Direct electrochemistry of glucose oxidase and sensing glucose using a screen-printed carbon electrode modified with graphite nanosheets and zinc oxide nanoparticles

We have studied the direct electrochemistry of glucose oxidase (GOx) immobilized on electrochemically fabricated graphite nanosheets (GNs) and zinc oxide nanoparticles (ZnO) that were deposited on a screen printed carbon electrode (SPCE). The GNs/ZnO composite was characterized by using scanning electron microscopy and elemental analysis. The GOx immobilized on the modified electrode shows a well-defined redox couple at a formal potential of ?0.4 V. The enhanced direct electrochemistry of GOx (compared to electrodes without ZnO or without GNs) indicates a fast electron transfer at this kind of electrode, with a heterogeneous electron transfer rate constant (K_s) of 3.75 s^?1. The fast electron transfer is attributed to the high conductivity and large edge plane defects of GNs and good conductivity of ZnO-NPs. The modified electrode displays a linear response to glucose in concentrations from 0.3 to 4.5 mM, and the sensitivity is 30.07 μA mM^?1 cm^?2. The sensor exhibits a high selectivity, good repeatability and reproducibility, and long term stability. Figure

Graphical representation for the fabrication of GNs/ZnO composite modified SPCE and the immobilization of GOx     

Kinetics and mechanism of electron transfer reaction of single and double chain surfactant cobalt(III) complex by Fe2+ ions in liposome (dipalmitoylphosphotidylcholine) vesicles: effects of phase transition

In this study, we report the kinetics of reduction reactions of single and double chain surfactant cobalt(III) complexes of octahedral geometry, cis-[Co(en)₂(4AMP)(DA)](ClO₄)₃ and cis-[Co(dmp)₂(C₁₂H₂₅NH₂)₂](ClO₄)₃ (en = ethylenediamine, dmp = 1,3-diaminopropane, 4AMP = 4-aminopropane, C₁₂H₂₅NH₂ = dodecylamine) by Fe²⁺ ion in dipalmitoylphosphotidylcholine (DPPC) vesicles at different temperatures under pseudo first-order conditions. The kinetics of these reactions is followed by spectrophotometry method. The reactions are found to be second order and the electron transfer is postulated as outer sphere. The remarkable findings in the present investigation are that, below the phase transition temperature of DPPC, the rate decreases with an increase in the concentration of DPPC, while above the phase transition temperature the rate increases with an increase in the concentration of DPPC. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes. Besides, comparing the values of rate constants of these outer-sphere electron transfer reactions in the absence and in the presence of DPPC, the rate constant values in the presence of DPPC are always found to be greater than in the absence of DPPC. This is ascribed to the double hydrophobic fatty acid chain in the DPPC that gives the molecule an overall tubular shape due to the intervesicular hydrophobic interaction between vesicles surface and hydrophobic part of the surfactant complexes more suitable for vesicle aggregation which facilitates lower activation energy, and consequently higher rate is observed in the presence of DPPC. The activation parameters (ΔS^# and ΔH^#) of the reactions at different temperatures have been calculated which corroborate the kinetics of the reaction.     

Kinetics of the Oxidation of Fluoren-9-olsby 4-Nitro-1-chlorobenzotriazole

Summary.  The oxidation of fluoren-9-ols by 4-nitro-1-chlorobenzotriazole in aqueous acetic acid is a second order reaction with a reaction constant of −1.4. The oxidation exhibits a kinetic deuterium isotope effect indicating the cleavage of the 9-C*H bond in the rate-determining step. The title oxidation is compared with that by 1-chlorobenzotriazole. Received October 23, 2000. Accepted (revised) January 22, 2001     

The Structural Characterization of Folded Peptides Containing the Conformationally Constrained β‐Amino Acid Residue β2,2Ac6c

