Multiple-steering QM-MM calculation of the free energy profile in chorismate mutase

A novel technique for computing free energy profiles in enzymatic reactions using the multiple steering molecular dynamics approach in the context of an efficient QM-MM density functional scheme is presented. The conversion reaction of chorismate to prephenate catalyzed by the Bacillus subtilis enzyme chorismate mutase has been chosen as an illustrative example.     

Preconcentration Systems Using Polyurethane Foam/Me-BDBD for Determination of Copper in Food Samples

A new reagent, 6-[2′-(6′-methyl-benzothiazolylazo)]-1,2-dihydroxy-3,5-benzenedisulfonic acid (Me-BDBD), was synthesised and used in on-line and off-line systems for copper preconcentration by solid-phase extraction. Spectrophotometry and Flame Atomic Absorption Spectrometry (FAAS) were the detection techniques. Polyurethane foam loaded with Me-BDBD packed in a minicolumn was used as sorbent in both systems. The spectral characteristics of Me-BDBD were investigated. The optimum pH values for maximum sorption of the metal are between 7.0 and 8.5. Copper was desorbed with 0.05 and 0.50 mol L⁻¹ hydrochloric acid solutions, for on-line and off-line systems, respectively. The effects of several foreign substances on the adsorption of copper are reported. The enrichment factors obtained were 7 (on-line) and 26 (off-line) for the systems. The proposed procedures allowed the determination of copper with detection limits of 3.4 and 1.4 μg L⁻¹ (0.85 and 0.35 μg per gram of sample) for on-line and off-line systems, respectively. The precision of the procedures was also calculated: 3.2 (on-line) and 1.9% (off-line). The validation of the procedures was carried out by analysis of certified reference material. The copper contents in corn and rice flour and black tea samples were determined by applying the proposed procedures.     

The influence of the shape of Au nanoparticles on the catalytic current of fructose dehydrogenase

Graphite electrodes were modified with triangular (AuNTrs) or spherical (AuNPs) nanoparticles and further modified with fructose dehydrogenase (FDH). The present study reports the effect of the shape of these nanoparticles (NPs) on the catalytic current of immobilized FDH pointing out the different contributions on the mass transfer–limited and kinetically limited currents. The influence of the shape of the NPs on the mass transfer–limited and the kinetically limited current has been proved by using two different methods: a rotating disk electrode (RDE) and an electrode mounted in a wall jet flow-through electrochemical cell attached to a flow system. The advantages of using the wall jet flow system compared with the RDE system for kinetic investigations are as follows: no need to account for substrate consumption, especially in the case of desorption of enzyme, and studies of product-inhibited enzymes. The comparison reveals that virtually identical results can be obtained using either of the two techniques. The heterogeneous electron transfer (ET) rate constants (k_S) were found to be 3.8 ± 0.3 s⁻¹ and 0.9 ± 0.1 s⁻¹, for triangular and spherical NPs, respectively. The improvement observed for the electrode modified with AuNTrs suggests a more effective enzyme-NP interaction, which can allocate a higher number of enzyme molecules on the electrode surface.

The shape of gold nanoparticles has a crucial effect on the catalytic current related to the oxidation of D-(-)-fructose to 5-keto-D-(-)-fructose occurring at the FDH-modified electrode surface. In particular, AuNTrs have a higher effect compared with the spherical one.

     

31P{1H}-n.m.r. as a tool for identification of ruthenium isomers containing PPh3 or 1,4-bis(diphenylphosphino)butane ligands. X-ray structures of the cis-{RuCl2(PPh3)2 [4,4′-(-X)2-2,2′-bipy]} complexes [X=-H, -Me, -SMe and (-Cl, -Me)]

A series of cis-{RuCl₂(PPh₃)₂[4,4-(X)₂-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH₂Cl₂ solution by ³¹P{¹H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)₂-2,2-bipy ligands.     

Flow injection determination of cobalt after its sorption onto polyurethane foam loaded with 2-(2-thiazolylazo)-p-cresol (TAC)

An electrochemically stable monolayer of tris(2,2'-bipyridyl)ruthenium(II) was obtained for the first time. It was based on the electrostatic attachment of Ru(bpy)(3)(2+) to the benzene sulfonic acid monolayer film, which was covalently bound onto glassy carbon electrode by the electrochemical reduction of diazobenzene sulfonic acid. The surface-confined Ru(bpy)(3)(2+) underwent reversible surface process, and reacted with the coreactant, tripropylamine, to produce electrochemiluminescence. In view of the stability of the electrode, the results strongly suggested that light was emitted from the surface-confined Ru(bpy)(3)(2+), not from the detached Ru(bpy)(3)(2+). The Ru(bpy)(3)(2+) modified electrode was used to the determination of tripropylamine. It showed good linearity in the concentration range from 5 muM to 1 mM with a detection limit of 1 muM (S/N=4). The good stability of the Ru(bpy)(3)(2+) modified electrode also showed that the benzene sulfonic acid monolayer film prepared can be served as an excellent support to construct multilayers.     

