10 M at 25 °C and pH 7.0. The fluorescence response to Zn was not influenced by Pb, Cd, Mn, Fe, Fe, Mg, Ca, K and Na ions and only slightly influenced by Co and Ni. Copper(ii), at concentrations as low as 5 μM, caused a strong quenching of both free and Zn complexed dH3w. The determination of the binding parameters for Cu has shown that one copper ion binds to one dH3w molecule with an association constant of 1.2 × 10 M thus confirming the higher affinity of peptide for Cu than for Zn. Finally, we demonstrated that dH3w can penetrate into HeLa cells and could thus be used for the determination of intracellular Zn and Cu concentrations.