Dimethyldioxirane oxidises nitrogen-containing substrates. The sites of oxidation are generally the sp3 nitrogen atoms in the molecules although other reactive groups can be oxidised if present. An indication of the reactivity of different dioxiranes was obtained qualitatively from the polarographic peak reduction potentials and quantitatively by reaction with the model substrate 4-nitro-N,N-dimethylaniline. The polarographic peak potentials were shown to be of a similar order to those of typical acyclic peroxides. The rank order in terms of reactivity was shown to be methyl(trifluoromethyl)dioxirane > dimethyldioxirane > ethylmethyldioxirane. The rate of the reaction was not influenced by pH or ionic strength but was accelerated greatly by the presence of water. An explanation for this observation was proposed through consideration of dielectric constant and hydrogen bonding effects.

Recombination of chlorine atoms in the gas phase ; The effect of quaternary ammonium salts on the stabilities of initial and transition states in SN1 reactions.

Made available in DSpace on 2014-06-11T19:29:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2012-02-28Bitstream added on 2014-06-13T18:38:52Z : No. of bitstreams: 1 bastos_ac_me_ilha.pdf: 818148 bytes, checksum: 000c63a72e266556fa261b7eca0bf7b3 (MD5) / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Neste trabalho o 3(2-amino-1,3,4-tiadiazol) propil sílica gel, ATDSG, foi caracterizado por análise elementar de C, H, e N; área superficial; espectroscopia de absorção na região do infravermelho e RMN de 13 C e de 29 Si. O ATDSG foi utilizado na adsorção de espécies MX2 (M= Cu2+, Co2+ e Ni2+e X = Cl-, Br- e ClO4 -), possibilitando a construção de isotermas de adsorção. O ATDSG apresentou maior adsorção dos sais cloreto de cobre, cloreto de cobalto e cloreto de níquel em meio cetônico do que em etanol e água. Este efeito pode ser atribuído ao fato de que o cátion e o ânion estão mais associados neste solvente. Também é devido a menor polaridade da acetona em relação aos outros dois solventes, reduzindo assim a competição entre o cátion e o solvente pelos sítios de adsorção da superfície, e, portanto facilitando o processo de adsorção. Foram aplicados os modelos de Langmuir, Freundlich, Temkin e Dubinin-Radushkevich (D-R) para o ajuste, via o método de mínimos quadrados, dos dados experimentais de adsorção em equilíbrio. O modelo que obteve maior concordância com os resultados experimentais foi o de Langmuir. Foram testados modelos de cinética de adsorção para as espécies CuCl2 e CoCl2, em meio etanólico, especificamente, os modelos de pseudo-primeira ordem; pseudo-segunda ordem e Elovich. O modelo que melhor se ajustou aos dados experimentais para as espécies CuCl2 e CoCl2 foi o de Elovich e o pseudo-primeira ordem, respectivamente. Analisou-se também o transporte das espécies iônicas CuCl2 e CoCl2 através da superfície do ATDSG, em meio etanólico, pelo mecanismo de difusão, o qual pode ser do tipo difusão pelicular ou difusão intrapartícula. O resultado obtido sugere que a difusão destas espécies iônicas se adequou melhor para o modelo de difusão pelicular / In this study 3-(2-amino-1,3,4-thiadiazol)-propyl silica gel ATDSG characterized through elemental analysis of C, H and N; surface area; absorption spectroscopy in the infrared region and NMR of 13 C and the 29 Si ATDSG was used in the adsorption of species MX2 (M = Cu 2+ , Ni2+ and Co2+ and X = Cl-, Br-, and ClO4-), allowing the construction of adsorption isotherms. The ATDSG showed higher adsorption of salts copper chloride, cobalt chloride and nickel chloride in acetone medium than ethanol and water. This effect can be attributed to the fact that the metal ion and counter ion are more associated in this solvent. It is also due to lower polarity of acetone in regarding the other two solvents, thus reducing competition between the metal ion and the solvent by adsorption sites of the surface and therefore facilitating the adsorption process. We applied the models of Langmuir, Freundlich, Temkin and Dubinin Radushkevich(DR) to fit, via the method of least squares, experimental data of adsorption at equilibrium. The model we obtained better agreement with experimental results was the Langmuir. We tested models of adsorption kinetics for the species CuCl2 and CoCl2 in ethanolic medium, specifically, the pseudo first-order model, pseudo second order and Elovich. The model that best fitted the experimental data for the species CuCl2 and CoCl2 was the Elovich and the pseudo first-order, respectively. Was also examined the transport of CuCl2 and CoCl2 ionic species across the surface of ATDSG in ethanolic medium by diffusion mechanism, which may be the type pellicular diffusion or intraparticle diffusion. The result obtained suggests that diffusion of these ionic species adapt themselves better to pellicular the diffusion model

No description available.

