Bidentate Binding Research Articles

Effect of PDI ligand binding pattern on the electrocatalytic activity of two Ru(II) complexes for CO2 reduction

The electrochemical properties of two complexes, [RuII(η3‐(N,N,N)‐OMePDI)Cl2(PPh3)]0 (1) and [RuII(η2‐(C,N)‐OMePDI‐H)Cl (PPh3)2]0 (2), were studied. In octahedral complex 1, bis(imino)pyridine (PDI) is a tridentate η3‐N,N,N‐coordinated ligand, whereas in trigonal‐bipyramidal complex 2, the deprotonated PDI ligand adopts the unusual bidentate binding mode η2‐C,N to coordinate to the central Ru(II) ion. Bulk electrolysis in two electrolyte solutions of acetonitrile (MeCN) and tetrahydrofuran (THF) suggests that complexes 1 and 2 have very different electrocatalytic CO2 reduction activities. In MeCN solution, complex 1 can selectively electrocatalytic CO2 reduction to CO with a Faradaic efficiency of about 50% and a turnover frequency (TOF) of 4.4 s−1, whereas complex 2 can perform electrocatalytic of CO2 reduction with a Faraday efficiency of ~22% and a TOF of 0.3 S−1. The electrocatalytic CO2 reduction selectivity and activity of the two complexes are poor when the solvent is changed to THF. Combined with the results of the density functional theory calculation, we propose that the binding pattern of the redox‐active ligand OMePDI has a significant effect on the electrocatalytic activity for the two Ru(II)PDI complexes.

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array.

The development of synthetic receptors for the selective binding and discrimination of anions in water requires an understanding of how anions interact with these synthetic receptors. Molecules designed to differentiate nucleoside phosphate anions (e.g. ATP, ADP, GTP, GDP, UDP) under physiological conditions could underpin exciting new sensing tools for biomedical research and drug discovery, but it is very challenging due to the similarities in anion structure, size and charge. We present a series of lanthanide-based anion receptors and establish key structural elements that impact on nucleoside phosphate anion binding and sensing. Structural evidence of anion binding using X-ray crystallographic and NMR data, supported by DFT calculations indicate the binding modes between the lanthanide complexes and certain phosphoanions, revealing a bidentate (α-, γ-) binding mode to ATP. We further use four of the receptors to allow discrimination of eight nucleoside phosphate anions in the first array-based assay using lanthanide complexes, taking advantage of the multiple emission bands and long emission lifetimes associated with luminescent lanthanide complexes.

Synthesis, Spectral, Thermal and DPPH Scavenging Studies of the Hydrazone Derived Transition Metal Complexes

Schiff base ligand 2,2-{benzene-1,4-diylbis[(E)methylylidene(1E,2E)hydrazine-2,1-diylidene(E)methylidene]}diphenol (H2-BHMD) was synthesized by reacting together the ortho vaniline and 1,4-bis(hydrazonomethyl)benzene. It was characterized by analytical and spectroscopic techniques including 1H-NMR, elemental analysis, FT-IR, and UV-Visible studies. Different transition metal ions like Co (II), Ni (II), Cu (II) and Zn (II) were reacted with well characterized ligand to obtain the metal derivatives. AAS, elemental analysis, FT-IR, molar conductance and UV-Visible methods were used to study the metal complexes. The analytical and spectroscopic technique suggested the formation of coordination polymers. The FT-IR results confirmed the bidentate binding of the ligand involving phenolic oxygen and azomethine nitrogen. The UV-visible studies suggested the formation of distorted square planar geometries for all the metal complexes. Hydrazone Schiff bases are known for their antioxidant activity, therefore the synthesized hydrazone and the corresponding metal complexes were also studied for their antioxidant capabilities utilizing DPPH free radicals. Among all complexes, the Co-BHMD demonstrated noteworthy percent hindrance to the free radicals, yet much lower than the standard medication utilized. The metal subsidiaries were likewise studied for their thermal degradation. The respective TG bands were utilized for dynamic and thermodynamic parameters estimation utilizing Horowitz-Metzger strategy.

