Imaging of zinc ions across diverse biological samples with a quinoline-based tris(2-pyridylmethyl)amine fluorescent probe

Zinc ions (Zn2⁺) is actively involved in diverse biological processes. Therefore, the precise detection of Zn2⁺ ion is an important object of increasing investigation. Although numerous fluorescent zinc ion detection probes have been developed, simple, biocompatible, and sensitive probes are still urgently needed. Herein, we reported two novel fluorescent probes, ZnTP1 and ZnTP2, by incorporating a quinoline fluorophore into a membrane-permeable zinc chelator tris(2-pyridylmethyl)amine. ZnTP1 exhibited a significant fluorescence enhancement in the presence of zinc ions through chelation-enhanced fluorescence (CHEF) processes, whereas probe ZnTP2 did not show any significant change in fluorescence due to the insertion of the carbonyl group. Further investigations revealed that ZnTP1 can effectively penetrate cell membranes and detect Zn2+ with high sensitivity in diverse biological samples, including living cells, plant tissues, and animal model zebrafish. This work suggests that ZnTP1 as a simple and efficient chemical probe has great potential for zinc ions detection in various biological contexts, thus providing a new tool for probing zinc ions in biosystems.

Mol-ecular structure of tris-[(6-bromo-pyridin-2-yl)meth-yl]amine

Coordination compounds of polydentate nitro-gen ligands with metals are used extensively in research areas such as catalysis, and as models of complex active sites of enzymes in bioinorganic chemistry. Tris(2-pyridyl-meth-yl)amine (TPA) is a tripodal tetra-dentate ligand that is known to form coordination compounds with metals, including copper, iron and zinc. The related compound, tris-[(6-bromo-pyridin-2-yl)meth-yl]amine (TPABr3), C18H15Br3N4, which possesses a bromine atom on the 6-position of each of the three pyridyl moieties, is also known but has not been heavily investigated. The mol-ecular structure of TPABr3 as determined by X-ray diffraction is reported here. The TPABr3 molecule belongs to the triclinic, P space group and displays interesting intermolecular Br⋯Br interactions that provide a stabilizing influence within the molecule.

Evaluation of oxygen-containing pentadentate ligands with pyridine/quinoline/isoquinoline binding sites via the structural and electrochemical properties of mononuclear copper(II) complexes

Eighteen mononuclear copper(II) complexes with oxygen-containing N4O1 pentadentate ligands were prepared. The ligand library consists of 2-aminoethanol derivatives ((Ar1CH2)(Ar2CH2)NCH2CH2OCH2Ar3) bearing three nitrogen-containing heteroaromatics (Ars) including pyridine, quinoline and isoquinoline via a methylene linker. Systematic replacements of pyridine binding sites with quinolines and isoquinolines reveal the general trends in the perturbation of bond distances and angles, the redox potential and the absorption maximum wavelength of the copper(II) complexes, depending on the position and number of (iso)quinoline heteroaromatics. The small effect on the redox potentials resulting from quinoline substitution at the Ar3 position (near oxygen) of the ligand comes from the steric hindrance of the peri hydrogen atom in the quinoline moiety at this position, which removes the counter anion to enhance the coordination of quinoline nitrogen and ether oxygen atoms to the metal centre. In the absorption spectra of copper(II) complexes in the d-d transition region, the quinoline substitution at this site (Ar3) exhibits an opposite effect to those at the Ar1 and Ar2 sites. The electronic and steric contributions of the heteroaromatic binding sites to the ligand properties are comprehensively discussed.

Structure and electrochemical properties of (μ-O)2Mn2(iii,iii) and (μ-O)2Mn2(iii,iv) complexes supported by pyridine-, quinoline-, isoquinoline- and quinoxaline-based tetranitrogen ligands

Seven new bis(μ-oxo)dimanganese complexes with Mn₂(iii,iii) or Mn₂(iii,iv) oxidation states were prepared using quinoline- and isoquinoline-based tetraamine ligands. The structures of the ligands include ethylenediamine, trans-1,2-cyclohexanediamine and tripodal amine, bearing two or three nitrogen-containing heteroaromatics. Regardless of the skeleton and number of aliphatic nitrogen atoms in the ligands, quinoline complexes stabilize the Mn₂(iii,iii) oxidation state, whereas, isoquinoline ligands afford Mn₂(iii,iv) complexes. A systematic comparison of the differences in structural parameters and redox potentials of a total of 14 complexes with a (μ-O)₂Mn₂ diamond core, which includes corresponding pyridine and quinoxaline derivatives as supporting ligands, highlights the distinct deviation of quinoline and tripodal amine motifs in this ligand series.