No description available.

The kinetics of the coordination reaction between the carboxyl group of the acetate radical and the trivalent chromium ion has been examined by independent methods at a temperature selected to yield reasonably accurate rate data. The study has been extended to an examination of the rate of chromium "fixation" by the carboxyl groups of hide collagen in a series of miniature tannages carried out under similar conditions for comparative purposes. The effect of olation of the chromium ions in solution on the kinetics of t he coordination reaction has also been examined. The rate of coordination of the acetate radical was followed by solvent extraction and spectrophotometric techniques over a range of concentration and pH levels. The rate data revealed a reaction having typical mass-action characteristics, the rate of reaction depending on the concentration levels of the chromium ion and the ionised acetate radical. An attempt made to distinguish a differential reaction rate in the case of a parallel reaction series using boiled and aged chromium nitrate reactant solutions, failed to reveal any significant differences between the series. The reactant solutions gave absorption spectra characteristic of the trivalent chromium ion with maxima in the 420 mµ and 570 mµ wavelength regions. The changes in optical density occurring in the vicinity of the maxima were followed spectrophotometrically, the height of the 570 mµ being found to increase during the course of reaction, while the height of the 420 mµ peak decreased. The variation in optical density at the 570 mµ peak was found to be directly related to the increase in concentration of the product complex in solution in accordance with the Baer-Bougher law, while the decrease in the height of the 420 mµ peak was related to the properties of unolated OH groups associated with the chromium complex as governed by pH and olation changes during the course of reaction. Boiled and aged chromium nitrate solutions gave reactant solutions having initial optical densities greater than those of the correspending fresh reactant solutions at the 570 mµ peak, and less than those of the fresh solutions at the 420 mµ peak. During the course of reaction, however, the two series converged to the same values. The equilibrium reaction solutions were subjected to paper electrophoretic study which indicated that all tho chromium was present in the form of cationic species. This finding was in accordance with stoicheiometric indications in the solvent extraction studies where coordination in a l : 1 ratio was reflected, giving rise to positively charged complex ions only. Making due allowance for band spreading the numbers of species predicted from the kinetic studies correspond with the electrophoretic patterns found. Application of the classical second order kinetic expression to the solvent extraction rate data, yielded plots having two distinctly linear sections, apparently indicating consecutive second order processes occurring in solution, contrary to the findings of previous workers who assign a first order mechanism to the reaction. This finding was consistent with the view that in the case of both the fresh and aged series, reaction consisted of successive coordination of an acetate radical to each chromium atom of a diol complex. Second order rate "constants" were calculated for each step and their dependency on pH level demonstrated. The findings of Hamm et al that these were first order reactions independent of acetate co'ncentration are believed to be due to their use of a large excess of acetate in their experiments. The kinetic study was further extended to an examination of the rate of "fixation"of trivalent chromium by the carboxyl groups of hide collagen under comparable conditions in a series of miniaturv tanning experiments in which the tannage substrate was provided in each of two physical forms:- (a) As hide powder where surface development was large, and (b) as prepared pelt pieces in which the fibrous weave pattern was retained. In view of the heterogeneity of the tannage systems, remarkable similarity was observed in the reaction course when compared with that of the acetate coordination studies, particularly in the case of the hide powder tannages. The dependency of the tannage rate data upon concentration and pH conditions was also found to be the same as in the case of the pure chemical system. The exact correlation between the rate data for the tannage and pure chemical systems was demonstrated by means of correlation plots. Close correlation was revealed in the case of hide powder tannage while the smaller degree of correlation observed in the case of the pelt tannage systems was attributed to the modifying effects of diffusion, particularly on the initial reaction velocity. The effect of using boiled and aged chromium solutions in the tannages was to increase the initial rate of chroniill!l "fixation" apparently due to tho coordination of a dial species at each coordination site. After the initial reaction period, the two series showed a tendency to converge as in the case of the spectrophotometric studies on the acetate reaction. The convergence trend was regarded as indicative of the tendency for the chromium in fresh solutions to under go rapid olation to the ame level as in the boiled and aged solutions. The experimental observations made on the various systems lead to the following conclusions:- (a) The mechanism of coordination of the acetate radical ttrivalent chromium appears to involve the successive coordination of ligand to each of the chromium atoms of a diol complex, both coordination steps proceeding by second order reaction mechanisms. (b) At the pH levels at which the reaction is carried out, the formation of elated bodios is rapid so that reaction in the case of both fresh and aged solutions essentially occurs between an elated species and the ionised acetate radical. (c) Modificati0ns in the absorption spectrum of the reactant solution at the 570 mµ peak are directly related to coordination, while changes at the 420 mµ peak are related to the formation of loosely-bound hydroxo chromium compounds, the concentrations of which depend on the pH level. (d) The mechanism of chromium fixation in hide is essentially similar to that operating in the case of coordination of the acetate radical to chromium, involving attachment of chromium to the side chain acid r sidues of collagen. (e ) The effect of olation is to enhance the rate of chromium fixation by hide protein during the initial reaction stages and to render possible bridge formation between adjacent side chains by secondary coordination during the latter reaction stages. (f) Where pelt pieces are used instead of hide powder, the initial rate of chromium fixation is dominated by the rate of diffusion of chromium into the fibrous structure. It should be stressed that the observations made on the various reaction systems cannot be regarded as exhaustive and the conclusions are subject to further confirmation. Consequently, the present study is essentially of a preliminary nature, but it is felt that the need for further investigation along similar lines has been demonstrated.