Synthesis, Spectral, Thermal and DPPH Scavenging Studies of the Hydrazone Derived Transition Metal Complexes

Schiff base ligand 2,2-{benzene-1,4-diylbis[(E)methylylidene(1E,2E)hydrazine-2,1-diylidene(E)methylidene]}diphenol (H2-BHMD) was synthesized by reacting together the ortho vaniline and 1,4-bis(hydrazonomethyl)benzene. It was characterized by analytical and spectroscopic techniques including 1H-NMR, elemental analysis, FT-IR, and UV-Visible studies. Different transition metal ions like Co (II), Ni (II), Cu (II) and Zn (II) were reacted with well characterized ligand to obtain the metal derivatives. AAS, elemental analysis, FT-IR, molar conductance and UV-Visible methods were used to study the metal complexes. The analytical and spectroscopic technique suggested the formation of coordination polymers. The FT-IR results confirmed the bidentate binding of the ligand involving phenolic oxygen and azomethine nitrogen. The UV-visible studies suggested the formation of distorted square planar geometries for all the metal complexes. Hydrazone Schiff bases are known for their antioxidant activity, therefore the synthesized hydrazone and the corresponding metal complexes were also studied for their antioxidant capabilities utilizing DPPH free radicals. Among all complexes, the Co-BHMD demonstrated noteworthy percent hindrance to the free radicals, yet much lower than the standard medication utilized. The metal subsidiaries were likewise studied for their thermal degradation. The respective TG bands were utilized for dynamic and thermodynamic parameters estimation utilizing Horowitz-Metzger strategy.

Ethylene carbonate adsorption on the major surfaces of lithium manganese oxide Li1-xMn2O4 spinel (0.000 < x < 0.375): a DFT+U-D3 study.

Understanding the surface reactivity of the commercial cathode material LiMn2O4 towards the electrolyte is important to improve the cycling performance of secondary lithium-ion batteries and to prevent manganese dissolution. In this work, we have employed spin-polarized density functional theory calculations with on-site Coulomb interactions and long-range dispersion corrections [DFT+U-D3-(BJ)] to investigate the adsorption of the electrolyte component ethylene carbonate (EC) onto the (001), (011) and (111) surfaces of the fully lithiated and partially delithiated Li1-xMn2O4 spinel (0.000 < x < 0.375). The surface interactions were investigated by evaluating the adsorption energies of the EC molecule and the surface free energies. Furthermore, we analyzed the impact of EC adsorption on the Wulff crystal morphologies, the molecular vibrational frequencies and the adsorbate/surface charge transfers. The adsorption energies indicate that the EC molecule strongly adsorbs on the (111) facet, which is attributed to a bidentate binding configuration. We found that EC adsorption enhances the stability of the (111) facet, as shown by the Wulff crystal morphologies. Although a negligible charge transfer was calculated between the spinel surfaces and the EC molecule, a large charge rearrangement takes place within the surfactant upon adsorption. The wavenumbers of the C[double bond, length as m-dash]O stretching mode for the interacting EC molecule are red-shifted with respect to the isolated adsorbate, suggesting that this bond becomes weaker. The surface free energies show that both the fully lithiated and partially delithiated forms of the LiMn2O4 surfaces are stabilized by the EC molecule.

Computational Screening of Metal–Organic Framework-Supported Single-Atom Transition-Metal Catalysts for the Gas-Phase Hydrolysis of Nerve Agents

Recent studies have suggested that the gas-phase hydrolysis of nerve agents by Zr-based metal–organic frameworks (MOFs) may be limited by product inhibition resulting from strong bidentate binding of the hydrolysis products to the Zr6-nodes. A potential method to avoid this problem is to deposit single-atom catalysts on the nodes so that the products bind in a more favorable monodentate mode. Such catalytic active sites can be characterized with atomic precision, enabling detailed computational mechanistic studies. Thus, we used density functional theory to perform a comprehensive screening of single-atom transition-metal catalysts, in varying oxidation states, deposited on NU-1000 nodes for the gas-phase hydrolysis of the nerve agent sarin. By calculating the complete reaction pathways for M–NU-1000 systems, we discovered that the highest reaction barrier varies between catalysts, highlighting the need to consider more than a single reaction step when screening a large number of diverse materials. The si...

NMR Quantification of the Effects of Ligands and Counterions on Lewis Acid Catalysis.

