Ultrasensitive mercury ion and biothiol detection based on Dansyl-His-Pro-Gly-Asp-NH2 fluorescent sensor

Mercury is an environmental contaminant, which is highly toxic even at extremely low concentrations. Long-term accumulation of mercury in human body will damage the central nervous system or digestive tract system. Here, a new fluorescent chemical sensor Dansyl-His-Pro-Gly-Asp-NH₂ (D-P4) was synthesized for the determination of Hg²⁺. The D-P4 sensor exhibits excellent selectivity and sensitivity to Hg²⁺ in aqueous solution with a 'turn-off' fluorescence response. Furthermore, D-P4-Hg system displays a good 'turn-on' fluorescence response to biothiols. The calculated binding constant for the 1:1 complex of D-P4 with Hg^{2 +} is 1.07 × 10⁵ M^-1, which also confirms the high affinity of D-P4 for Hg²⁺. Results indicate that the detection limit of D-P4 for Hg²⁺ is 61.0 nM, and that of D-P4-Hg system for Cys is as low as 80.0 nM.

Cadmium transport activity of four mercury transporters (MerC, MerE, MerF and MerT) and effects of the periplasmic mercury-binding protein MerP on Mer-dependent cadmium uptake

Mercury superfamily proteins, i.e. inner membrane-spanning proteins (MerC, MerE, MerF and MerT) and a periplasmic mercury-binding protein (MerP), transport mercury into the cytoplasm. A previous study demonstrated that a Mer transporter homolog exhibits cadmium transport activity; based on this, the present study aimed to evaluate the cadmium transport activity of MerC, MerE, MerF and MerT and the effects of MerP co-expression in Escherichia coli. Bacteria expressing MerC, MerE, MerF or MerT without MerP were more sensitive to cadmium and significantly absorbed more cadmium than did the control strain. Expression of MerP in combination with MerC, MerE, MerF or MerT increased the bacterial sensitivity to cadmium and cadmium accumulation compared to a single expression of MerC, MerE, MerF or MerT. Cadmium uptake mediated by MerC, MerE, MerF or MerT was inhibited under cold or acidic conditions. These findings suggest that MerC, MerE, MerF and MerT are broad-spectrum heavy metal transporters that mediate both mercury and cadmium transport into cells and that MerP accelerates the cadmium transport ability of MerC, MerE, MerF and MerT.

Metal Exchange in the Interprotein ZnII -Binding Site of the Rad50 Hook Domain: Structural Insights into CdII -Induced DNA-Repair Inhibition

CdII is a major genotoxic agent that readily displaces ZnII in a multitude of zinc proteins, abrogates redox homeostasis, and deregulates cellular metalloproteome. To date, this displacement has been described mostly for cysteine(Cys)-rich intraprotein binding sites in certain zinc finger domains and metallothioneins. To visualize how a ZnII -to-CdII swap can affect the target protein's status and thus understand the molecular basis of CdII -induced genotoxicity an intermolecular ZnII -binding site from the crucial DNA repair protein Rad50 and its zinc hook domain were examined. By using a length-varied peptide base, ZnII -to-CdII displacement in Rad50's hook domain is demonstrated to alter it in a bimodal fashion: 1) CdII induces around a two-orders-of-magnitude stabilization effect (log  K 12 Zn II =20.8 vs. log  K 12 Cd II =22.7), which defines an extremely high affinity of a peptide towards a metal ion, and 2) the displacement disrupts the overall assembly of the domain, as shown by NMR spectroscopic and anisotropy decay data. Based on the results, a new model explaining the molecular mechanism of CdII genotoxicity that underlines CdII 's impact on Rad50's dimer stability and quaternary structure that could potentially result in abrogation of the major DNA damage response pathway is proposed.

Selective mercury(ii) detection in aqueous solutions upon the absorption changes corresponding to the transition moments polarized along the short axis of an azobenzene chemosensor

A completely water-soluble azobenzene chemosensor shows selective Hg²⁺ detection properties in wide pH ranges and under different light conditions.

