Palladium complexes with Pd-->B dative bonds: analysis of the bonding in the palladaboratrane compound [kappa4-B(mimBut)3]Pd(PMe3)

The dinuclear complex {[mu-kappa(1),kappa(3)-B(mim(Bu(t)))(3)]Pd}(2), which features a Pd-->B dative bond, may be obtained by the reaction of [Tm(Bu(t))]K with Pd(OAc)(2); treatment of {[mu-kappa(1),kappa(3)-B(mim(Bu(t)))(3)]Pd}(2) with PMe(3) affords the mononuclear boratrane derivative [kappa(4)-B(mim(Bu(t)))(3)]Pd(PMe(3)), for which a molecular orbital analysis indicates that the palladium center possesses a d(8) configuration.     

Carboxymethyl chitosan-coated capillary and its application in CE of proteins

A hydrophilic basic polysaccharide, carboxymethyl chitosan (CMC) as a capillary coating is presented with a simple preparation procedure. The CMC-coated capillary showed a long lifetime of more than 100 runs, and had good tolerance to some organic solvents, 0.1 M HCl, 0.1 M NaOH, and 5 M urea. The run-to-run, day-to-day, and capillary-to-capillary RSDs for the CMC-coated capillary were all below 2.0% for the determination of EOF. Moreover, the coatings with different concentrations and molecular weights of CMC were also investigated. The CMC-coated capillary was successfully applied to separate basic proteins and recombinant human erythropoietin (rhEPO). Furthermore, several experimental parameters, such as the concentration and pH of the running buffer, temperature, and applied voltage, were optimized for the separation of rhEPO glycoforms. Comparison of an uncoated capillary with chitosan- and CMC-coated capillaries for the separation of rhEPO glycoforms was also discussed. The results demonstrated that rhEPO glycoforms can be well separated by a CMC-coated capillary within 8 min with good reproducibility and resolution. Finally, the volatile BGE HAc-NH4Ac was utilized to separate rhEPO for its further application with CE-MS, achieving a satisfactory result.     

Mutagenesis of morphinone reductase induces multiple reactive configurations and identifies potential ambiguity in kinetic analysis of enzyme tunneling mechanisms

We have identified multiple reactive configurations (MRCs) of an enzyme-coenzyme complex that have measurably different kinetic properties. In the complex formed between morphinone reductase (MR) and the NADH analogue 1,4,5,6-tetrahydro-NADH (NADH4) the nicotinamide moiety is restrained close to the FMN isoalloxazine ring by hydrogen bonds from Asn-189 and His-186 as determined from the X-ray crystal structure. Molecular dynamic simulations indicate that removal of one of these hydrogen bonds in the N189A MR mutant allows the nicotinamide moiety to occupy a region of configurational space not accessible in wild-type enzyme. Using stopped-flow spectroscopy, we show that reduction of the FMN cofactor by NADH in N189A MR is multiphasic, identifying at least four different reactive configurations of the MR-NADH complex. This contrasts with wild-type MR in which hydride transfer occurs by environmentally coupled tunneling in a single kinetic phase [Pudney et al. J. Am. Chem. Soc. 2006, 128, 14053-14058]. Values for primary and alpha-secondary kinetic isotope effects, and their temperature dependence, for three of the kinetic phases in the N189A MR are consistent with hydride transfer by tunneling. Our analysis enables derivation of mechanistic information concerning different reactive configurations of the same enzyme-coenzyme complex using ensemble stopped-flow methods. Implications for the interpretation from kinetic data of tunneling mechanisms in enzymes are discussed.     

Voltammetric determination of DNA based on regulation of DNA strand displacement using an allosteric DNA toehold

An electrochemical biosensor for determination of DNA is described that is based on the reaction of regulated DNA (reg-DNA) first with substrated DNA (subs-DNA) to form a reaction intermediate. The intermediate binds target DNA (T) by hybridization and initiates a branch migration leading to the production of complex of substrated DNA and target DNA (TC). Once TC is produced, it reacts with assisted DNA (ass-DNA) through a toehold exchange mechanism, yielding the product complex of substrated DNA and assisted DNA (CS). The target is then released back into the solution and and catalyzes the next cycle of toehold-exchange with the reaction intermediate of substrated DNA and regulated DNA (CPR). Unlike in a conventional DNA toehold that is hardwired with the branch migration domain, the allosteric DNA toehold is designed into a reg-DNA which is independent of the branch migration domain. Under the optimal experimental conditions and at a working potential as low as 0.18 V, response to DNA is linear in the 1 fM to 1000 pM concentration range, and the detection limit is 0.83 fM. The assay is highly specific and can discriminate target DNA even from a single-base mismatch. It was applied to the analysis of DNA spiked plasma samples.

