A Comparative Study of Electrochemical Kinetic Parameters by the Potential Step Method. Electrode Reactions of CyDTA and EDTA Complexes of Cu(II) at DME

By using the same current-time (I-t) curves, electrochemical kinetic parameters are determined by two methods, (a) using the ratio of current at a given potential to the diffusion-controlled limiting current and (b) curve fitting method, for the reduction of Cu(II)–CyDTA complex. The analysis by the method (a) shows that the rate determining step involves only one electron although the overall reduction of the complex involves two electrons suggesting thereby the stepwise reduction of the complex. The nature of I-t curves suggests the adsorption of intermediate species at the electrode surface. Under these circumstances more reliable kinetic parameters can be obtained by the method (a) compared to that of (b). Similar observations are found in the case of reduction of Cu(II)–EDTA complex.

Polarographic and redox potential studies on copper(I) and copper(II) and their complexes. Part III. Copper (I and II)-ethylene diamine and edta complexes and theory of formation of step reduction waves in cupric copper systems

Polarographic and redox potential measurements on the cupric and cuprous complexes of ethylenediamine and EDTA have been carried out. From the ratio of the stability constants of the cupric and cuprous complexes, and the stability constant of the cupric complex, the stability constant of the cuprous-ethylenediamine complex is obtained. In the case of the EDTA complex it has been possible to obtain only βic/β2ous from the equilibrium concentrations of the cuprous and cupric complexes and the disproportionation constant. The inequalities for the appearance of step reduction waves have been given. The values of the stability constants of the cupric and cuprous complexes determined by the polarographic-redox potential method have been used to explain the appearance of step reduction waves in some systems and the non-appearance in other systems.

Electrochemical reduction of Cu(II)-EDTA complex at dropping mercury electrode. An impedance approach

The study of electrochemical reduction of Cu(II)-EDTA system by phase sensitive a.c. impedance method at dropping mercury electrode reveals several interesting features. The complex plane polarograms exhibit loop like shape in contrast to the classical zinc ion reduction where crest like shape is found. Again, the relative placement of peaks of in-phase and quadrature components, and the relative placement of portions before and after the peaks of complex plane polarograms are different from that of zinc ion reduction. The complex plane plots suggest that electrochemical reduction of Cu-EDTA is charge transfer controlled.

Synthesis and characterization of barium bis(citrato) oxozirconate(iv) tetrahydrate--A new molecular precursor for fine particle BaZrO3

Barium metazirconate (BaZrO3) fine powder has been produced by thermally decomposing a molecular precursor, barium bis(citrato)oxozirconate(IV) tetrahydrate at about 700-degrees-C. The precursor, Ba[ZrO(C6H6O7)2] . 4H2O (BZO) has been synthesized and characterized by employing a combination of spectroscopic and thermoanalytical techniques. The precursor undergoes thermal decomposition in three major stages: (i) dehydration to give an anhydrous barium zirconyl citrate, (ii) decomposition of the anhydrous citrate in a multistep process to form an ionic oxycarbonate intermediate, Ba2Zr2O5CO3, and (iii) decomposition of the oxycarbonate to produce BaZrO3 fine powder. The particle size of the resultant BaZrO3 is about 0.2 mum, and the surface area is found to be 4.0 m2 g-1.

Requirement Of A Diperoxovanadate-Derived Intermediate for the Interdependent Oxidation Of Vanadyl And Nadh

Oxygen release accompanying oxidation of vanadyl by diperoxovanadate was suppressed on addition of NADH. The added NADH was rapidly oxidized, oxygen in the medium was consumed, and the reaction terminated on exhaustion of either NADH or vanadyl. The consumption of oxygen and disappearance of NADH needed small concentrations of diperoxovanadate to initiate and increased with increase in the concentration of vanadyl and NADH or decrease of pH. The products of the reaction were found to be NAD(+) from NADH and vanadate oligomers from vanadyl and oxygen. The reaction was insensitive to catalase and was not dependent on H2O2. The reaction was inhibited by superoxide dismutase, cytochrome c, EDTA, Mn2+, histidine, and DMPO, but not by hydroxyl radical scavengers such as ethanol and benzoate, The ESR spectrum of the reaction mixture showed the presence of the 1:2:2:1 quartet signal typical of a DMPO-OH adduct, but this was not modified by ethanol, This oxygen radical species, possibly of (OV)-O-. type derived from diperoxovanadate, is proposed to have a role in the reactions of oxygen release and NADH oxidation