Genetically encoded FRET-based optical sensor for Hg2+ detection and intracellular imaging in living cells

Due to the potential toxicity of mercury, there is an immediate need to understand its uptake, transport and flux within living cells. Conventional techniques used to analyze Hg²⁺ are invasive, involve high cost and are less sensitive. In the present study, a highly efficient genetically encoded mercury FRET sensor (MerFS) was developed to measure the cellular dynamics of Hg²⁺ at trace level in real time. To construct MerFS, the periplasmic mercury-binding protein MerP was sandwiched between enhanced cyan fluorescent protein (ECFP) and venus. MerFS is pH stable, offers a measurable fluorescent signal and binds to Hg²⁺ with high sensitivity and selectivity. Mutant MerFS-51 binds with an apparent affinity (K_d) of 5.09 × 10^-7 M, thus providing a detection range for Hg²⁺ quantification between 0.210 µM and 1.196 µM. Furthermore, MerFS-51 was targeted to Escherichia coli (E. coli), yeast and human embryonic kidney (HEK)-293T cells that allowed dynamic measurement of intracellular Hg²⁺ concentration with a highly responsive saturation curve, proving its potential application in cellular systems.

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.

Fluorescent peptide dH3w: A sensor for environmental monitoring of mercury (II)

Heavy metals are hazardous environmental contaminants, often highly toxic even at extremely low concentrations. Monitoring their presence in environmental samples is an important but complex task that has attracted the attention of many research groups. We have previously developed a fluorescent peptidyl sensor, dH3w, for monitoring Zn2+ in living cells. This probe, designed on the base on the internal repeats of the human histidine rich glycoprotein, shows a turn on response to Zn2+ and a turn off response to Cu2+. Other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Pb2+ and Cd2+) do not interfere with the detection of Zn2+ and Cu2+. Here we report that dH3w has an affinity for Hg2+ considerably higher than that for Zn2+ or Cu2+, therefore the strong fluorescence of the Zn2+/dH3w complex is quenched when it is exposed to aqueous solutions of Hg2+, allowing the detection of sub-micromolar levels of Hg2+. Fluorescence of the Zn2+/dH3w complex is also quenched by Cu2+ whereas other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Cd2+, Pb2+, Sn2+ and Cr3+) have no effect. The high affinity and selectivity suggest that dH3w and the Zn2+/dH3w complex are suited as fluorescent sensor for the detection of Hg2+ and Cu2+ in environmental as well as biological samples.

A peptide-based multifunctional fluorescent probe for Cu2+, Hg2+ and biothiols

A peptide-based fluorescent probe (Dansyl-His-Pro-Gly-Trp-NH₂, D-P4) bearing the dansyl fluorophore and tryptophan residue has been developed for the detection of Hg²⁺, Cu²⁺ and biothiols (–SH).

Sensitive ratiometric detection of Al(iii) ions in a 100% aqueous buffered solution using a fluorescent probe based on a peptide receptor

The peptidyl bioprobe detected Al(iii) ions sensitively in a 100% aqueous buffered solution through ratiometric response.

A simple cost effective carbazole–thiobarbituric acid conjugate as a ratiometric fluorescent probe for detection of mercury(ii) ions in aqueous medium

A multimode ratiometric Hg²⁺ selective chemosensor based on a carbazole–thiobarbituric acid conjugate was developed. It responds to the presence of Hg²⁺ by colorimetry and by fluorescence. The sensor action is demonstrated in both aqueous homogeneous or microheterogeneous media.

Highly sensitive and selective detection of Al(III) ions in aqueous buffered solution with fluorescent peptide-based sensor

A fluorescent sensor based on a tripeptide (SerGluGlu) with a dansyl fluorophore detected selectively Al(III) among 16 metal ions in aqueous buffered solutions without any organic cosolvent. The peptide-based sensor showed a highly sensitive turn on response to aluminium ion with high binding affinity (1.84×10(4)M(-1)) in aqueous buffered solutions. The detection limit (230nM, 5.98ppb) of the peptide-based sensor was much lower than the maximum allowable level (7.41μM) of aluminium ions in drinking water demanded by EPA. The binding mode of the peptide sensor with aluminium ions was characterized using ESI mass spectrometry, NMR titration, and pH titration experiments.

Selective and Sensitive Detection of Heavy Metal Ions in 100% Aqueous Solution and Cells with a Fluorescence Chemosensor Based on Peptide Using Aggregation-Induced Emission

A fluorescent peptidyl chemosensor for the detection of heavy metal ions in aqueous solution as well as in cells was synthesized on the basis of the peptide receptor for the metal ions using an aggregation-induced emission fluorophore. The peptidyl chemosensor (1) bearing tetraphenylethylene fluorophore showed an exclusively selective turn-on response to Hg(2+) among 16 metal ions in aqueous buffered solution containing NaCl. The peptidyl chemosensor complexed Hg(2+) ions and then aggregated in aqueous buffered solution, resulting in the significant enhancement (OFF-On) of emissions at around 470 nm. The fluorescent sensor showed a highly sensitive response to Hg(2+), and about 1.0 equiv of Hg(2+) was enough for the saturation of the emission intensity change. The detection limit (5.3 nM, R(2) = 0.99) of 1 for Hg(2+) ions was lower than the maximum allowable level of Hg(2+) in drinking water by EPA. Moreover, the peptidyl chemosensor penetrated live cells and detected intracellular Hg(2+) ions by the turn-on response.