Backbone alkylation has been shown to result in a dramatic reduction in the conformational space that is sterically accessible to α‐amino acid residues in peptides. By extension, the presence of geminal dialkyl substituents at backbone atoms also restricts available conformational space for β and γ residues. Five peptides containing the achiral β^2,2‐disubstituted β‐amino acid residue, 1‐(aminomethyl)cyclohexanecarboxylic acid (β^2,2Ac₆c), have been structurally characterized in crystals by X‐ray diffraction. The tripeptide Boc‐Aib‐β^2,2Ac₆c‐Aib‐OMe ( 1 ) adopts a novel fold stabilized by two intramolecular H‐bonds (C₁₁ and C₉) of opposite directionality. The tetrapeptide Boc‐[Aib‐β^2,2Ac₆c]₂‐OMe ( 2 ) and pentapeptide Boc‐[Aib‐β^2,2Ac₆c]₂‐Aib‐OMe ( 3 ) form short stretches of a hybrid αβ C₁₁ helix stabilized by two and three intramolecular H‐bonds, respectively. The structure of the dipeptide Boc‐Aib‐β^2,2Ac₆c‐OMe ( 5 ) does not reveal any intramolecular H‐bond. The aggregation pattern in the crystal provides an example of an extended conformation of the β^2,2Ac₆c residue, forming a ‘polar sheet’ like H‐bond. The protected derivative Ac‐β^2,2Ac₆c‐NHMe ( 4 ) adopts a locally folded gauche conformation about the C_β? C_α bonds (θ=?55.7°). Of the seven examples of β^2,2Ac₆c residues reported here, six adopt gauche conformations, a feature which promotes local folding when incorporated into peptides. A comparison between the conformational properties of β^2,2Ac₆c and β^3,3Ac₆c residues, in peptides, is presented. Backbone torsional parameters of H‐bonded αβ/βα turns are derived from the structures presented in this study and earlier reports.     

A sensitive amperometric detection of dopamine agonist drug pramipexole at functionalized multi-walled carbon nanotubes (f-MWCNTs) modified electrode

The electrochemical detection of dopaminergic agonist drug pramipexole dihydrochloride monohydrate (PPX) has been investigated by cyclic voltammetric (CV) and amperometric i–t techniques at functionalized multi-walled carbon nanotubes-modified glassy carbon electrode. For the first time, a sensitive and rapid electrochemical method was developed for the determination of PPX. The surface morphological characteristics of the proposed electrode have been studied by using transmission electron microscopy (TEM); further, electrochemical impedance spectroscopy (EIS) and Fourier transform infrared spectroscopy (FTIR) have been employed. PPX shows an irreversible anodic peak, which may be ascribed to the oxidation of the –NH groups of PPX. The proposed method was showing good sensitivity of 0.993 μA μM^?1 cm^?2 with a linear range of 5 to 340 μM by amperometric i–t and CV technique shows a linear range of 12.5 to 313 μM with a sensitivity of 1.92 μA μM^?1 cm^?2. The recovery of PPX from blood serum samples was found 100.6 and 98.9 %, respectively. Furthermore, the proposed method has been demonstrated for the determination of PPX in commercially available pharmaceutical samples and good agreement of results obtained.     

Synthesis,CMC Determination,Antimicrobial Activity and Nucleic Acid Binding of A Surfactant Copper(II) Complex Containing Phenanthroline and Alanine Schiff-Base

A new water-soluble surfactant copper(II) complex [Cu(sal-ala)(phen)(DA)] (sal-ala = salicylalanine, phen = 1,10-phenanthroline, DA = dodecylamine), has been synthesized and characterized by physico-chemical and spectroscopic methods. The critical micelle concentration (CMC) values of this surfactant–copper(II) complex in aqueous solution were obtained from conductance measurements. Specific conductivity data (at 303, 308, 313. 318 and 323 K) served for the evaluation of the temperature-dependent CMC and the thermodynamics of micellization (ΔG⁰m, ΔH⁰m and ΔS⁰m). The interaction of this complex with nucleic acids (DNA and RNA) has been explored by using electronic absorption spectral titration, competitive binding experiment, cyclic voltammetry, circular dichroism (CD) spectra, and viscosity measurements. Electronic absorption studies have revealed that the complex can bind to nucleic acids by the intercalative binding mode which has been verified by viscosity measurements. The DNA binding constants have also been calculated (K_b?=?1.2?×?10⁵ M^?1 for DNA and K_b?=?1.6?×?10⁵ M^?1 for RNA). Competitive binding study with ethidium bromide (EB) showed that the complex exhibits the ability to displace the DNA-bound-EB indicating that the complex binds to DNA in strong competition with EB for the intercalative binding site. The presence of hydrophobic ligands, alanine Schiff-base, phenanthroline and long aliphatic chain amine in the complex were responsible for this strong intercalative binding. The surfactant–copper (II) complex was screened for its antibacterial and antifungal activities against various microorganisms. The results were compared with the standard drugs, amikacin(antibacterial) and ketokonazole(antifungal).