The relative Lewis acidity of a variety of metal-ligand catalyst complexes is quantified using 31P NMR spectroscopy. Three 31P NMR probes, including two new bidentate binding probes, are compared on the basis of different binding modes (i.e., monodentate vs bidentate) and the relative scale of their downfield shift upon binding to Lewis acid complexes. Bidentate coordination of catalyst complexes including metal catalysts, ligands, and counterions were assessed due to their importance to asymmetric catalysis. The effect of ligands, counterions, and additives on Lewis acidity is quantified and correlated to reaction yield at an early time point as an approximation for catalytic activity/efficiency and chelation mode in two organic transformations. Binding studies were performed under catalytically relevant conditions, giving further applicability to synthesis. Insight into activation modes are revealed through this analysis.

Structural and Biochemical Investigations of the [4Fe-4S] Cluster-Containing Fumarate Hydratase from Leishmania major.

Class I fumarate hydratases (FHs) are central metabolic enzymes that use a [4Fe-4S] cluster to catalyze the reversible conversion of fumarate to S-malate. The parasite Leishmania major, which is responsible for leishmaniasis, expresses two class I FH isoforms: mitochondrial LmFH-1 and cytosolic LmFH-2. In this study, we present kinetic characterizations of both LmFH isoforms, present 13 crystal structures of LmFH-2 variants, and employ site-directed mutagenesis to investigate the enzyme’s mechanism. Our kinetic data confirm that both LmFH-1 and LmFH-2 are susceptible to oxygen-dependent inhibition, with data from crystallography and electron paramagnetic resonance spectroscopy showing that oxygen exposure converts an active [4Fe-4S] cluster to an inactive [3Fe-4S] cluster. Our anaerobically conducted kinetic studies reveal a preference for fumarate over S-malate. Our data further reveal that single alanine substitutions of T467, R421, R471, D135, and H334 decrease kcat values 9–16000-fold without substantially affecting Km values, suggesting that these residues function in catalytic roles. Crystal structures of LmFH-2 variants are consistent with this idea, showing similar bidentate binding to the unique iron of the [4Fe-4S] cluster for substrate S-malate as observed in wild type FH. We further present LmFH-2 structures with substrate fumarate and weak inhibitors succinate and malonate bound in the active site and the first structure of an LmFH that is substrate-free and inhibitor-free, the latter showing increased mobility in the C-terminal domain. Collectively, these data provide insight into the molecular basis for the reaction catalyzed by LmFHs, enzymes that are potential drug targets against leishmaniasis.

A Chemical Probe for Tudor Domain Protein Spindlin1 to Investigate Chromatin Function.

Modifications of histone tails, including lysine/arginine methylation, provide the basis of a "chromatin or histone code". Proteins that contain "reader" domains can bind to these modifications and form specific effector complexes, which ultimately mediate chromatin function. The spindlin1 (SPIN1) protein contains three Tudor methyllysine/arginine reader domains and was identified as a putative oncogene and transcriptional coactivator. Here we report a SPIN1 chemical probe inhibitor with low nanomolar in vitro activity, exquisite selectivity on a panel of methyl reader and writer proteins, and with submicromolar cellular activity. X-ray crystallography showed that this Tudor domain chemical probe simultaneously engages Tudor domains 1 and 2 via a bidentate binding mode. Small molecule inhibition and siRNA knockdown of SPIN1, as well as chemoproteomic studies, identified genes which are transcriptionally regulated by SPIN1 in squamous cell carcinoma and suggest that SPIN1 may have a role in cancer related inflammation and/or cancer metastasis.

New Evidence of the Bidentate Binding Mode in 3-MBA Protected Gold Clusters: Analysis of Aqueous 13-18 kDa Gold-Thiolate Clusters by HPLC-ESI-MS Reveals Special Compositions Aun(3-MBA)p, (n = 48-67, p = 26-30).

