Development of novel LpxC inhibitors: chiral-pool synthesis of C-triazolyl glycosides

The Zn²⁺-dependent deacetylase LpxC plays an important role in the biosynthesis of the cell wall of Gram-negative bacteria and therefore represents an interesting target for the development of novel antibiotics. In a 10-step, chiral pool synthesis starting from d-mannose (3), a series of C-aryl furanosidic hydroxamic acids bearing a 1,4-disubstituted triazole ring in α-configuration at the furanose moiety was stereoselectively synthesized and tested for inhibitory activity against LpxC. The key step of the synthesis comprises a Cu(I) catalyzed Huisgen cycloaddition of terminal alkyne 10 with various azides to introduce diversity to the potential LpxC inhibitors. The X-ray crystal structure of the click product 11e proves the stereochemistry at the anomeric center and the substitution pattern of the triazole ring. The synthesized compounds did not inhibit LpxC.     

Design of bisquinolinyl malonamides as Zn2+ ion-selective fluoroionophores based on the substituent effect

A series of malonamides possessing two quinoline moieties were synthesized and characterized as fluoroionophores for the Zn²⁺ ion. We focused on the relationship between the substituents introduced to the C2-position of the malonamides and their Zn²⁺ ion-selectivity, exploiting the structural effect of the substituents in the design of the fluoroionophores with high selectivity. The substituents introduced to the malonamides were the methyl, benzyl and naphthalenylmethyl groups. In dimethyl sulfoxide solvent, all substituted bisquinolinyl malonamides showed excellent fluorescence sensing for the Zn²⁺ ion, while unsubstituted bisquinolinyl malonamide 1 displayed ratiometric sensing for the Co²⁺ ion. N,N′-Bis(8-quinolyl)-2-methyl-2-naphthalenylmethyl malonamide 4 exhibited the highest Zn²⁺ ion-selectivity against the Cd²⁺ ion. Although the substituents introduced into the C2-position are spatially distant from the quinoline recognition moiety, this study indicated that they greatly influenced the ion selectivities of the bisquinolinyl malonamides. Furthermore, it was demonstrated that visible fluorescence analyses could be performed on malonamide 4.     

C-Triazolyl β-d-furanosides as LpxC inhibitors: stereoselective synthesis and biological evaluation

C-Triazolyl β-d-furanosides 10a–f were synthesized in a stereocontrolled way, starting from d-mannose. In the key steps of the synthesis a diastereoselective reduction of hemiketal 14 and a Cu(I) catalyzed [3+2]-cycloaddition of central building block 18 with various azides were performed. The synthesized hydroxamic acids were tested for their inhibitory activity against LpxC, a Zn²⁺-dependent deacetylase playing an important role in the biosynthesis of lipid A and therefore representing an interesting target for the development of novel antibiotics against Gram-negative bacteria. The C-triazolyl glycosides 10a–f did not exhibit antibiotic activity. However, the described synthesis is a versatile way to access C-triazolyl β-d-furanosides bearing all of their substituents at the same side of the tetrahydrofuran ring.     

Conformationally regulated fluorescent sensors. Study of the selectivity in Zn versus Cd sensing

The Zn²⁺ and Cd²⁺ complexing properties of four ligands containing a 4,4′-substituted biphenyl moiety are described. Ligands 1 and 3, containing only one 1-aza-18-crown-6 cavity, lead to selective complexation of Cd²⁺ versus Zn²⁺. Ligand 4, with two crown cavities linked to a tetramethylbenzidine unit, is able to form 1:1 complexes with Zn²⁺ and Cd²⁺, showing a higher complexing constant with Zn²⁺ than with Cd²⁺, probably due to enthalpic factors. Several complementary experiments suggest that the 1:1 complexes formed by ligand 4 involve both crown cavities acting together to give rise to clamp structures. The formation of this type of zinc complex gives rise to red shifted emission bands and distinct quenching of the fluorescence. A similar situation is observed with cadmium but the change is then less pronounced. When mixtures of both salts are used, ligand 4 selectively responds to zinc. Finally, ligand 2, which also has two crown cavities but contains nitro rather than amino groups in the biphenyl moiety, shows no propensity to form clamp complexes and, for this reason, it complexes cadmium much more strongly than zinc and binds the former selectively when mixtures of both salts are used in complexing experiments.     

