Di- and trinuclear copper(II) complexes of polyaza macrocycles and cryptands as anion receptors

Enhancement of Alkali Metal Ion Recognition by Metalation of a Tris(saloph) Cryptand Having Benzene Rings at the Bridgeheads

A cryptand derivative, H₆L, which has three H₂saloph arms connected by two benzene ring bridgeheads, was synthesized and converted into the trinuclear metallocryptand, LNi₃. The nonmetalated host, H₆L, was found to bind to alkali metal ions (Na⁺, K⁺, Rb⁺, Cs⁺; logK_a = 3.37-6.67) in its well-defined cavity in DMSO/chloroform (1:9). The binding affinity was enhanced by 1-2 orders of magnitude upon the conversion into the metallocryptand, LNi₃, which can be explained by the more polarized phenoxo groups in the [Ni(saloph)] arms. The guest binding affinity of Na⁺ < K⁺ < Rb⁺ ≈ Cs⁺ was clearly demonstrated by the ¹H NMR competition experiments. The DFT calculations suggested that the Rb⁺ ion most suitably fit into the benzene-benzene spacing with a cation-π interaction and that only the largest Cs⁺ ion can almost equally interact with all six phenoxo oxygen donor atoms. The metallocryptand, LNi₃, also showed a strong binding affinity to Ag⁺ by taking advantage of cation-π interactions, which was confirmed by spectroscopic titrations and crystallographic analysis as well as DFT calculations. Thus, the well-defined three-dimensional cavity of LNi₃ was found to be suitable for strong binding with alkali metal ions as well as Ag⁺.

Chiral anion recognition using calix[4]arene-based ureido receptors in a 1,3-alternate conformation

The introduction of chiral alkyl substituents into the lower rim of calix[4]arene immobilised in the 1,3-alternate conformation led to a system possessing a preorganised ureido cavity hemmed with chiral alkyl units in the near proximity. As shown by the ¹H NMR titration experiments, these compounds can be used as receptors for chiral anions in DMSO-d ₆. The chiral recognition ability can be further strengthened by the introduction of another chiral moiety directly onto the urea N atoms. The systems with double chiral units being located around the binding ureido cavity showed better stereodiscrimination, with the highest selectivity factor being 3.33 (K _L/K _D) achieved for N-acetyl-ʟ-phenylalaninate. The structures of some receptors were confirmed by single crystal X-ray analysis.

Monitoring inorganic pyrophosphatase activity with the fluorescent dizinc(ii) complex of a macrocycle bearing one dansylamidoethyl antenna

The dizinc(ii) complexes of L were used for the recognition of anions by fluorescence spectroscopy (L is a heteroditopic hexaazamacrocycle with two diethylenetriamine coordination heads with 2-methylpyridyl and dansylamido ethyl arms, and m-xylyl spacers). The protonation of L and stability constants of its zinc(ii) complexes were determined in aqueous solution, at 298.2 ± 0.1 K and I = 0.10 ± 0.01 M in KNO₃. At a 2 : 1 Zn²⁺/L ratio, the dinuclear complexes clearly dominate. The ligand alone does not display fluorescence changes upon increasing the pH value, but in the presence of Zn²⁺ the emission reaches a maximum at pH ≅ 7.5, at which 95% of the ligand is in the dinuclear complex form. The emission appears concomitantly with the [Zn₂H_-1L]³⁺ species formation, which supports that the latter complex corresponds to the metal-promoted deprotonation of dansylamide NH. The [Zn₂H_-1L]³⁺ complexes were used for the recognition of phosphate and polyphosphate anions in aqueous solution buffered at pH 7.5 with 2 mM PIPPS, at 298.2 K. The binding of anions causes a decrease of the emission. The association constant determination revealed that HPPi^3- is the strongest bound anion (log K_app = 5.57), followed by HATP^3- (two times weaker), and the remaining anions show lower binding constants, with HPO₄^2- having the weakest uptake by the receptor. The observed selectivity of the [Zn₂H_-1L]³⁺ receptor for PPi in relation to HPO₄^2-, and the fact that the formation of the [Zn₂H_-1L]³⁺ complex is not disturbed by the presence of Mg²⁺, allowed monitoring of the PPi hydrolysis by using inorganic pyrophosphatase in real-time.

Zinc(ii) and copper(ii) complexes as tools to monitor/inhibit protein phosphorylation events

A perspective on the advance of copper(ii) and zinc(ii) complexes of varied ligand architectures as binders of phosphorylated peptides/proteins and as sensors of phosphorylation reactions is presented.