Gold clusters protected by 3-MBA ligands (MBA = mercaptobenzoic acid, –SPhCO2H) have attracted recent interest due to their unusual structures and their advantageous ligand-exchange and bioconjugation properties. Azubel et al. first determined the core structure of an Au68-complex, which was estimated to have 32 ligands (3-MBA groups). To explain the exceptional structure-composition and reaction properties of this complex, and its larger homologs, Tero et al. proposed a “dynamic stabilization” via carboxyl O–H––Au interactions. Herein, we report the first results of an integrated liquid chromatography/mass spectrometer (LC/MS) analysis of unfractionated samples of gold/3-MBA clusters, spanning a narrow size range 13.4 to 18.1 kDa. Using high-throughput procedures adapted from bio-macromolecule analyses, we show that integrated capillary high performance liquid chromatography electrospray ionization mass spectrometer (HPLC-ESI-MS), based on aqueous-methanol mobile phases and ion-pairing reverse-phase chromatography, can separate several major components from the nanoclusters mixture that may be difficult to resolve by standard native gel electrophoresis due to their similar size and charge. For each component, one obtains a well-resolved mass spectrum, nearly free of adducts or signs of fragmentation. A consistent set of molecular mass determinations is calculated from detected charge-states tunable from 3− (or lower), to 2+ (or higher). One thus arrives at a series of new compositions (n, p) specific to the Au/3-MBA system. The smallest major component is assigned to the previously unknown (48, 26); the largest one is evidently (67, 30), vs. the anticipated (68, 32). Various explanations for this discrepancy are considered. A prospective is given for the various members of this novel series, along with a summary of the advantages and present limitations of the micro-scale integrated LC/MS approach in characterizing such metallic-core macro-molecules, and their derivatives.

Synthesis, structures and reactivity of cobalt(II) complexes supported by N,N,N′,N″-tetradentate N′-substituted bis((1H-pyrazol-1-yl)methyl)amine

A series of cobalt(II) complexes, namely [LnCo(NO3)]+Y− (Ln = LA–LD; Y = NO3−, BPh4−) was prepared by the reaction of the corresponding metal starting materials with N,N-bis((1H-pyrazol-1-yl)methyl)-2-(piperidin-1-yl)ethanamine (LA), N,N-bis((1H-pyrazol-1-yl)methyl)-2-morpholinoethanamine (LB), N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-2-(piperidin-1-yl)ethanamine(LC) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-2-morpholinoethanamine (LD), respectively. X-ray diffraction studies revealed that the coordination geometry around the Co(II) centre in [LnCo(NO3)]BPh4 (Ln = LA, LB and LD) can be best described as distorted trigonal bipyramidal, whereas [LCCo(NO3)]NO3 showed a distorted octahedral geometry around the Co(II) centre due to the bidentate binding mode of the ligand NO3−. Specifically, the presence of methyl substituents at the pyrazole moieties plays an influential role in determining the anionic exchange capabilities of these complexes. The anion exchange reaction was found to be highly selective; the substitution of the smaller anion NO3− to the bigger anion BPh4− was achieved easily, whereas the attempted anion exchange of BPh4− with NO3− proved unsuccessful.

How Do Actinyls Interact with Hyperphosphorylated Yolk Protein Phosvitin?

The development of the nuclear industry has raised multiple questions about its impact on the biotope and humans. Proteins are key biomolecules in cell machinery and essential in deciphering toxicological processes. Phosvitin was chosen as a relevant model for phosphorylated proteins because of its important role as an iron, calcium, and magnesium storage protein in egg yolk. A multitechnique spectroscopic investigation was performed to reveal the coordination geometry of two oxocations of the actinide family (actinyl UVI , NpV ) in speciation with phosvitin. IR spectroscopy revealed phosphoryl groups as the main functional groups interacting with UVI . This was confirmed through laser luminescence spectroscopy (U) and UV/Vis absorption spectroscopy (Np). For UVI , X-ray absorption spectroscopy at the LIII edge revealed a small contribution of bidentate binding present, along with predominantly monodentate binding of phosphoryl groups; for NpV , uniquely bidentate binding was revealed. As a perspective to this work, X-ray absorption spectroscopy speciation of UVI and NpV in the extracted yolk of living eggs of the dogfish Scyliorhinus canicula was determined; this corroborated the binding of phosphorous together with a reduction of the actinyl moiety. Such data are essential to pinpoint the mechanisms of heavy metals (actinyls) accumulation and toxicity in oviparous organisms, and therefore, contribute to a shift from descriptive approaches to predictive toxicology.