Secondary N-nitrosamines: diphenylnitrosamine (DPhNA), dimethylnitrosamine (DMNA), diethylnitrosamine (DENA), dipropylnitrosamine (DPNA), dibutylnitrosamine (DBNA), diethanolnitrosamine (DEtNA), methylnitrosoglycine (MNGly), nitrosopyrrolidine (NPyr), nitrosomorpholine (NMor) and nitrosopiperidine (NPip) were synthesised and their interaction with acetylcholinesterase (AChE) was investigated. Analyses of kinetic results show that DMNA (Ki=34.78 μM); DENA (Ki=54.24 μM); DPNA(Ki=60.36 μM); DBNA(Ki=95.54 μM); DEtNA(Ki=43.68 μM)MNGly (Ki=30.18 μM); NPip (Ki=123 μM); NPyr (Ki=66.07 μM), NMor (Ki=73.93 μM) and DPhNA (Ki=20.32 μM) are competitive and reversible inhibitors of acetylcholinesterase, with respect to the substrate, acetylthiocholine chloride, ATChCl. With time they act as irreversible covalent inhibitors with dipropy1nitrosamine producing 72% inactivation after 60 minutes. Scatchard analyses of f1uorometric titrations, (Kd=0.75mM-4.09mM); gel chromatography (Kd=O. 80mM-4. 60mM) and equilibrium dia1ysis (Kd=O. 71mM- 4.21mM) for MNG1y, DMNA, DEtNA, DENA, DPNA, NPyr, DSNA, NMor and NPip show that these compounds have weaker affinity for the enzyme, as compared to the much tightly binding aromatic DPhNA, Kd values (0.65mM, 0.68mM and 0.68mM) for fluorometric experiments, gel chromatography and equilibrium dialysis respectively. In all cases, the number of binding sites of acetylcholinesterase averaged to four.