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Graphical abstract Schematic illustration of the electrochemical strategy for target DNA detection based on regulation of DNA strand displacement using an allosteric DNA toehold strategy. It can be used to analyze DNA-spiked plasma samples and has a low detection limit of 0.83 fM.

     

Amperometric detection of nitrite based on Dawson-type vanodotungstophosphate and carbon nanotubes

A nitrite sensor based on Dawson vanodotungstophosphates α₂-K₇P₂VW₁₇O₆₂·18H₂O (P₂W₁₇V) and carbon nanotubes (CNTs) was prepared by electrostatic layer-by-layer self-assembly technique. The sensor {PEI/PSS/[PDDA/P₂W₁₇V-CNTs]_n} was characterized by UV–vis spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray photoelectron spectra (XPS). The electron transfer and sensing ability of this sensor were explored using cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) technology. The results show that the incorporation of CNTs and P₂W₁₇V into the composite film endowed the modified electrode with fast transfer rate and high electrocatalytic activity towards oxidation of nitrite. This nitrite sensor with 10 bilayers has a broad linear range of 5 × 10⁻⁸ to 2.13 × 10⁻³ M, a low detection limit of 0.0367 μM (S N⁻¹ = 3), a high sensitivity of 0.35 mA mM⁻¹ NO₂⁻, an excellent anti-interference property in the presence of other potential interfering species and a good stable. It was successfully employed for determination of nitrite in real towards.     

Tetra­aqua(2,2′‐bi­pyridine)­nickel(II) terephthalate

The reaction of nickel(II) nitrate with terephthalic acid and 2,2′‐bi­pyridine in di­methyl­form­amide solution gives the title complex, [Ni(C₁₀H₈N₂)(H₂O)₄](C₈H₄O₄). The Ni^II ion is octahedrally coordinated to one 2,2′‐bi­pyridine and four water mol­ecules and does not coordinate to the terephthalate anion. Hydro­gen bonds between the terephthalate anions and the [Ni(2,2′‐bipy)(H₂O)₄]²⁺ cations produce a two‐dimensional hydrogen‐bonding architecture with double sheets.     

Effect of promoters on the behavior and properties of VPO catalysts for the selective oxidation of n-butane to maleic anhydride

A series of vanadyl pyrophosphate (VPO) modified by several additives has been investigated with the aim to reveal the promotion mechanism of the additives for selective conversion of n-butane to maleic anhydride (MA). Catalyst performance and various physicochemical characterization including XPS, XRD, FT-IR, DTA and BET indicated that the properties of additives, such as electronegativity and ion radius, contributed much to the effect of the additives on the structure and surface characteristics of VPO catalysts, and then influenced the catalytic behavior. The remarkable effects of additives on the surface vanadium and oxygen species were discussed. It was shown that maximum MA selectivity could be found with a moderate V⁵⁺ /V⁴⁺ ratio, and the catalytic activity and selectivity were improved, respectively, with the increase of surface active oxygen species, as well as as their amount and stability.     

A new carotenoid glycoside from Rehmannia glutinosa

A new carotenoid glycoside, namely neo-rehmannioside (1), together with five known compounds, 6-O-seco-hydroxyaeginetoyl ajugol (2), oxyrehmaionoside B (3), ajugol (4), geniposidic acid (5) and geniposide (6) was isolated from the 95% ethanol extract of dry roots of Rehmannia glutinosa. The structure of the new compound (1) was determined based on MS, IR, 1-D and 2-D NMR spectral data.     

High-throughput protein analysis by multiplexed sodium dodecyl sulfate capillary gel electrophoresis with UV absorption detection

We have developed a novel, high-throughput approach for molecular mass determination of proteins from 14 200 to 116 000 based upon multiplexed, absorbance-based capillary electrophoresis. Via capillary multiplexing, 96 samples were analyzed simultaneously within 30 min. Detection with ultraviolet light obviates the need for protein staining or derivatization. The detection limit of the system was estimated at 5 microg/ml bovine serum albumin (BSA) when sampled from 12.5 mM Tris-HCl. The linear dynamic range was over two orders of magnitude from 5 microg/ml to 1000 microg/ml for BSA. Better than 5% sizing accuracy for protein molecular mass determination and excellent run-to-run and day-to-day reproducibility was obtainable with the described method.