Recognition of phosphopeptides by a dinuclear copper(ii) macrocyclic complex in a water : methanol 50 : 50 v/v solution

A new triethylbenzene-derived tetraazamacrocycle containing pyridyl spacers, L, was prepared and its dinuclear copper(ii) complex was used as a receptor for the recognition of phosphorylated peptides in aqueous solution. A detailed study of the acid-base behaviour of L and its copper(ii) complexation properties as well as of the cascade species with phosphorylated anions including two peptidic substrates was carried out in a H₂O/MeOH (50 : 50 v/v) solution using different techniques, such as potentiometry, X-band EPR and DFT calculations. The association constants of the dinuclear receptor with the phosphorylated peptides and other anionic species revealed a clear preference towards phenylic phosphorylated substrates, with values ranging 3.96-5.35 log units. Single-crystal X-ray diffraction determination of the dicopper(ii) complex of L showed the copper centres at a distance of 5.812(1) Å from one another, with the phosphate group of the PhPO₄^2- substrate well accommodated between them. X-band EPR studies indicated a similar structure for this cascade complex and for the other cascade complexes with the phosphorylated anions studied. DFT studies of the [Cu₂L(μ-OH)]³⁺ complex revealed a different conformation of the ligand that brings the two copper centres at a very short distance of 3.94 Å aided by the presence of a bridging hydroxide anion that provides a CuOCu angle of 167.3°. This complex is EPR silent, in line with the singlet ground state obtained using CASSCF(2,2) calculations and DFT calculations with the broken-symmetry approach. This species coexists in solution with a complex in a different conformation, and having a CuCu distance of 6.63 Å, in lower percentage.

Highly selective and sensitive macrocycle-based dinuclear foldamer for fluorometric and colorimetric sensing of citrate in water

The selective detection of citrate anions is essential for various biological functions in living systems. A quantitative assessment of citrate is required for the diagnosis of various diseases in the human body; however, it is extremely challenging to develop efficient fluorescence and color-detecting molecular probes for sensing citrate in water. Herein, we report a macrocycle-based dinuclear foldamer (1) assembled with eosin Y (EY) that has been studied for anion binding by fluorescence and colorimetric techniques in water at neutral pH. Results from the fluorescence titrations reveal that the 1·EY ensemble strongly binds citrate anions, showing remarkable selectivity over a wide range of inorganic and carboxylate anions. The addition of citrate anions to the 1·EY adduct led to a large fluorescence enhancement, displaying a detectable color change under both visible and UV light in water up to 2 μmol. The biocompatibility of 1·EY as an intracellular carrier in a biological system was evaluated on primary human foreskin fibroblast (HF) cells, showing an excellent cell viability. The strong binding properties of the ensemble allow it to be used as a highly sensitive, detective probe for biologically relevant citrate anions in various applications.

Highly selective iodide sensing ability of an anthraquinone-derived Schiff base in semi-aqueous medium and its performance in antioxidation, anti-inflammation and HRBC membrane protection

In this study, a new iodide (I⁻) ion selective chromogenic receptor (3) was developed by reacting 9,10-anthraquinone with 2,4-dinitrophenylhydrazine.

Polyamide-Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide-polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc^2-) through glutarate (glu^2-), α-ketoglutarate (kglu^2-), adipate (adi^2-), pimelate (pim^2-), suberate (sub^2-), to azelate (aze^2-) have shown adipate preference with association constant value of K = 4900 M^-1 in a H₂O/DMSO (50:50 v/v) binary solvent mixture. The binding constant increases from glu^2- to adi^2- and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes to the stability of the associations along with electrostatic interactions. Negative-mode electrospray ionization of aqueous solutions of host-guest complexes shows clear evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies.