PEP activity has been described in several locations and has mostly been linked to a variety of neurological disorders such as schizophrenia, amnaesia, depression as well as other disease states such as anorexia nervosa, bulimia nervosa and blood pressure regulation. The enzyme has also been previously isolated from a variety of archae, microorganisms and several eukaryotic species but no prolyl endopeptidases have been isolated from plants. Plants have high levels of proline and glutamine rich peptides in seeds. We therefore hypothesize plants must express PEPs during germination. Bioinformatics tools were used to identify known PEPs and putative plant PEPs. A global sequence alignment of putative plant PEPs and other known PEPs indicated that the active site amino acids Ser, His and Asp are conserved in putative plant PEP sequences. Furthermore, putative plant PEPs showed similar secondary structures to known PEPs and when a rice PEP was modelled onto porcine brain PEP structure, a high degree of similarity was found. Germination studies of wheat seed showed an increase of PEP activity over time with maximum PEP activity reached after 4 days of germination, which remained at this level until 9 days of germination, implying a function for PEP in plant seed germination. Wheat PEP was purified using ion exchange and gel filtration chromatography with a final yield of less than 1 percent and a relative purity (only 2 bands detected by SDS-PAGE). The purified wheat PEP had a molecular weight of approximately 55kDa, substrate specificity for chymotrypsin-like substrates (N-Suc-Ala-Ala-Pro-Phe-pNa, Km value of 0.58 mM, Kcat of 29.37 s–1; Kcat /Km 50813.14s–1 M–1); a pH optimum of 7.9; temperature optima of 37oC and a high sensitivity to temperature as indicated by loss of activity at temperatures above 40oC. Inhibition studies using E64, Leupeptin and PMSF confirmed that the wheat PEP is from the Serine protease family and is most likely a trypsin-like protease.

No description available.

The kinetics of the electron transfer reaction U(IV) + T1(III) → U(VI) + T1(I) were examined in aqueous perchloric acid solution. The rate law was found to be of the form [ Formula omitted ] The two rate constants were identified with reaction paths involving activated complexes of the compositions (U.OH.T1)⁶⁺ and (U.O.T1)⁵⁺ , respectively. The corresponding heats and entropies of activation, evaluated from rate measurements over the temperature range 16 to 25°, are ΔH₁± = 24.6 kcal/mole, ΔH₂± = 21.7 kcal/mole, ΔS₁±= 16 e.u. and ΔS₂±= 7 e.u. The effect of ionic strength and the specific effects of various anions and cations on the rate were examined. The results suggest, but do not prove, that the reaction occurs through a single two-equivalent step rather than through successive one electron changes. The homogeneous oxidation of carbon monoxide by metal ions in aqueous solutions was studied. At temperatures below 80° only Hg²⁺ and MnO₄¯ were found to oxidize carbon monoxide. The ions Cu²⁺ , Ag⁺ , Hg₂²⁺ , Fe³⁺ , T1³⁺ and Cr₂O₇²¯, were inactive. Kinetic measurements of the reaction 2Hg²⁺+ CO + H₂O → Hg₂²⁺+ + CO₂ + 2H⁺ in dilute perchloric acid over the temperature range 26 to 54° yielded the rate law [ Formula omitted ] The activation parameters are ΔH± = 14.6 kcal/mole and ΔS± =-13 e.u. It is believed that the reaction proceeds by a mechanism which involves the insertion of CO between Hg²⁺ and a coordinated water molecule [ Formula omitted ] The oxidation of carbon monoxide by MnO₄¯ was found to proceed readily over the temperature range 28 to 50°. The rate law was found to be [ Formula omitted ] with ΔH± = 13 kcal/mole and ΔS± = -17 e.u., both substantially constant over the pH range 1 to 13. The rate determining step is considered to be the formation of hypomanganate MnO₄¯+ CO + H₂O → MnO₄³¯ + CO₂ + 2H which then undergoes further fast reactions to yield MnO₄²¯ in basic solution and MnO₂ in acid and neutral solutions. A remarkable feature of the latter reaction is its very marked sensitivity to catalysis by Ag⁺ and Hg²⁺ (but not by Cu²⁺, Cd²⁺, Fe³⁺, or T1³⁺ ). The rate law of the catalyzed path is, in each case, [ Formula omitted ] where M = Ag⁺ or Hg²⁺. For Ag⁺, k at 0° is 1.10 x 10⁵ M¯² sec¯¹ with ΔH± = 1.3 kcal/mole and ΔS± = -30 e.u. For Hg²⁺, k at 0° is 1.09 x 10³ M¯² sec¯¹ with ΔH± = 6.5 kcal/mole and ΔS± = -21 e.u. It is suggested that the remarkably high reactivities exhibited by carbon monoxide in these catalytic reactions are related to favourable oxidation paths involving intermediates such as [ Formula omitted ] / Science, Faculty of / Chemistry, Department of / Graduate