Synergistic effects of macrocyclic polyethers and converging anionic binding sites : Synthesis of biphenyl crown ethers and lipophilization of divalent cations

The synthesis of nine novel macrocyclic polyethers with a 1,1'-biphenyl subunit is described. Crown ethers 2,4,5 and 7 have substituents with terminal acid groups at the 4- and 4'- positions of the 1,1'-biphenyl subunit and crown ethers 9, 11, 12 and 14 have similar substituents at the 3- and 3'-positions. In the ionic form these crown ethers extract divalent cations (Ca²⁺, Sr²⁺ and Ba²⁺) from basic aqueous solutions into chloroform. The degree of lipophilization varies with the size of the cation and of the crown ether cavity, with the position and the length of the substituents and with the nature of the terminal acid groups. ¹H NMR spectroscopic data of the complexes in chloroform are in agreement with a structure of the complexes in which the cation is encapsulated by oxygen atoms and anionic groups.     

Reaction of Zn(II) with a BINOL-amino-acid Schiff base: An unusual off-on-off fluorescence response

An amino-acid based Schiff base (S)-2 is prepared from the condensation of (S)-3,3′-diformyl BINOL (BINOL?=?1,1′-bi-2-naphthol) with l-valine in the presence of tetrabutylammonium hydroxide in methanol. This compound is found to exhibit off-on-off fluorescence response toward Zn²⁺. The spectroscopic studies reveal that (S)-2 reacts with 1 equiv Zn²⁺ to form a dimeric [2+2] complex with greatly enhanced fluorescence. Excess amount of Zn²⁺ might cause dissociation of this dimeric complex to give significantly reduced fluorescence.     

New regioisomeric naphthol-substituted thiazole based ratiometric fluorescence sensor for Zn2+ with a remarkable red shift in emission spectra

Ratiometric fluorescent chemosensors based on the position of ring annulation of the naphthol–thiazole moiety for quantification of zinc ions in aqueous ethanol were synthesized and investigated. It was found that sensor 3 exhibited a remarkably large red shift of 140 nm in emission upon complexation with Zn²⁺. A TD-B3LYP/6-31G (d,p) calculation was performed to characterize the nature of the fluorescence behavior of sensor 3 upon Zn²⁺ complexation. The combination of experimental and computational analyses provides a more complete understanding of the molecular level origin of these unique photophysical properties of this type of chemosensor.     

The synthesis of luminescent lanthanide-based chemosensors for the detection of zinc ions

Herein, we report the synthesis of two lanthanide-based chemosensors, Tb-5 and Eu-6, designed to sense free zinc ions (Zn²⁺) in aqueous solutions. The Tb-5 complex features a bis(2-pyridinylmethyl)amine moiety as a zinc(II)-responsive lanthanide-sensitising ‘antenna’, while Eu-6 incorporates a quinoline-based moiety for this purpose. Luminescence enhancements of 210% and 340% are observed upon addition of Zn²⁺ ions to Tb-5 and Eu-6, respectively. Both sensors are selective for Zn²⁺ ions over several other cations of environmental significance.     

A highly selective ratiometric fluorescent chemosensor for Zn2+ ion based on a polyimine macrocycle

A new ratiometric fluorescent chemosensor based on a polyimine macrocycle ligand 1 has been synthesized. The chemosensor can exhibit a pronounced fluorescence response and high selectivity to Zn²⁺ ion over other 15 metal ions, including Cd²⁺. Sensor 1 appears an emission peak at 370 nm. Upon the addition of Zn²⁺ ion, the typical emission peak for 1 at 370 nm is obviously quenched, but a new emission peak at around 470 nm appears and shows a large enhancement due to the formation of a 1:1 Zn²⁺-1 complex. In addition, there is a good linear relationship between the fluorescence ratio I_470nm/I_370nm and the concentration of Zn²⁺, which makes a ratiometric assay of Zn²⁺ ion possible.     