A regenerative electrochemical biosensor for mercury(II) by using the insertion approach and dual-hairpin-based amplification

A simple and effective biosensor for Hg(2+) determination was investigated. The novel biosensor was prepared by the insertion approach that the moiety-labeled DNA inserted into a loosely packed cyclic-dithiothreitol (DTT) monolayer, improving the hybridization efficiency. Electrochemical impedance spectroscopy studies of two biosensors (single-hairpin and dual-hairpin structure DNA modified electrodes) used for Hg(2+) detection indicated that the dual-hairpin modified electrode had a larger electron transfer resistance change (ΔRct). Consequently, the dual-hairpin structure was used as a signal amplifier for the preparation of a selective Hg(2+) biosensor. This biosensor exhibited an excellent selectivity toward Hg(2+) over Cd(2+), Pd(2+), Co(2+) etc. Also, a linear relation was observed between the ΔRct and Hg(2+) concentrations in a range from 0.1 nM to 5 μM with a detection limit of 28 pM under optimum conditions. Moreover, the biosensor can be reused by using L-cysteine and successfully applied for detecting Hg(2+) in real samples.

Pyrene Excimer-Based Peptidyl Chemosensors for the Sensitive Detection of Low Levels of Heparin in 100% Aqueous Solutions and Serum Samples

Fluorescent chemosensors (1 and 2, Py-(Arg)nGlyGlyGly(Arg)nLys(Py)-NH2, n = 2 and 3) bearing two pyrene (Py) labeled heparin-binding peptides were synthesized for the sensitive ratiometric detection of heparin. The peptidyl chemosensors (1 and 2) sensitively detected nanomolar concentrations of heparin in aqueous solutions and in serum samples via a ratiometric response. In 100% aqueous solutions at pH 7.4, both chemosensors exhibited significant excimer emission at 486 nm as well as weak monomer emission in the absence of heparin. Upon the addition of heparin into the solution, excimer emission increased with a blue shift (10 nm) and monomer emission at 376 nm decreased. The chemosensors showed a similar sensitive ratiometric response to heparin independent of the concentration of the chemosensors. The peptidyl chemosensors were applied to the ratiometric detection of heparin over a wide range of pH (1.5-11.5) using the excimer/momomer emission changes. In the presence of serum, 1 and 2 displayed significant monomer emission at 376 nm with relatively weak excimer emission and the addition of heparin induced a significant increase in excimer emission at 480 nm and a concomitant decrease in monomer emission. The enhanced ratiometric response to heparin in the serum sample was due to the interactions between the peptidyl chemosensors and serum albumin in the serum sample. The detection limits of 2 for heparin were less than 1 nM in 100% aqueous solutions and serum samples. The peptidyl chemosensors bearing two heparin-binding sites are a suitable tool for the sensitive ratiometric detection of nanomolar concentrations of heparin in 100% aqueous solutions and serum samples.

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.

Highly selective colorimetric and fluorescent detection for Hg2+ in aqueous solutions using a dipeptide-based chemosensor

A dipeptide-based chemosensor selectively and sensitively detected Hg^II in aqueous solutions by colorimetric change and fluorescent change.

Highly sensitive colorimetric detection of HgII and CuII in aqueous solutions: from amino acids toward solid platforms

A chemosensor (NBD-H) based on an amino acid with 7-nitro-2,1,3-benzoxadiazole was used for selective detection of Hg^II and Cu^II among 15 metal ions in aqueous solutions by a colorimetric change and fluorescence change.

Carbamodithioate-based dual functional fluorescent probe for Hg(2+) and S(2-)

Carbamodithioate-based compound T1 was designed and synthesized as a dual-functional probe for Hg(2+) ions and S(2-) anions. The underlying signaling mechanism was intramolecular charge transfer (ICT). It could serve as a direct probe towards Hg(2+) ions through "on-off" fluorescence changes and an indirect probe towards S(2-) anions through "on-off-on" fluorescence changes.

Ratiometric fluorescence sensing of sugars via a reversible disassembly and assembly of the peptide aggregates mediated by sugars

An amphiphilic dipeptide (1) bearing pyrene and phenylboronic acid was demonstrated as a unique example of a ratiometric sensing system for sugars by reversibly converting the peptide aggregates into the monomer form of the complex with sugars in aqueous solutions.