Gossypol mediated DNA degradation

Gossypol, a polyphenolic compound isolated from cotton plant was found to degrade pBR322 DNA Image in a reaction which required the presence of a metal ion, a reducing agent (2-mercaptoethanol) and oxygen as revealed after agarose gel electrophoresis. Fe3+ and Co2+ showed maximum degradation whereas addition of Ca2+ and Mg2+ prevented the gossypol mediated DNA damage. Gossypol caused degradation of rat liver DNA incubated Image even in the absence of added metal ions and 2-mercaptoethanol. Incubation of intact rat liver nuclei with gossypol reveled DNA degradation and nuclei isolated from rats treated with gossypol Image showed higher succestibility to DNA fragmentation when incubated with gossypol Image than control nuclei. EcoRl and AIuI digestion of DNA isolated from gossypol treated rats gave clear cut evidence for DNA degradation. These observations indicate that gossypol is genotoxic and considereable care has to be exercised in its use. SDS, sodium dodecayl sulphate; TE buffer, Tris-HCL-EDTA buffer.

Sacrificial photolysis of water on TiO2 fine powders prepared by the hydrothermal method

The often discussed role of surface hydroxylation of TiO2 particles as an essential characterestics for their photocatalytic activity can be verified by preparing TiO2 powders by hydrothermal method since hydroxylated surface layers will be better retained on these particles formed in superheated water. Thus, fine powders of TiO2 (rutile) with high degree of crystallinity are formed from titanium oxychloride in the mixed solvent of water and 2-propanol at 160–230°C and 20–120 atm. The anatase phase is produced from the same medium when sulfate ion impurity is present, with Image . TiO2 powders are washed free of anions and 2-propanol by ultrafiltration and are Pt mounted by a photochemical method. Aqueous suspensions of both forms of TiO2 neither as such nor after Pt-loading, do not produce H2 on band gap illumination whereas, H2 is generated in presence of hole scavengers such as EDTA, TEOA, sulfite or hypophosphite. The effects of hole scavenger concentration, Pt : TiO2 ratio, particulate suspension density and the nature of hole scavengers on H2 production are presented. Platinised rutile powders are equally active as anatase in sacrificial systems.

Role of glutamine synthetase in citric acid fermentation byAspergillus niger

The activity of glutamine synthetase fromAspergillus niger was significantly lowered under conditions of citric acid fermentation. The intracellular pH of the organism as determined by bromophenol blue dye distribution and fluorescein diacetate uptake methods was relatively constant between 6•0–6•5, when the pH of the external medium was varied between 2•3–7•0.Aspergillus niger glutamine synthetase was rapidly inactivated under acidic pH conditions and Mn2+ ions partially protected the enzyme against this inactivation. Mn2+-dependent glutamine synthetase activity was higher at acidic pH (6•0) compared to Mg2+-supported activity. While the concentration of Mg2+ required to optimally activate glutamine synthetase at pH 6•0 was very high (≥ 50 mM), Mn2+ was effective at 4 mM. Higher concentrations of Mn2+ were inhibitory. The inhibition of both Mn2+ and Mg2+-dependent reactions by citrate, 2-oxoglutarate and ATP were probably due to their ability to chelate divalent ions rather than as regulatory molecules. This suggestion was supported by the observation that a metal ion chelator, EDTA also produced similar effects. Of the end-products of the pathway, only histidine, carbamyl phosphate, AMP and ADP inhibitedAspergillus niger glutamine synthetase. The inhibitions were more pronounced when Mn2+ was the metal ion activator and greater inhibition was observed at lower pH values. These results permit us to postulate that glutamine synthesis may be markedly inhibited when the fungus is grown under conditions suitable for citric acid production and this block may result in delinking carbon and nitrogen metabolism leading to acidogenesis.