Inhibition of the STAT3 Protein by a Dinuclear Macrocyclic Complex

A new diethylenetriamine-derived macrocycle bearing 2-methylpyridyl arms and containing m-xylyl spacers, L, was prepared, and its dinuclear copper(II) and zinc(II) complexes were used as receptors for the recognition in aqueous solution of a phosphorylated peptide derived from a sequence of the STAT3 protein. A detailed study of the acid-base behavior of L and of its complexation properties as well as of the association of the phosphorylated peptide to the receptor was carried out by potentiometry in aqueous solution at 298.2 K and I = 0.10 M in KNO3. The data revealed that the receptor forms stable associations with several protonated forms of the substrate, with constant values ranging from 3.32 to 4.25 log units. The affinity of the receptor for the phosphorylated substrate studied is higher at a pH value where the receptor is mainly in the [Cu2L](4+) form and the pY residue of the substrate is in the dianionic form (pH 6.55). These results, also supported by (31)P NMR studies, showed that the phosphopeptide is bound through the phosphoryl group in a bridging mode. Additionally, the receptor inhibited binding between active (phosphorylated) STAT3 and its target DNA sequence in a dose-dependent manner (IC50 63 ± 3.4 μM) in human nuclear extracts in vitro. Treatment of whole cells with the inhibitor revealed that it is bioactive in living cells and has oncostatic properties that could be interesting for the fight against cancer and other pathologies involving the STAT3 protein.

Advantages of Organic Halogen Bonding for Halide Recognition

The study of hydrogen bonding organocatalysis is rapidly expanding. Much research has been directed at making catalysts more active and selective, with less attention on fundamental design strategies. This study systematically increases steric hindrance at the active site of pH switchable urea organocatalysts. Incorporating strong intramolecular hydrogen bonds from protonated pyridines to oxygen stabilizes the active conformation of these ureas thus reducing the entropic penalty that results from substrate binding. The effect of increasing steric hindrance was studied by single crystal X-ray diffraction and by kinetics experiments of a benchmark reaction.

A tetraphenylethene-based zinc complex as a sensitive DNA probe by coordination interaction

We developed a new DNA probe by utilizing the coordination interaction of Zn(2+) with DNA and the consequent emission. Because the coordination interactions do not depend on the length of the DNA, the new probe exhibited much higher sensitivity for the detection of short ssDNA than the corresponding probe based on electrostatic interactions.

"3 + 1 = 6 + 2" in Cu(II) coordination chemistry of 1H-pyrazole aza cryptands

A polyazamacrocycle formed from two tris(2-aminoethyl)amine units connected by 1H-pyrazole units shows unique hexanuclear Cu(ii) complexes by combination of two binuclear Cu(ii) cryptand complexes through pyrazolate moieties belonging to both cryptands. The formation of these dimeric entities has been proven both in solution by potentiometric studies and mass spectroscopy and in the solid state by X-ray diffraction of crystals of three different batches of formulae [Cu6(H-3)2(H2O)2](TsO)6·22H2O (), [Cu6(H-3)2(NO3)2](NO3)4·2H2O () and [Cu6(H-3)2Cl2]Cl4·(C4H5N3O2)2·14.35H2O (). The hexanuclear unit in and can be viewed like three magnetically independent binuclear complexes with J = -366(3) cm(-1), g = 2.08(1) for and J = -360(3) cm(-1), g = 2.07(1) for .

Colorimetric and Optical Discrimination of Halides by a Simple Chemosensor

A thiophene-based tripodal copper(II) complex has been synthesized as a new colorimetric and optical chemosensor for naked-eye discrimination of halides in acetonitrile and an acetonitrile-water mixture. The binding interactions of the new receptor with several anions were analyzed by UV-Vis titrations, electrospray ionization mass spectrometric (ESI-MS) experiments and density functional theory (DFT) calculations. The results from UV-Vis titrations indicate that the coordinative unsaturated copper(II) complex strongly binds a halide at its vacant copper(II) centre via a metal-ligand bond forming a 1:1 complex, exhibiting binding affinities in the order of fluoride > chloride > bromide > iodide. The interactions of the receptor with halides were further confirmed by ESI-MS, showing a distinct signal corresponding to a 1:1 complex for each halide, suggesting that the noncovalent interactions also exist in the gas phase. In addition, time-dependent DFT (TD-DFT) calculations were also carried out to understand the excited-state properties of the chemosensor complexes. A detailed analysis of the TD-DFT calculations shows a consistent red-shift in the first optically-allowed transition, consistent with the observed colorimetric experiments.

A highly efficient dinuclear Cu(II) chemosensor for colorimetric and fluorescent detection of cyanide in water

A novel dinuclear copper chemosensor selectively binds cyanide over a wide range of inorganic anions, enabling it to detect cyanide in water up to 0.02 ppm which is 10 times lower than the EPA standard for drinking water.

Colourimetric and fluorescent detection of oxalate in water by a new macrocycle-based dinuclear nickel complex: a remarkable red shift of the fluorescence band

A new macrocycle-based dinuclear nickel chemosensor selectively binds oxalate anions both in solution and the solid state, displaying a remarkable red shift of the fluorescence band with a visible colour change in water at physiological pH in the presence of an external dye.