Probing Conformation Change and Binding Mode of Metal Ion–Carboxyl Coordination Complex through Resonant Surface-Enhanced Raman Spectroscopy and Density Functional Theory

Understanding carboxyl-metal ligand interaction has great significance in analytical chemistry. Herein, we use resonant surface-enhanced Raman scattering (SERS) to probe the physiochemical interaction and conformation change in several metal ion-carboxyl coordination complex systems adsorbed on the surface of plasmonically resonant metal nanostructures. Our SERS results and density function theory calculations jointly reveal that low-valence metal ions (such as K+ and Pb2+) tend to bind to the carboxyl active site of a Raman tag molecule, 4-mercaptobenzoic acid (4-MBA), in a unidentate binding mode of low binding energy whereas high-valence metal ions (such as Fe3+) favor a bidentate binding mode of relatively high binding energy. Particularly, Pb2+-ion concentration-dependent SERS suggests a repulsive interaction among the coordination complex leading to a tilted configuration of 4-MBA on the metal surface. This work indicates the resonant SERS approach is suitable not only for studying the carboxyl-metal ligand interaction but also for detecting various types of heavy metal ions at low concentrations.

Syntheses, crystal structures and DNA binding potential of copper(II) carboxylates

Copper(II) carboxylate complexes [Cu2{CH3CH(C6H5)CH2COO}4]n (1) and [Cu2{CH3CH(C6H5)CH2COO}4(3-MePy)2] (2) were synthesized and characterized by FT-IR and single crystal X-ray analyses. Bridging bidentate binding modes of the carboxylate ligands were first suggested by FT-IR spectra and later on confirmed by x-ray diffraction. Complexes 1 and 2 exhibited polymeric and discrete molecular units, respectively, with a paddle wheel arrangement of the bridging carboxylate ligands around the copper centers with octahedral coordination geometry. Both the complexes have shown hypochromism with no (complex 2) or a very small red shift of 3 nm (complex 1) on addition of SSDNA suggesting a mixed binding mode consisting of partial intercalation of the complexes into the base pairs of DNA together with groove binding.Molecular docking studies have shown increase in steric descriptors including octanol-water partition coefficient (slogP), molar refractivity (MR), and surface volume (Vsurf) with increasing binding strength of the complexes.

Comprehensive understanding of multiple binding of D-penicillamine with Cu2+-hexa aqua complex: a DFT approach

The multiple-ligand-binding of D-penicillamine with [Cu(H2O)6]2+ has been explored computationally using density functional theory (DFT). Because of the implementation of bulk aqueous medium and considering the pH at physiological level, both neutral as well as de-protonated analogues of D-penicillamine are taken into account to study the binding phenomena with Cu2+. In doing so, at first, we have studied the binding of both neutral and de-protonated analogues of two D-penicillamine with Cu2+ in bi-dentate mode by replacing four molecules of H2O (di-amino complex) and afterward, the binding of three D-penicillamine with Cu2+ has been investigated by substituting all six molecules of H2O (tri-amino complex). Apart from bi-dentate binding, the de-protonated form of D-penicillamine can also bind in tri-dentate mode and in that case, all six H2O molecules are substituted during di-amino complex formation. Based on the coordinating modes of the artificial amino acids, for each di- and tri-amino complex, more than one isomer has been detected and the isomers are designated accordingly. By analyzing the optimized geometries, it is noticed that most of the di- and tri-amino complexes are distorted hexa-coordinated in nature and in few cases, they adopt penta-coordinated geometry. To analyze the stability of the complexes, we have determined the binding energy (BE) in both DCM and CDCM mechanisms for each di- and tri-amino complex. Overall, the present study is arranged in such a way so that it can provide a complete understanding about the binding process of the aforementioned artificial amino acid with Cu2+-aqua complex.