Studies on nucleotidases in plants Dimerization of the crystalline mung bean nucleotide pyrophosphatase by 5-adenosine monophosphate and the properties of the dimerized enzyme

The addition of AMP to the crystalline and homogeneous mung bean nucleotide pyrophosphatase [EC 3.6.1.9]altered its electrophoretic mobility. AMP was tightly bound to the enzyme and was not removed on passage through a column of Sephadex G-25 or on electrophoresis. The molecular weight of the native and AMP-modified enzymes were 65,000 and 136,000, respectively. The properties of the native enzyme such as the pH (9.4) and temperature (49 °C) optima, inhibition by EDTA, reversal of EDTA-inhibition by Zn2+ and Co2+, were not altered on dimerization by AMP. The AMP-modified enzyme had a linear time-course of reaction, unlike the native enzyme which exhibited a biphasic time-course of reaction. The AMP-modified enzyme was irreversibly denatured by urea. AMP concentrations larger than 100 μM inhibited linearly the activity of the AMP-modified enzyme. ADP and ATP inhibited the activity in a sigmoidal manner. Km and V of the native and AMP-modified enzymes were, 0.25 mImage and 0.58 mImage ; and 3.3 and 2.5, respectively.

Purification, properties and induction of a specific benzoate-4-hydroxylase from Aspergillus niger (UBC 814)

An inducible benzoate-4-hydroxylase has been partially purified from crude extracts of the mycelial felts of Aspergillus niger. This enzyme catalyzes the transformation of benzoate to p-hydroxybenzoate with equimolar consumption of NADPH and O2. It requires tetrahydropteridine as a prosthetic group. The optimum activity was found at pH 6.2 with a Km value at 30°C of 1.6 · 10−4 M for NADPH and 1.3 · 10−4 M for benzoate. Fe2+ (iron) is required for the enzyme activity. The enzyme is stabilized by the inclusion of benzoate, EDTA and glutathione in the extracting buffer. The enzyme is specific for benzoate as substrate. Sulfhydryl group(s) are essential for enzyme activity as indicated by p-chloromercuri-benzoate and N-ethylmaleimide inactivation. Benzoate-4-hydroxylase activity is decreased in the mycelial felts of Aspergillus niger grown in the presence of higher concentrations of benzoate. Maximum activity of the enzyme was observed at 36 h after inoculation.

Reactivity of µ-Peroxo-Bridged Dimeric Vanadate in Bromoperoxidation

Diglycyl triperoxodivanadate [V2O2(O2)3(Gly H)2(H2O)2], a synthetic compound with μ-peroxo-bridge derived from H2O2and vanadate, oxidized bromide to a bromination-competent intermediate in phosphate buffer and physiological pH. This is in contrast to the requirement of acid medium with H2O2as the oxidant. Addition of its solid to bromide solution instantly produced a 262-nm-absorbing compound that converted phenol red (a trap) to its 592-nm-absorbing bromo-derivative. The high bromination activity was lost on dissolving this compound in water and the solution showed the presence of peroxovanadates (mono and di) and vanadates (V1and oligomeric V10) in51V-NMR spectrum. Of these, diperoxovanadate and vanadate together supported slow bromination activity by a second set of reactions including bromide-assisted reductive formation of vanadyl. Bromination activity dependent on vanadyl was sensitive to oxidation by excess H2O2and to complexation by EDTA, whereas that of triperoxodivanadate was relatively insensitive. Vanadyl and diperoxovanadate are capable of forming a μ-peroxo-bridged complex that is essentially similar to the synthetic vanadate dimer used in the present experiments. It appears that a μ-peroxo-intermediate is the proximal oxidant of bromide in vanadium-catalyzed bromoperoxidation.

Structure of Sesbania mosaic virus at 3 angstrom resolution

Sesbania mosaic virus (SMV) is an isometric, ss-RNA plant virus found infecting Sesbania grandiflora plants in fields near Tirupathi, South India. The virus particles, which sediment at 116 S at pH 5.5, swell upon treatment with EDTA at pH 7.5 resulting in the reduction of the sedimentation coefficient to 108 S. SMV coat protein amino acid sequence was determined and found to have approximately 60% amino acid sequence identity with that of southern bean mosaic virus (SBMV). The amino terminal 60 residue segment, which contains a number of positively charged residues, is less well conserved between SMV and SBMV when compared to the rest of the sequence. The 3D structure of SMV was determined at 3.0 Å resolution by molecular replacement techniques using SBMV structure as the initial phasing model. The icosahedral asymmetric unit was found to contain four calcium ions occurring in inter subunit interfaces and three protein subunits, designated A, B and C. The conformation of the C subunit appears to be different from those of A and B in several segments of the polypeptide. These observations coupled with structural studies on SMV partially depleted of calcium suggest a plausible mechanisms for the initiation of the disassembly of the virus capsid.

