Photochemistry and vibrational spectra of matrix-isolated methyl 4-chloro-5-phenylisoxazole-3-carboxylate

Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier (<0.3 kJ mol(-1)). In situ UV irradiation (λ > 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization.     

Protein unfolding studied by fluorescence methods and electrical impedance spectroscopy: the cases of Cratylia mollis and Concanavalin A

This work is dedicated to the investigation of the prevailing molecular interactions between Cratylia mollis (Cramoll) and Concanavalin A (Con A) lectins and ionic (sodium dodecylsulfate, SDS) and non-ionic (Triton X-100, TX-100) surfactants, where we have used electrical impedance spectroscopy to map the dielectric characteristics of mixed lectin/surfactant solutions. The disorder induced in the lectin conformation is proportional to the extent of the access of the surfactant to the fluorophore present in the protein, resulting in its progressive unfolding. The complete unfolding of the lectin is associated to the formation of micelles in the core of the protein, each one of them containing a large number of detergent molecules, and therefore the process can be accompanied by measuring the electrical response of the binary surfactant/lectin system. For instance, the change in the real part of the impedance as a function of the relative concentration of the surfactant in the binary solution exhibits a breaking in its linear behavior that can be taken as indicative of a qualitative change in the environment surrounding the protein residue. We consider these results strong evidence in favor of using impedance spectroscopy methods for the analysis of protein-surfactant associations and for the characterization of the interactions that must prevail when the protein unfolds as the relative surfactant concentration is increased in aqueous solutions of these binary systems.     

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ? S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).     

Functionalized-graphene modified graphite electrode for the selective determination of dopamine in presence of uric acid and ascorbic acid

Graphene is chemically synthesized by solvothermal reduction of colloidal dispersions of graphite oxide. Graphite electrode is modified with functionalized-graphene for electrochemical applications. Electrochemical characterization of functionalized-graphene modified graphite electrode (FGGE) is carried out by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The behavior of FGGE towards ascorbic acid (AA), dopamine (DA) and uric acid (UA) has been investigated by CV, differential pulse voltammetry (DPV) and chronoamperommetry (CA). The FGGE showed excellent catalytic activity towards electrochemical oxidation of AA, DA and UA compared to that of the bare graphite electrode. The electrochemical oxidation signals of AA, DA and UA are well separated into three distinct peaks with peak potential separation of 193mv, 172mv and 264mV between AA-DA, DA-UA and AA-UA respectively in CV studies and the corresponding peak potential separations in DPV mode are 204mv, 141mv and 345mv. The FGGE is successfully used for the simultaneous detection of AA, DA and UA in their ternary mixture and DA in serum and pharmaceutical samples. The excellent electrocatalytic behavior of FGGE may lead to new applications in electrochemical analysis.     

Influence of Cu(II) in the SrSnO<Subscript>3</Subscript> crystallization

Perovskite type oxides have been intensively studied due to their interesting optical, electrical, and catalytic properties. Among perovskites the alkaline earth stannates stand out, being strontium stannates (SrSnO₃) the most important material in ceramic technology among them due to their wide application as dielectric component. SrSnO₃ has also been applied as stable capacitor and humidity sensor. In the present work, SrSnO₃:Cu was synthesized by polymeric precursor method and heat treated at 700, 800, and 900 °C for 4 h. After that, the material was characterized by thermal analysis (TG/DTA), X-ray diffraction (XRD), infrared spectroscopy, and UV–vis spectroscopy. Results indicated three thermal decomposition steps and confirmed the presence of strontium carbonate and Cu²⁺ reduction to Cu⁺ at higher dopant amounts. XRD patterns indicated that the perovskite crystallization started at 700 °C with strontiatite (SrCO₃) and cassiterite (SnO₂) as intermediate phases, disappearing at higher temperatures. The amount of secondary phase was reduced with the increase in the Cu concentration.     

Reactivity of 2-halo-2H-azirines. 1. Reactions with nucleophiles.

Nucleophilic substitution reactions of 2-halo-2H-azirines 1a, 1b, 1d, and 1e with potassium phthalimide and aniline allowed the preparation of new substituted 2H-azirines 2-5. The reactions of 2-bromo-2H-azirine 1a with methylamine led to the synthesis of alpha-diimines 7 and 8. 2-Halo-2H-azirines were also established as building blocks for the synthesis of a range of heterocyclic compounds, namely, quinoxalines 10a-10d, 3-oxazoline 14, and 2H-[1,4]oxazines 18 and 20. X-ray crystal structures of alpha-diimine 7, 3-oxazoline 14, and 2H-[1,4]oxazine 18 are reported.     

Effect of pretreatment on pyrolysis products of Pennisetum purpureum Schum. by Py-GC/MS

Journal of Thermal Analysis and Calorimetry - Elephant grass (Pennisetum purpureum Schum.) is one such resource with a great amount of carbohydrate and lignin polymers that together represent high...     

Recovery and Purity of High Molar Mass Bio-hyaluronic Acid Via Precipitation Strategies Modulated by pH and Sodium Chloride

Applied Biochemistry and Biotechnology - The effects of ethanol/broth proportions and the number of steps at varying pH in the presence or absence of sodium chloride (NaCl) were studied as...     

Energy potential and thermogravimetric study of pyrolysis kinetics of biomass wastes

The use of agricultural wastes for energy conversion has been widely studied as renewable and carbon neutral energy sources. This paper aims to evaluate the energetic potential of six agricultural wastes—sugarcane bagasse, bean pods, corn stover, pineapple crown leaves, white cotton and natural coloured cotton stalks, through their characterization and pyrolysis kinetic study. The energetic potential of biomasses was evaluated by ultimate and proximate analysis, higher heating value (HHV), apparent density, and kinetic parameters of conversion and apparent activation energy (E_a) determined by Model-Free kinetics though thermogravimetric analysis data. The results indicate energetic density for dry basis biomasses, such as moisture content less than 7%, volatiles higher than 77% and moderate ash content. The HHVs were higher for the biomass with low O:C ratio. The E_a values increased with increasing O:C ratio and were also influenced by the biomass ash content. Among the studied biomasses, PCL are less explored for energy application, although the results confirm its potential for application in thermochemical processes such as pyrolysis or combustion.

     

Surface properties of sensors based on aminophenol-polymerized film

In this work, carbon electrodes modified with aminophenols were developed for the production of pesticides biosensors based on acetylcholinesterase. The polymers were potentiodynamically deposited on a graphite electrode surface by the oxidation of monomers, 2-aminophenol, 3-aminophenol and 4-aminophenol. The electrochemical behaviour and surface analysis of the electrodes modified by polyaminophenols non-immobilized and immobilized on acetylcholinesterase were studied by cyclic voltammetry, electrochemical impedance spectroscopy and atomic force microscopy. Roughness values obtained for graphite electrodes modified with poly(4-aminophenol) and poly(4-aminophenol)/acetylcholinesterase were 174 and 86 nm, respectively. The acetylcholinesterase enzyme was immobilized on a graphite and a graphite modified with poly(4-aminophenol), and these electrodes were coupled in the flow system. Potentiometric response due to hydrogen ions generated by an enzymatic system in the presence of acetylcholine chloride substrate was evaluated. The results showed that the graphite/poly(4-aminhophenol) sensor presents high sensitivity to hydrogen ions when compared with other graphite/polyaminophenols sensors. The biosensor coupled in a continuous flow system was employed for the detection of dichlorvos. The detection and quantification limits were 0.8 and 2.4 μmol L⁻¹ dichlorvos, respectively. This sensor reveals an efficient and promising material for biomolecules immobilization.