Hg2+ and Cd2+ binding of a bioinspired hexapeptide with two cysteine units constructed as a minimalistic metal ion sensing fluorescent probe

Hg²⁺ and Cd²⁺ 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 Hg²⁺-binding ability of the ligand was also described in an immobilized form, where the peptides were anchored to a hydrophilic resin. Hg²⁺ was demonstrated to form a 1 : 1 complex with the ligand even at pH = 2.0 while Cd²⁺ 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 (Cd²⁺) and 7.0 (Hg²⁺). Fluorescence studies demonstrated a Hg²⁺ induced concentration-dependent quenching of the Tyr fluorescence until a 1 : 1 Hg²⁺ : DY ratio. The fluorescence emission intensity decreases linearly with the increasing Hg²⁺ concentration in a range of over two orders of magnitude. The fact that this occurs even in the presence of 1.0 eq. of Cd²⁺ per ligand reflects a complete displacement of the latter metal ion by Hg²⁺ from its peptide-bound form. The immobilized peptide was also shown to bind Hg²⁺ 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 Hg²⁺ per immobilized DY molecule under Hg²⁺-excess conditions. Experiments performed with a mixture of four metal ions, Hg²⁺, Cd²⁺, Zn²⁺ and Ni²⁺, indicate that this molecular probe may potentially be used in Hg²⁺-sensing systems under acidic conditions for the measurement of μM range concentrations.

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-NH₂ (dH3w), showed a selective fluorescence turn-on response to Zn²⁺ 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_a = 5.7 × 10⁵ M^-1 at 25 °C and pH 7.0. The fluorescence response to Zn²⁺ was not influenced by Pb²⁺, Cd²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Mg²⁺, Ca²⁺, K⁺ and Na⁺ ions and only slightly influenced by Co²⁺ and Ni²⁺. Copper(ii), at concentrations as low as 5 μM, caused a strong quenching of both free and Zn²⁺ complexed dH3w. The determination of the binding parameters for Cu²⁺ has shown that one copper ion binds to one dH3w molecule with an association constant of 1.2 × 10⁶ M^-1 thus confirming the higher affinity of peptide for Cu²⁺ than for Zn²⁺. Finally, we demonstrated that dH3w can penetrate into HeLa cells and could thus be used for the determination of intracellular Zn²⁺ and Cu²⁺ concentrations.

Peptide-based biosensors

Peptides have been used as components in biological analysis and fabrication of novel biosensors for a number of reasons, including mature synthesis protocols, diverse structures and as highly selective substrates for enzymes. Bio-conjugation strategies can provide an efficient way to convert interaction information between peptides and analytes into a measurable signal, which can be used for fabrication of novel peptide-based biosensors. Many sensitive fluorophores can respond rapidly to environmental changes and stimuli manifest as a change in spectral characteristics, hence environmentally-sensitive fluorophores have been widely used as signal markers to conjugate to peptides to construct peptide-based molecular sensors. Additionally, nanoparticles, fluorescent polymers, graphene and near infrared dyes are also used as peptide-conjugated signal markers. On the other hand, peptides may play a generalist role in peptide-based biosensors. Peptides have been utilized as bio-recognition elements to bind various analytes including proteins, nucleic acid, bacteria, metal ions, enzymes and antibodies in biosensors. The selectivity of peptides as an enzymatic substrate has thus been utilized to construct enzyme sensors or enzyme-activity sensors. In addition, progress on immobilization and microarray techniques of peptides has facilitated the progress and commercial application of chip-based peptide biosensors in clinical diagnosis.