Synthesis, structure and properties of ruthenium(II) complexes with quinolinedione derivatives as chelate ligands: Crystal structure of [Ru(CO)2Cl2(6-methoxybenzo[g]quinoline-5,10-dione)]

The reaction of the dimer complex [{Ru(CO)₃Cl₂}₂] with the ligands 4,6-dichloroquinoline-5,8-dione and 6-methoxybenzo[g]quinoline-5,10-dione in ethanol solution led to the neutral mononuclear complexes of general formula [Ru(CO)₂Cl₂(κ²-quinolinedione-N,O)]. The complexes were characterized by elemental analysis, IR and RMN spectroscopy, and the molecular structure of [Ru(CO)₂Cl₂(6-methoxybenzo[g]quinoline-5,10-dione)] was determined by single-crystal X-ray diffraction. The redox chemistry of ligands and complexes was investigated by cyclic voltammetry, and their potential antitumor activity was also evaluated.     

Study of reaction processes by in-line near-infrared spectroscopy in combination with multivariate curve resolution: Esterification of myristic acid with isopropanol

The acid-catalysed esterification of myristic acid with isopropanol was studied by using near-infrared spectroscopy (NIR) in combination with soft-modeling curve resolution (MCR) methodology with a view to establishing the effect of experimental variables on the reaction kinetics. The reaction was conducted at temperatures above the boiling point of the alcohol, with continuous addition of an isopropanol/water mixture to the reactor. Spectral and concentration profiles were determined by applying soft-modeling curve resolution methodology to a column-wise augmented data matrix containing the spectra for the pure components. MCR profiles were compared with reference values and found to depart from then by less than 3% as %RSE for concentrations and to exhibit correlation above 0.999 for spectra.The reaction kinetics as estimated from the concentration profiles was found to be pseudo-first-order. Also, the pseudo-first-order rate constant was found to depend on the flow-rate of the isopropanol/water mixture and its water content; although the constant decreased with increase in the proportion of water, a content of ca. 15% could be used without important retarding effects on the kinetics. The proposed NIR-MCR method allows the rate constant and the influence of the initial water content to be determined with a view to minimizing consumption of the raw materials and optimizing the experimental conditions.     

Voltammetric behavior of zaleplon and its differential pulse polarographic determination in capsules

In this work both the electrochemical behavior and the analysis of the hypnotic pyrazolopyrimidine derivative zaleplon were studied. Zaleplon in ethanol-0.1M Britton Robinson buffer solution (30-70) showed 2 irreversible, well-defined cathodic responses in the pH range of 2-12 using differential pulse polarography (DPP), tast polarography, and cyclic voltammetry. From chronocoulometric studies, it was possible to conclude that one electron was transferred in each reduction peak or wave. For analytical purposes, the DPP technique working at pH 4.5 for peak I was selected, which exhibited adequate repeatability, reproducibility, and selectivity. The recovery was 99.97 +/- 1.52%, and the detection and quantitation limits were 5.13 x 10(-7)M and 1.11 x 10(-6)M, respectively. The DPP method was applied successfully to the individual assay of capsules in order to verify the content uniformity of zaleplon. Treatment of the sample is not required because the excipients do not interfere, the method is not time consuming, and it is less expensive than column liquid chromatography.     

Processes associated with ionic current rectification at a 2D-titanate nanosheet deposit on a microhole poly(ethylene terephthalate) substrate

Films of titanate nanosheets (approx. 1.8-nm layer thickness and 200-nm size) having a lamellar structure can form electrolyte-filled semi-permeable channels containing tetrabutylammonium cations. By evaporation of a colloidal solution, persistent deposits are readily formed with approx. 10-μm thickness on a 6-μm-thick poly(ethylene-terephthalate) (PET) substrate with a 20-μm diameter microhole. When immersed in aqueous solution, the titanate nanosheets exhibit a p.z.c. of − 37 mV, consistent with the formation of a cation conducting (semi-permeable) deposit. With a sufficiently low ionic strength in the aqueous electrolyte, ionic current rectification is observed (cationic diode behaviour). Currents can be dissected into (i) electrolyte cation transport, (ii) electrolyte anion transport and (iii) water heterolysis causing additional proton transport. For all types of electrolyte cations, a water heterolysis mechanism is observed. For Ca²⁺ and Mg²⁺ions, water heterolysis causes ion current blocking, presumably due to localised hydroxide-induced precipitation processes. Aqueous NBu₄⁺ is shown to ‘invert’ the diode effect (from cationic to anionic diode). Potential for applications in desalination and/or ion sensing are discussed.