A highly selective turn-on fluorescent chemosensor for copper(II) ion

A new pyrene derivative (1) containing a diaminomaleonitrile moiety exhibits high selectivity for Cu²⁺ detection. Significant fluorescence enhancement was observed with chemosensor 1 in the presence of Cu²⁺. However, the metal ions Ag⁺, Ca²⁺, Cd²⁺, Co²⁺, Fe²⁺, Fe³⁺, Hg²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ produced only minor changes in fluorescence values for the system. The apparent association constant (K_a) of Cu²⁺ binding in chemosensor 1 was found to be 5.55×10³ M⁻¹. The maximum fluorescence enhancement caused by Cu²⁺ binding in chemosensor 1 was observed over the pH range 5-7.5.     

Proton- and metal cation-enhanced excited state intramolecular proton transfers of 2-(2-hydroxyfluorophenyl)benzoxazole having imidazole moiety

2-(2-Hydroxyfluorophenyl)benzoxazole having an imidazole moiety 1 was synthesized by the two step reactions starting from 2-(pentafluorophenyl)benzoxazole. Protonation at the 3-imidazole nitrogen atom of 1 enhances the green emission around 500 nm, where the positive character caused by the protonation is inductively communicated to the hydroxy group, to recover its intramolecular hydrogen bonding, leading to the ESIPT process. Addition of Al³⁺ or Zn²⁺ to 1 enhances both the green emission and the blue emission around 450 nm in chloroform-acetonitrile.     

Three new turn-on fluorescent sensors for the selective detection of Zn2+: Synthesis,properties and DFT studies

The design and synthesis of three new 1,8-naphthalimide-based fluorescent sensors (1–3) for the detection of Zn²⁺ in aqueous solution is described. The structural architect of these sensors contains 1,8-naphthalimide scaffold as a fluorophore attached to 2,2′-dipicolylamine (DPA) and bis(2-quinolinylmethyl)amine (DQA) receptors through an amide linkage. The addition of Zn²⁺ to the solutions of sensors (1–3) led to enhanced fluorescence intensity, ranging between 2.5 and 14 folds. At physiological pH (pH = 7.4), these sensors exhibited high selectivities for Zn²⁺ over a wide range of competing metal cations, displaying high sensitivities with a limit of detections of 120, 81.7 and 79.2 nM, respectively. This suggests that these sensors can detect chronic Zn²⁺ concentration for freshwater (>1.84 μM), designated by the U.S. Environmental Protection Agency. DFT simulations performed on the more stable stacked conformations of unbound and Zn²⁺ bounded states suggested that the latter display higher density of excited states than the unbound sensors. Moreover, the stacked conformer of sensor 3 was significantly more stable as compared to sensors 1 and 2, which was attributed to a stronger Van Der Waals (VDW) interaction between DQA and 1,8-naphthalimide. The Zn²⁺ binding leads to enhanced electronic coupling between the HOMOs and LUMOs, making excited states more populated which then undergoes geometric relaxation before emitting light and relaxing back to the ground states. The lower energy separation (5.0 eV) between the HOMO and the first Zn²⁺ d-orbital in sensor 3 as compared to sensors 1 and 2 results in enhanced density of the generated states and subsequently higher intensity upon binding with Zn²⁺.     

Exploiting the deprotonation mechanism for the design of ratiometric and colorimetric Zn fluorescent chemosensor with a large red-shift in emission

The design, synthesis, and photophysical evaluation of a new naphthalimide-based fluorescent chemosensor, N-butyl-4-[di-(2-picolyl)amino]-5-(2-picolyl)amino-1,8-naphthalimide (1), were described for the detection of Zn²⁺ in aqueous acetonitrile solution at pH 7.0. Probe 1 showed absorption at 451 nm and a strong fluorescence emission at 537 nm (Φ_F=0.33). The capture of Zn²⁺ by the receptor resulted in the deprotonation of the secondary amine conjugated to 1,8-naphthalimide so that the electron-donating ability of the N atom would be greatly enhanced; thus probe 1 showed a 56 nm red-shift in absorption (507 nm) and fluorescence spectra (593 nm, Φ_F=0.14), respectively, from which one could sense Zn²⁺ ratiometrically and colorimetrically. The deprotonated complex, [(1-H)/Zn]⁺, was calculated at m/z 619.1800 and measured at m/z 618.9890. In contrast to these results, the emission of 1 was thoroughly quenched by Cu²⁺, Co²⁺, and Ni²⁺. The addition of other metal ions such as Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Fe³⁺, Mn²⁺, Al³⁺, Cd²⁺, Hg²⁺, Ag⁺, and Pb²⁺ produced a nominal change in the optical properties of 1 due to their low affinity to probe 1. This means that probe 1 has a very high fluorescent imaging selectivity to Zn²⁺ among metal ions.     