A dual sensor of fluorescent and colorimetric for the rapid detection of lead

This study demonstrated a simple and reliable method to rapidly detect Pb(2+) in aqueous solution, exploiting gold nanoparticles as a lead ion probe; the results indicated that the dual channels sensor showed high selectivity and sensitivity for Pb(2+) as low as ppm levels in aqueous environment.

Ratiometric detection of Cr3+ and Hg2+ by a naphthalimide-rhodamine based fluorescent probe

Newly synthesized rhodamine derivatives, L(1) and L(2), are found to bind specifically to Hg(2+) or Cr(3+) in presence of large excess of other competing ions with associated changes in their optical and fluorescence spectral behavior. These spectral changes are significant enough in the visible region of the spectrum and thus, allow the visual detection. For L(1), the detection limit is even lower than the permissible [Cr(3+)] or [Hg(2+)] in drinking water as per standard U.S. EPA norms; while the receptor, L2 could be used as a ratiometric sensor for detection of Cr(3+) and Hg(2+) based on the resonance energy transfer (RET) process involving the donor naphthalimide and the acceptor Cr(3+)/Hg(2+)-bound xanthene fragment. Studies reveal that these two reagents could be used for recognition and sensing of Hg(2+)/Cr(3+). Further, confocal laser microscopic studies confirmed that the reagent L(2) could also be used as an imaging probe for detection of uptake of these ions in A431 cells.

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.

Homogeneous competitive assay of ligand affinities based on quenching fluorescence of tyrosine/tryptophan residues in a protein via Főrster-resonance-energy-transfer

A new homogeneous competitive assay of ligand affinities was proposed based on quenching the fluorescence of tryptophan/tyrosine residues in a protein via Főrster-resonance-energy-transfer using a fluorescent reference ligand as the acceptor. Under excitation around 280 nm, the fluorescence of a protein or a bound acceptor was monitored upon competitive binding against a nonfluorescent candidate ligand. Chemometrics for deriving the binding ratio of the acceptor with either fluorescence signal was discussed; the dissociation constant (K(d)) of a nonfluorescent candidate ligand was calculated from its concentration to displace 50% binding of the acceptor. N-biotinyl-N'-(1-naphthyl)-ethylenediamine (BNEDA) and N-biotinyl-N'-dansyl-ethylenediamine (BDEDA) were used as the reference ligands and acceptors to streptavidin to test this new homogeneous competitive assay. Upon binding of an acceptor to streptavidin, there were the quench of streptavidin fluorescence at 340 nm and the characteristic fluorescence at 430 nm for BNEDA or at 525 nm for BDEDA. K(d) of BNEDA and BDEDA was obtained via competitive binding against biotin. By quantifying BNEDA fluorescence, K(d) of each tested nonfluorescent biotin derivative was consistent with that by quantifying streptavidin fluorescence using BNEDA or BDEDA as the acceptor. The overall coefficients of variation were about 10%. Therefore, this homogeneous competitive assay was effective and promising to high-throughput-screening.

A highly sensitive and selective detection of Hg(II) in 100% aqueous solution with fluorescent labeled dimerized Cys residues

A simple design of a ratiometric fluorescent sensor for detecting Hg(II) ion in 100% aqueous solution was demonstrated, based on the structure of dimerized Cys residues with two dansyl fluorophores. The sensor highly sensitively and selectively detected mercury ion (K(d) = 41 nM) in 100% aqueous solution via a turn-on and ratiometric response. The sensor showed no interferences of other metal ions and satisfied for monitoring the maximum allowable level (2 ppb) of mercury ion in drinking water demanded by EPA via a turn-on response.

A new approach to design ratiometric fluorescent probe for mercury(II) based on the Hg(2+)-promoted deprotection of thioacetals

On the basis of the protection reaction between ethanethiol and aldehyde, we designed and synthesized two new ratiometric fluorescent chemosensors, 3 and 4, by using intramolecular charge transfer (ICT) as a signaling mechanism. Upon the addition of Hg(2+) ion, both probes displayed apparent luminescence color changes, which could be observed by naked eyes under a UV lamp. Unexpectedly, both chemosensors also gave response to the addition of trace silver ions, making this kind of chemosensors as the first example of ratiometric fluorescent probe that showed dual channel fluorescence for both Hg(2+) and Ag(+). The test strips experiments suggested that 3 and 4 could serve as practical fluorescent probes for rapid detection of Hg(2+) ion.