(Invited) Atomic-Scale Investigations of TiO2 surfaces in Ambient and Solution Environments

Earth-abundant metal oxide nanocrystals have attracted a great deal of attention for environmental remediation and photocatalysis. These materials are attractive because they are inexpensive, they are typically very stable under reactive conditions, and their semiconducting nature enables efficient generation of long-lived photocarriers that can initiate chemical reactions. Nevertheless, an atomic-scale understanding of their reactivity in ambient and solution environments has proven elusive for two reasons. First, the complex milieu of reactants in these environments makes the discrimination of surface chemistry from adventitious contamination difficult. Second, the small size of nanocatalysts prohibits face-specific studies with most surface science techniques. In this talk, I will review our work in developing “new eyes” for nanocatalysis that address both issues. Over the past few years, researchers around the world have observed the formation of mysterious molecularly ordered structures on the surface of TiO2 photocatalysts in air and solution. These structures have been attributed to many causes, including a new ordered state of adsorbed H2O — persistent in vacuum at room temperature — in which one H2O molecule adsorbs to every other surface Ti atom. Using a combination of atomic-scale microscopy and spectroscopy, we show that these structures are due to the highly selective adsorption of atmospheric formic and acetic acids that are typically present in parts-per-billion concentrations. These surfaces effectively repel other adsorbates, such as alcohols, present in much higher concentrations. The high affinity of the surface towards carboxylic acids is attributed to their bidentate binding. The self-assembled organic acid monolayers have the unusual property of being both hydrophobic and highly water soluble, which may contribute to the self-cleaning properties of TiO2. Interestingly, these monolayers block the undercoordinated surface cation sites typically implicated in photocatalysis. In related work, we have shown that hydrothermal crystal growth techniques can produce anatase nanocrystals suitable for study at the atomic-scale with STM and other surface science techniques, as shown in the accompanying figure. Despite being synthesized in solution, the nanocrystal surfaces are very clean and passivated by a protecting monolayer. Using these crystals, we have shown that the most commonly used functionalization chemistry for oxide nanocatalysts, a carboxylic acid solution, causes the spontaneous reorganization of the nanocatalyst, leading to a five-fold increase in the number of reactive sites and revealing the atomic-scale origins of catalyst activation. Finally, we have investigated the structure of solution-deposited monolayers on TiO2 and demonstrated a rational approach to tuning intermolecular interactions and enabling long-range ordering. We show that simple electrostatic insights can be used to engineer away unfavorable intermolecular interactions, producing monolayers with exceptional long-range ordering. Quantitative measurements of the intermolecular interaction energies from molecularly resolved STM images are a factor of ~7 larger than those predicted by dispersion-corrected density functional theory (DFT). The discrepancy between experiment and simulation is not currently understood. This finding suggests a new path to the production of highly ordered monolayers and superstructures of large molecules. Figure 1

Photochemical behavior of ferrihydrite-oxalate system: Interfacial reaction mechanism and charge transfer process

Heterogeneous photochemical reactions associated with natural iron (hydr)oxides and oxalic acid have attracted a great deal of scientific attention in the application of organic pollutants degradation. However, the reaction mechanism is still unclear due to the complicated iron cycles and reactive oxygen species (ROS) generation. In this study, the in situ attenuated total reflectance-Fourier transform infrared spectroscopy was implemented to investigate the adsorption process and photochemical behavior of oxalic acid on the surface of ferrihydrite. A comprehensive reaction mechanism from the perspective of charge transfer process, including homogeneous-heterogeneous iron cycling and ROS generation, was illustrated in detail. We found that oxalic acid was first adsorbed on the surface of ferrihydrite with a mononuclear bidentate binding geometry. Interestingly, this mononuclear bidentate complex on the surface of ferrihydrite was stable under visible light irradiation. Subsequently, the whole complex departed from ferrihydrite surface through non-reduction dissolution with the form of Fe(C2O4)+. In the solution, the Fe(C2O4)+ complexes would quickly convert to Fe(C2O4)2− complexes. Under visible light irradiation, the electrons generated from the photolysis of Fe(C2O4)2− complex reacted with O2 to form O2•−/•OOH. Meanwhile, Fe(III) was reduced to Fe(II). Finally, the produced O2•−/•OOH could react with Fe(II) through a one-step way to generate •OH, which possessed higher •OH formation efficiency than that through the two-step way of H2O2 as the intermediates. This study helps us with understanding of in-situ photochemical reaction mechanism of ferrihydrite-oxalic acid system, and also provides guidance to effectively utilize widespread iron (hydr)oxides and organic acids in natural environment to develop engineered systems for water treatment.