Photochemistry of β, γ-unsaturated ketones-V: The direct irradiation of some γ-phenyl β, γ-enones

The photochemistry of some members of the two series of γ-phenyl substituted acyclic β, γ-unsaturated ketones 1 and 2 upon direct irradiation with γ 310nm has been investigated, viz 1c–1h and 2b+2c.The alkyl substituted (E)-5-phenyl-4-penten-2-ones 1c–1h yield the corresponding 1,3-acyl shift products and (Z)-isomers, and 1g and 1h in addition two decarbonylated products. 2b only yields the (Z)-isomer and some benzaldehyde, but 2c yields the 1,3-acyl shift product, the ODPM product, three hydrocarbons formed by disproportionation of the allyl radical, and some benzaldehyde. The β-phenyl β, γ-UK 3a proved to be photostable. The 1,3-acyl shift products of 1c–1h result mainly from the singlet excited state in a cage radical process. The exclusive formation of the (E)-configuration of the 1,3-acyl shift product is explained in terms of conformational preference of the intermediate allyl radical. It is proposed that the formation of the (Z)-isomer proceeds from ¹T(π -π^*) which is populated according to

. Evidence is presented which supports the proposed mechanism.The β,γ-UK 2b containing a benzoyl moiety leads to a higher degree of (E)-(Z) isomerization than the corresponding 1d which has an acetyl moiety.The triplet energies of (E)- and (Z)-1h are 56 and ca 70 kcal/mol respectively.     

Metallovesicular catalytic hydrolysis of p-nitrophenyl picolinate catalyzed by zinc(II) complexes of pyridyl ligands in vesicular solution

The hydrolysis of p-nitrophenyl picolinate (PNPP) catalyzed by the Zn(II) complexes of 6-(n-butyloxymethyl)-2-(hydroxymethyl)pyridine and 6-(n-dodecyloxymethyl)-2-(hydroxymethyl)pyridine was kinetically investigated by observation of the rates of release of p-nitrophenol in a vesicular solution at different pH values and temperatures. A 1:1 ligand:Zn²⁺ stoichiometry was found to produce the most active species based on a kinetic version of the Job plot analysis. Experimental results also showed that the complex formed from ligand 2 and Zn²⁺ exhibited a more remarkable rate acceleration effect on the hydrolysis of PNPP in vesicular solution than that formed from ligand 1 and Zn²⁺.     

Naphthalimide-based fluorescent Zn chemosensors showing PET effect according to their linker length in water

We have developed naphthalimide-based fluorescent chemosensors that exhibit fluorescence enhancement upon binding Zn²⁺ ion in 10 mM HEPES buffer (pH 7.4) at 25 °C. The fluorescence enhancement was induced by a PET inhibition process in which electron transfer from the nitrogen lone pair electrons of the Dpa unit to naphthalimide was blocked upon the binding of the sensor to Zn²⁺. The longer the linker length (n = 1-3) of the sensor, the less the PET efficiency becomes. Among the sensors (1, 2, and 3) examined, 1 shows the highest selectivity and sensitivity for Zn²⁺ over other transition metal ions and alkali metal ions in water.     

A functional applied material on recognition of metal ion zinc based on the double azine compound

A colorimetric and fluorescent probe L has been designed and synthesized, which bearing the double azine moiety and showing a detection limit of 2.725 × 10^?7 M towards Zn²⁺. Based on the basic recognition mechanism of ESIPT and CHEF effect, the L has high selectivity and sensitivity to only Zn²⁺ (not Fe³⁺, Hg²⁺, Ag⁺, Ca²⁺, Co²⁺, Ni²⁺, Cd²⁺, Pb²⁺, Cr³⁺, and Mg²⁺) within the physiological pH range (pH = 7.0–8.4) and showed a fluorescence switch. Moreover, this detection progress occured in the DMSO/H₂O ～ HEPES buffer (80/20, v/v; pH 7.23) solution which can conveniently used on test strip.     