Mammalian spermatid specific protein, TP2, is a zinc metalloprotein with two finger motifs

An analysis of the recently reported cDNA derived amino acid sequences of mouse (Kleene and Flynn, J. Biol. Chem. , 17272–17277, 1987) and rat (Luersson Image ,Nucl. Acids Res. Image , 3585, 1989). TP2 has revealed the presence of two potential zinc finger motifs involving cysteine and histidine residues. TP2, as purified from rat elongating spermatids, is shown here to contain 0.2 atoms of zinc bound per molecule of the protein by atomic absorption spectroscopy. On incubation with 10 μM ZnCl2, Image , and subsequent exhaustive dialysis, TP2 had 2 atoms of zinc bound per molecule. The involvement of cysteine residues of TP2 in coordination with zinc was also suggested by the observation that TP2 could be labeled, Image , with iodoacetamidofluorescein only after preincubation of spermatid nuclei with EDTA. The zinc finger domains of TP2 may play an important role in initiation of chromatin condensation and /or cessation of transcriptional activity during mammalian spermiogenesis. DTT, Dithiothreitol; IAF, Iodoacetamido-fluorescein; SDS, Sodium dodecyl sulfate; PAGE, Polyacrylamide gel electrophoresis; PMSF, Phynyl methyl sulfonyl fluoride

Metal-ion-mediated tuning of duplex DNA binding by bis(2-(2-pyridyl)-1H-benzimidazole)

Studies of double-stranded-DNA binding have been performed with three isomeric bis)2-(n-pyridyl)-1H-benzimidazole)s (n = 2, 3, 4). Like the well-known Hoechst 33258, which is a bisbenzimidazole compound, these three isomers bind to the minor groove of duplex DNA. DNA binding by the three isomers was investigated in the presence of the divalent metal ions Mg2+, Co2+, Ni2+, Cu2+, and Zn2+. Ligand-DNA interactions were probed with fluorscence and circular dichroism spectroscopy. These studies revealed that the binding of the 2-pyridyl derivative to DNA is dramatically reduced in the presence of Co2+, Ni2+, and Cu2+ ions and is abolished completely at a ligand/metal-cation ratio of 1:1. Control experiments done with the isomeric 3- and 4-pyridyl derivatives showed that their binding to DNA is unaffected by the aforementioned transition-metal ions. The ability of 2-(2-pyridyl)benzimidazole changes of the ligand associated with ion chelation probably ledto such unusual binding results for the ortho isomer. The addition of ethylenediaminetetraacetic acid (EDTA) reversed the effects completely.

Anthranilic acid oxidase system of Tecoma stans—II. : Studies on the properties of a purified anthranilic acid oxidase system and its separation into different enzymic components

An enzyme system which catalysed the conversion of anthranilic acid to catechol has been purified 20-fold from a cell-free leaf extract of Tecoma stans. The optimum substrate concentration was 10−3 M and optimum temperature for the reaction was 45°. The presence of a multi-enzyme system was inferred from inhibition studies. The formation of catechol was inhibited by Mg2+, Zn2+, and Co2+ ions, whereas anthranilic acid disappearance was not affected to the same extent. The effect of metal chelating agents like EDTA, cyanide and pyrophosphate showed a similar trend. PCMB inhibited catechol formation but had no effect on anthranilic acid disappearance. The reaction was not inhibited by catalase, nor was it activated by peroxide-donating systems. This ruled out the possibility of peroxidative type of reaction. The overall reaction is markedly activated by NADPH and THFA. This multi-enzyme was separated into three different components, by fractionation with Alumina Cγ and calcium phosphate gels. The overall reaction catalysed by these components can be represented as anthranilic acid→3-hydroxy anthranilic acid→o-aminophenol→catechol.