Dipicolylamino quinoline derivative as novel dual fluorescent detecting system for Hg2+ and Fe3+

Novel quinolinyl oxazolidinonyl dipicolylamine derivatives (QOD) were synthesized via halo-induced cyclization of t-butyl allyl(quinolinyl) carbamate followed by substitution of dipicolylamine (DPA). One of the quinoline derivatives, 6QOD, shows a remarkable fluorescent enhancement in both Hg2+ in aqueous media and Fe3+ in CH3CN over other metal ions with a detection limit of 1.01 and 0.22 μM, respectively. Moreover, the complexation was proved to be a 1:1 stoichiometric binding by Job's plot and MS data. 6QOD was confirmed to form a bidentate binding with Hg2+ (Ka = 6556 M−1) and Fe3+ (Ka = 27,700 M−1) as evidenced by the chemical shifts in 1H NMR experiments of the DPA protons and the oxazolidinonyl protons in an excellent agreement with the most stable complex structures for both metal ions revealed by the DFT study. Both sensing mechanisms probably involve PET inhibition between the DPA unit and quinoline. The advantage of this 6QOD probe is that it can effectively be applied as a selective Hg2+ and Fe3+ chemosensor by adapting the proper dual-sensing system.

Synthesis, spectral characterization, and biological activities of Cobalt(II) complexes of Schiff bases derived from &lt;i&gt;o&lt;/i&gt;-vanillin and &lt;i&gt;p&lt;/i&gt;-vanillin with 3-aminopyridine

Two biologically active Schiff base GL1 and GL2 were synthesized in equimolar reaction of 3-aminopyridine with o-vanillin and p-vanillin. The synthesized Schiff bases were reacted with cobalt acetate, cobalt chloride or cobalt chloride with trimethylamine by using a molar ratio of Schiff base: cobalt salt as 2:1. The characterization of Schiff bases and cobalt complexes was done by UV–visible in methanol, diffused reflectance, FTIR (mid and far), mass spectrometry studies while 1H NMR and 13C NMR were used to further characterize the Schiff bases. The FTIR results confirmed the bidentate binding of the Schiff bases with cobalt center after the formation of the complexes. The UV-visible spectra and diffused reflectance spectra data obtained were used to further investigate the electronic structure as well as the properties of Schiff bases and their cobalt complexes. On the basis of the spectral studies, distorted tetrahedral geometry has been assigned for the Co(II) complexes. The antimicrobial activities of the two Schiff bases GL1, GL2 and their cobalt(II) complexes were tested against laboratory isolated Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and one fungus strain, Aspergillus niger. The antimicrobial evaluation demonstrates significant activity of the Schiff bases and the cobalt complexes against the four known human pathogens. The brine shrimps lethality study confirmed that the compounds are concentration and geometry dependent. The studies showed the complexes were higher in biological activity than the corresponding Schiff bases.Keywords: 3-aminopyridine Schiff bases, cobalt(II), antimicrobial activity, brine shrimps test

Modeling Boronic Acid Based Fluorescent Saccharide Sensors: Computational Investigation of d-Fructose Binding to Dimethylaminomethylphenylboronic Acid.

Designing organic saccharide sensors for use in aqueous solution is a nontrivial endeavor. Incorporation of hydrogen bonding groups on a sensor's receptor unit to target saccharides is an obvious strategy but not one that is likely to ensure analyte-receptor interactions over analyte-solvent or receptor-solvent interactions. Phenylboronic acids are known to reversibly and covalently bind saccharides (diols in general) with highly selective affinity in aqueous solution. Therefore, recent work has sought to design such sensors and understand their mechanism for allowing fluorescence with bound saccharides. In past work, binding orientations of several saccharides were determined to dimethylaminomethylphenylboronic acid (DMPBA) receptors with an anthracene fluorophore; however, the binding orientation of d-fructose to such a sensor could not be determined. In this work, we investigate the potential binding modes by generating 20 possible bidentate and six possible tridentate modes between fructose and DMPBA, a simplified receptor model. Gas phase and implicit solvent geometry optimizations, with a myriad functional/basis set pairs, were carried out to identify the lowest energy bidentate and tridentate binding modes of d-fructose to DMPBA. An interesting hydrogen transfer was observed during selected bidentate gas phase optimizations; this transfer suggests a strong sharing of the hydrogen atom between the boronate hydroxyl and amine nitrogen.