DNAzyme-Mediated Genetically Encoded Sensors for Ratiometric Imaging of Metal Ions in Living Cells

Genetically encoded fluorescent proteins (FPs) have been used for metal ion detection. However, their applications are restricted to a limited number of metal ions owing to the lack of available metal-binding proteins or peptides that can be fused to FPs and the difficulty in transforming the binding of metal ions into a change of fluorescent signal. We report herein the use of Mg2+-specific 10-23 or Zn2+-specific 8-17 RNA-cleaving DNAzymes to regulate the expression of FPs as a new class of ratiometric fluorescent sensors for metal ions. Specifically, we demonstrate the use of DNAzymes to suppress the expression of Clover2, a variant of the green FP (GFP), by cleaving the mRNA of Clover2, while the expression of Ruby2, a mutant of the red FP (RFP), is not affected. The Mg2+ or Zn2+ in HeLa cells can be detected using both confocal imaging and flow cytometry. Since a wide variety of metal-specific DNAzymes can be obtained, this method can likely be applied to imaging many other metal ions, expanding the range of the current genetically encoded fluorescent protein-based sensors.

Quinoline- and isoquinoline-derived ligand design on TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) platform for fluorescent sensing of specific metal ions and phosphate species

Utilizing the unique metal-binding and fluorescent properties of methoxy-substituted (iso)quinolines, varieties of fluorescent probes were developed from TQEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)ethylenediamine) structure.

DNAzyme-Mediated Genetically Encoded Sensors for Ratiometric Imaging of Metal Ions in Living Cells

Genetically encoded fluorescent proteins (FPs) have been used for metal ion detection. However, their applications are restricted to a limited number of metal ions owing to the lack of available metal-binding proteins or peptides that can be fused to FPs and the difficulty in transforming the binding of metal ions into a change of fluorescent signal. We report herein the use of Mg²⁺ -specific 10-23 or Zn²⁺ -specific 8-17 RNA-cleaving DNAzymes to regulate the expression of FPs as a new class of ratiometric fluorescent sensors for metal ions. Specifically, we demonstrate the use of DNAzymes to suppress the expression of Clover2, a variant of the green FP (GFP), by cleaving the mRNA of Clover2, while the expression of Ruby2, a mutant of the red FP (RFP), is not affected. The Mg²⁺ or Zn²⁺ in HeLa cells can be detected using both confocal imaging and flow cytometry. Since a wide variety of metal-specific DNAzymes can be obtained, this method can likely be applied to imaging many other metal ions, expanding the range of the current genetically encoded fluorescent protein-based sensors.

Remote conformational responses to enantiomeric excess in carboxylate-binding dynamic foldamers

A crystallographically characterised zinc(ii)-capped foldamer can sense the enantiomeric excess of scalemic carboxylate solutions, including those produced by enantioselective organocatalysis, and can relay this input signal along the foldamer body to a remote glycinamide group, which then provides an NMR spectroscopic output.

A coumarin derivative as a "turn-on" fluorescence probe toward Cd2+ in live cells

A novel coumarin-derived Schiff base fluorescence probe (CTB) has been successfully designed and synthesized through exploiting tris-(2-aminothyl)-amine moiety as a recognition unit for the highly selective and sensitive detection of Cd²⁺. It is based on CN isomerization and the photo-induced electron transfer (PET) mechanism. The investigation into the sensing processes showed that CTB exhibited an excellent selectivity for Cd²⁺. The sensitivity exceeded that of other competing metal ions, and had a high sensitivity, a detection limit of 1.16 × 10^-7 M with the association constants of 1.37 × 10¹¹ M^-2. The experiments including Job's plot, UV-Vis titration, ¹H NMR titration and ESI-MS spectrum established that the probe CTB binds to Cd²⁺ in a 1:2 ratio. Further studies also demonstrated that probe CTB can be successfully applied to the fluorescence imaging of Cd²⁺ in HepG-2 cells.

A selective fluorescent chemosensor for Cd2+ based on 8-hydroxylquinoline-benzothiazole conjugate and imaging application

A 8-hydroxylquinoline-benzothiazole conjugate (HQ-BT) was facilely synthesized by two steps with >60% total reaction yield. The HQ-BT showed a weak fluorescence that could be strongly enhanced by coordination with various metal ions such as Al³⁺, Cd²⁺, Zn²⁺ in methanol containing 1% water. Interestingly, the selectivity toward Cd²⁺ was achieved by increasing water fraction to 30% aqueous methanol solution. Thus, the HQ-BT was developed as a new and selective fluorescent chemosensor for Cd²⁺ in aqueous solution with a broad pH region 4-12. A good linear relationship between the fluorescence intensity and the Cd²⁺ concentration was found in the range of 0-5 μM with a detection limit of 0.1 μM (S/N = 3). It was also succesfully used for fluorescence imaging of Cd²⁺ in living cells.