A quinoline-based selective ‘turn on’ chemosensor for zinc(II) via quad-core complex,and its application in live cell imaging

An efficient quinoline-based fluorescent chemosensor (QLNPY) was successfully developed for the detection of zinc ions (Zn²⁺). This novel chemosensor displayed higher sensitivity and selectivity toward Zn²⁺ over other competitive metal ions accompanying with obvious fluorescence enhancement. The QLNPY-Zn²⁺ complex can be further used as a new fluorescent “turn-off” sensor for pyrophosphate (PPi) and sulfur ion (S^2?) via a Zn²⁺ displacement approach. The limits of detection were calculated to be 3.8 × 10^?8 M for Zn²⁺, 3.7 × 10^?7 M for PPi and 4.9 × 10^?7 M for S^2?. The binding mechanism of QLNPY and Zn²⁺ was investigated through NMR, HR-MS analysis and further studied by crystallographic analysis. Additionally, further application of QLNPY for sequential bioimaging of Zn²⁺ and PPi was studied in HepG2 cells, suggesting that the quinoline-based chemosensor possesses great potential applications for the detection of intracellular Zn²⁺ and PPi in vivo.     

High-Affinity Ratiometric Fluorescence Probe Based on 6-Amino-2,2′-Bipyridine Scaffold for Endogenous Zn2+ and Its Application to Living Cells

Zinc is an essential trace element involved in many biological activities; however, its functions are not fully understood. To elucidate the role of endogenous labile Zn²⁺, we developed a novel ratiometric fluorescence probe, 5-(4-methoxyphenyl)-4-(methylsulfanyl)-[2,2′-bipyridin]-6-amine (6 (rBpyZ)) based on the 6-amino-2,2′-bipyridine scaffold, which acts as both the chelating agent for Zn²⁺ and the fluorescent moiety. The methoxy group acted as an electron donor, enabling the intramolecular charge transfer state of 6 (rBpyZ), and a ratiometric fluorescence response consisting of a decrease at the emission wavelength of 438 nm and a corresponding increase at the emission wavelength of 465 nm was observed. The ratiometric probe 6 (rBpyZ) exhibited a nanomolar-level dissociation constant (K_d = 0.77 nM), a large Stokes shift (139 nm), and an excellent detection limit (0.10 nM) under physiological conditions. Moreover, fluorescence imaging using A549 human lung adenocarcinoma cells revealed that 6 (rBpyZ) had good cell membrane permeability and could clearly visualize endogenous labile Zn²⁺. These results suggest that the ratiometric fluorescence probe 6 (rBpyZ) has considerable potential as a valuable tool for understanding the role of Zn²⁺ in living systems.     

Binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes with Schiff-bases derived from crown ether platforms: Rare examples of ether oxygen atoms bridging metal centers

Bibracchial lariat ethers L³ and L⁴, derived from the condensation of N,N′-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 or N,N′-bis(2-aminobenzyl)-4,13-diaza-18-crown-6 with salicylaldehyde, form binuclear complexes with Co(II), Ni(II), Cu(II) and Zn(II). Our studies show that the different denticity and crown moiety size of the two related receptors give rise to important differences on the structures of the corresponding complexes. Single crystal X-ray diffraction analysis shows that the [Ni₂(L³)(H₂O)₂]²⁺ and [Cu₂(L³)(NO₃)]⁺ complexes constitute a rare example in which an oxygen atom of the crown moiety is bridging the two six coordinate metal ions. In contrast, none of the oxygen atoms of the crown moiety is acting as a bridging donor atom in the [Co₂(L⁴)(CH₃CN)₂]²⁺, [Cu₂(L⁴)]²⁺ and [Zn₂(L⁴)]²⁺ complexes. This is attributed to the larger size the crown moiety and the higher denticity of L⁴ compared to L³. In [Co₂(L⁴)(CH₃CN)₂]²⁺ the metal ions show a distorted octahedral coordination, while in the Cu(II) and Zn(II) analogues the metal ions are five-coordinated in a distorted trigonal bipyramidal environment. In [Cu₂(L³)(NO₃)]⁺ the coordinated nitrate anion acts as a bidentate bridging ligand, which results in the formation of a 1D coordination polymer.