Synthesis of highly selective fluorescent peptide probes for metal ions: tuning selective metal monitoring with secondary structure

A Highly Selective and Sensitive Peptide-Based Fluorescent Ratio Sensor for Ag. J Fluoresc 2020;31:237-246. [PMID: 33215317 DOI: 10.1007/s10895-020-02653-5]

A fluorescence ratio sensor based on dansyl-peptide, Dansyl-Glu-Cys-Glu-Glu-Trp-NH₂ (D-P5), was efficiently synthesized by Fmoc solid phase peptide synthesis. The sensor exhibits high selectivity and sensitivity for Ag⁺ over 16 metal ions in 100 mM sodium perchlorate and 50 mM 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid buffer solution by fluorescence resonance energy transfer. The 1:1 binding stoichiometry of the sensor and Ag⁺ is measured by fluorescence ratio response and the job's plot. The dissociation constant of the sensor with Ag⁺ was calculated to be 6.4 × 10^-9 M, which indicates that the sensor has an effective binding affinity for Ag⁺. In addition, the limit of detection of the sensor for Ag⁺ was determined to be 80 nM, which also indicates that the sensor has a high sensitivity to Ag⁺. Result showed that the sensor is an excellent Ag⁺ sensor under neutral condition. Furthermore, this sensor displays good practicality for Ag⁺ detection in river water samples without performing tedious sample pretreatment, as well as for silver chloride detection.

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.

Label-free, Water-soluble Fluorescent Peptide Probe for a Sensitive and Selective Determination of Copper Ions

This study aimed to develop a label-free, sensitive, selective, and environment-friendly fluorescent peptide probe His-His-Trp-His (HHWH) for determining the concentration of copper ion (Cu²⁺) in aqueous solutions. The results demonstrated that the designed HHWH has a high selectivity and sensitivity for monitoring the concentration of free Cu²⁺ via quenching of the probe fluorescence upon a binding of Cu²⁺. The fluorescence intensity of the HHWH had a linear relationship with the concentration of Cu²⁺ between 10 nM and 10 μM, and the detection limit was 8 nM. Furthermore, HHWH could be regenerated with sulfide ions at least five times. The concentrations of Cu²⁺ in three different real water samples were detected using this probe, and the results were consistent with the one detected using an atomic absorption spectrometer. Thus, HHWH can be used as an accurate and feasible fluorescent peptide probe for detecting Cu²⁺ in aqueous solutions.

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.

A novel fluorescent chemosensor based on tetra-peptides for detecting zinc ions in aqueous solutions and live cells

A novel fluorescent chemosensor (HL) based on tetra-peptides conjugated with dansyl groups has been designed and synthesized, which is a promising analytical tool for detecting Zn²⁺ in aqueous solutions and live cells.

A peptide-based fluorescent chemosensor for measuring cadmium ions in aqueous solutions and live cells

A novel peptide fluorescent chemosensor (H2L) with a lysine backbone and both -NH2 sites conjugated with cysteine and dansyl groups has been designed and synthesized by solid phase peptide synthesis with Fmoc chemistry. This chemosensor is a promising analytical tool for detecting Cd(2+) based on the photo-induced electron transfer (PET) effect by turn-on response in 100% aqueous solutions. As designed, H2L exhibits excellent cell permeation and low biotoxicity as well as displaying relatively high selectivity and sensitivity. The chemosensor penetrated live HeLa cells and detected intracellular Cd(2+) by turn-on response. The binding stoichiometry and affinity, interference test, pH sensitivity, fluorescence quantum yield, quantum mechanical calculations, lifetimes, and cytotoxicity of the chemosensor H2L to Cd(2+) were also investigated. Moreover, H2L exhibits low biotoxicity with a limit of detection (LOD) for Cd(2+) of about 52 nM, implying that H2L can be used as a highly selective and sensitive peptide fluorescent chemosensor in biological systems.