A water-soluble BODIPY based 'OFF/ON' fluorescent probe for the detection of Cd2+ ions with high selectivity and sensitivity

A water-soluble dilithium salt BODIPY derivative (LiBDP) with appended dicarboxylate pseudo-crown ether [NO4] coordinating sites has been designed, synthesized and characterized successfully for the selective and sensitive recognition of Cd2+ in aqueous media. The chemosensor exhibits a remarkable increase in fluorescence intensity as well as a distinct color change upon the addition of Cd2+ over other environmentally and biologically relevant metal ions in H2O. The fluorometric response of LiBDP is attributed to the metal chelation-enhanced fluorescence (MCHEF) effect which has been confirmed by a strong association constant of 2.57 ± 1.06 × 105 M-1 and Job's plot, indicating 1 : 1 binding stoichiometry between LiBDP and Cd2+. Frontier molecular orbital analysis (obtained from DFT studies) also illustrates the turn-on fluorescence of the probe by blocking photoinduced electron transfer (PET) after coordination to Cd2+. The probe can detect Cd2+ in a competitive environment up to a submicromolar level in a biologically significant pH range. The sensor is proved to be reversible and reusable by the alternative addition of Cd2+ followed by S2-. The OFF/ON/OFF sensing behavior is utilized to construct an INHIBIT molecular logic gate based on the two inputs of Cd2+ and S2- and a fluorescence intensity at 512 nm as an output. The test paper experiment demonstrates the practical utility of LiBDP to monitor Cd2+ in an aqueous sample. Finally, the sensing probe was utilized to monitor Cd2+ in living cells.

Cadmium Compounds with an [N3C] Atrane Motif: Evidence for the Generation of a Cadmium Hydride Species

Tris(2-pyridylthio)methane ([Tptm]H) has been employed to synthesize a series of cadmium carbatrane compounds that feature an [N3C] coordination environment. Specifically, [Tptm]H reacts with Cd[N(SiMe3)2]2 to afford [Tptm]CdN(SiMe3)2, which thereby provides access to other derivatives. For example, [Tptm]CdN(SiMe3)2 reacts with (i) CO2 to form {[Tptm]Cd(μ-NCO)}2 and (ii) Me3SiOH and Ph3SiOH to form {[κ3-Tptm]Cd(μ-OSiMe3)}2 and [Tptm]CdOSiPh3, respectively. The siloxide compound {[κ3-Tptm]Cd(μ-OSiMe3)}2 reacts with Me3SiX (X = Cl, Br, O2CMe) to give [Tptm]CdX, while the reaction with PhSiH3 in the presence of CO2 generates the formate complex, [Tptm]CdO2CH, thereby providing evidence for the generation of a proposed cadmium hydride intermediate, {[Tptm]CdH}.

A highly selective colorimetric and fluorescent turn-on chemosensor for Al(3+) based on naphthalimide derivative

A new chemosensor L based on the naphthalimide moiety was synthesized and characterized. L exhibited the high selectivity and sensitivity for Al(3+) in CH3OH, along with colorimetric and fluorometric dual-signaling responses based on the joint contribution of the ICT and CHEF processes. A 1:1 stoichiometry for the L-Al(3+) complex was formed with an association constant of 7.6×10(4)M(-1), and the limit of detection for Al(3+) was determined as 6.9μM. In addition, L was successfully applied to the determination of Al(3+) in real water samples.

OFF–ON–OFF fluorescent response of N,N,N′,N′-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN) toward Zn2+

1-isoHTQHPN exhibits OFF–ON–OFF fluorescent response toward increasing concentration of Zn²⁺ions due to the specific intramolecular excimer formation in a mononuclear complex.

A highly sensitive benzimidazole-based chemosensor for the colorimetric detection of Fe(ii) and Fe(iii) and the fluorometric detection of Zn(ii) in aqueous media

A selective chemosensor 1 was developed for the colorimetric detection of Fe^2+/3+ and the fluorescent detection of Zn²⁺ in aqueous solution.

Tris(8-methoxy-2-quinolylmethyl)amine (8-MeOTQA) as a highly fluorescent Zn2+probe prepared by convenient C3-symmetric tripodal amine synthesis

The convenient synthesis ofC₃-symmetric tribenzylamines utilizing acetaldehyde ammonia trimer (1) revealed tris(8-methoxy-2-quinolylmethyl)amine (8-MeOTQA) as a fluorescent zinc sensor with high sensitivity.

A highly selective fluorescent chemosensor based on a quinoline derivative for zinc ions in pure water

A water-soluble fluorescent sensor with a low detection limit could be used to detect and quantify Zn²⁺in water samples.

TQPHEN (N,N,N′,N′-tetrakis(2-quinolylmethyl)-1,2-phenylenediamine) derivatives as highly selective fluorescent probes for Cd2+

The tetrakisquinoline derivatives with a 1,2-phenylenediamine (PHEN) scaffold exhibit a Cd²⁺-specific fluorescence enhancement, in contrast to the Zn²⁺-specific trans-1,2-diaminocyclohexane (DACH) counterpart.

An efficient vanillinyl Schiff base as a turn on fluorescent probe for zinc(ii) and cell imaging

Vanillinyl Schiff base (H₂L), a nontoxic probe and a fluorescent sensor to Zn²⁺, LOD 0.018 μM is used for living cell imaging.