Peptide-based ligand screening and functional analysis of protein kinase C

Protein kinase C (PKC) plays an important role in cell signaling pathways and is implicated in disorders ranging from cancer to Alzheimer's disease. Highly potent PKC ligands as therapeutic drugs have not been developed to date and useful methodologies for controlling PKC activation in defined areas are necessary to analyze precise PKC functions in cells. Studies focused on the development of PKC ligand screening systems and methods for regulation of PKC activation have been performed in our laboratory. In this review, our ligand screening methods involving synthetic peptides and solvatochromic fluorescent dye-labeled small compounds are summarized and the technique of spatio-temporal manipulation of PKC activation by caging strategies is introduced.

Competition of zinc(II) with cadmium(II) or mercury(II) in binding to a 12-mer peptide

Speciation of the complexes of zinc(II) with a dodecapeptide (Ac-SCPGDQGSDCSI-NH2), inspired by the metal binding domain of MerR metalloregulatory proteins, have been studied by pH-potentiometric titrations, UV, SRCD (synchrotron radiation circular dichroism) and (1)H NMR experiments. (MerR is a family of transcriptional regulators the archetype of which is the Hg(2+)-responsive transcriptional repressor-activator MerR protein.) The aim of the ligand-design was to retain the advantageous metal binding features of MerR proteins in a model peptide for the efficient capture of toxic metal ions. The peptide binds zinc(II) via two deprotonated Cys-thiol groups and one of the Asp-carboxylates in the ZnL parent complex, possessing a remarkably high stability (logK=9.93). In spite of the relatively long peptide loop, bis-complexes are also formed with the metal ion under basic conditions. In a competition with cadmium(II) or mercury(II), zinc(II) cannot prevent the binding of toxic metal ions by the thiolate donor groups of the ligand. Around neutral pH one equivalent of mercury(II) was shown to fully replace zinc(II) from the ZnL species. Partial replacement of zinc(II) from the peptide by one equivalent of cadmium(II), relative to zinc(II) and the ligand, is also presumable, nevertheless, spectroscopic data may suggest the formation of mixed metal ion complexes, as well. Based on the obtained results the investigated dodecapeptide can be a promising candidate for capturing toxic metal ions in practical applications.

A methionine-based turn-on chemical sensor for selectively monitoring Hg2+ ions in 100% aqueous solution

Dansyl-labeled methionine is synthesized by solid-phase synthesis, and found to be a highly sensitive and selective sensor for Hg(2+). The sensor sensitively detects Hg(2+) ions in aqueous solution by a turn-on response; however, the sensor detects Hg(2+) ions by a turn-off response in organic and mixed aqueous-organic solutions. We investigated the binding stoichiometry, binding constant, and binding mode of the sensor under various solvent conditions. In 100% aqueous solution, 2 : 1 complexation of the sensor with Hg(2+) ions is more favorable than 1 : 1 complexation, whereas the sensor preferentially forms a 1 : 1 complex in 100% CH(3)CN or in 50% CH(3)CN-aqueous solutions. Results reveal that the stoichiometry of the sensor-Hg(2+) complex plays an important role in the type of response to Hg(2+) ions, and that 2 : 1 complexation is required for a turn-on response to Hg(2+) ions in aqueous solution.

Fluorescent-responsive synthetic C1b domains of protein kinase Cδ as reporters of specific high-affinity ligand binding

Protein kinase C (PKC) is a critical cell signaling pathway involved in many disorders such as cancer and Alzheimer-type dementia. To date, evaluation of PKC ligand binding affinity has been performed by competitive studies against radiolabeled probes that are problematic for high-throughput screening. In the present study, we have developed a fluorescent-based binding assay system for identifying ligands that target the PKC ligand binding domain (C1 domain). An environmentally sensitive fluorescent dye (solvatochromic fluorophore), which has been used in multiple applications to assess protein-binding interactions, was inserted in proximity to the binding pocket of a novel PKCδ C1b domain. These resultant fluorescent-labeled δC1b domain analogues underwent a significant change in fluorescent intensity upon ligand binding, and we further demonstrate that the fluorescent δC1b domain analogues can be used to evaluate ligand binding affinity.