Models of pesticides inside cavities of molecular dimensions. A role of the guest inclusion in the dechlorination process

The reduction mechanism of the pesticide vinclozoline (3-(3,5-dichlorophenyl)-5-methyl-5-vinyl-1,3-oxazolidine-2,4-dione) was studied in nonaqueous solvents in the confined environment of a cyclodextrin (CD) cavity. The effect of the cavity dimensions on the mechanism of the redox process was evaluated using glucose as a reference and using three cyclodextrin molecules of different cavity sizes, namely, alphaCD, betaCD, and gammaCD. In the absence of CD the main reduction product of vinclozoline in the first reduction step is dichloroaniline. An addition of glucose leads to a quantitative change of mechanism with 10 products in total. Addition of CD, however, leads exclusively to dechlorination of the phenyl ring. The degree of dechlorination depends strongly on the choice of cyclodextrin molecule. The importance of the complex formation equilibria in the change of the mechanism is supported by a series of semiempirical AM1 quantum-mechanical calculations. Very good correlation (correlation coefficient 0.995) was obtained between the complex stabilization energy of the inclusion complex and the degree of pesticide dechlorination. Additionally, we were able to show that the complexes are stabilized by the formation of intermolecular hydrogen bonds between the host and guest species. CD molecules do not simply act as proton donors in a nonaqueous environment, but also protect parts of the molecule included within the cavity and steer the degradation process toward fewer products.     

Study of dehalogenation of halogenoanilines using Raney Al–Ni alloy in aqueous medium at room temperature

Abstract  

Dehalogenation of halogenated anilines by action of powdered aluminium–nickel alloy in aqueous alkaline solution at room temperature has been studied. The reaction course was monitored by means of ¹H nuclear magnetic resonance (NMR) spectroscopy. The rates of dehalogenation of 4-bromo-, 4-chloro-, 4-fluoro- and 3-chloroaniline were compared under conditions of minimum necessary excess of Al–Ni alloy as reducing agent in edetane buffer medium at pH 10.9. The dehalogenation rates of halogenated anilines decreased in the following order: 3-chloroaniline > 4-bromoaniline ≥ 4-chloroaniline > 4-fluoroaniline. On the basis of the results obtained in buffers, the dehalogenation method of aqueous solutions of halogenoanilines was verified using NaOH as the base.     

Laser Desorption Ionization of As<Subscript>2</Subscript>Ch<Subscript>3</Subscript> (Ch = S,Se, and Te) Chalcogenides Using Quadrupole Ion Trap Time-of-Flight Mass Spectrometry: A Comparative Study

Laser desorption ionization using time-of-flight mass spectrometer afforded with quadrupole ion trap was used to study As₂Ch₃ (Ch = S, Se, and Te) bulk chalcogenide materials. The main goal of the study is the identification of species present in the plasma originating from the interaction of laser pulses with solid state material. The generated clusters in both positive and negative ion mode are identified as 10 unary (S _p ^+/– and As _m ^+/– ) and 34 binary (As _m S _p ^+/– ) species for As₂S₃ glass, 2 unary (Se _q ^+/– ) and 26 binary (As _m Se _q ^+/– ) species for As₂Se₃ glass, 7 unary (Te _r ^+/– ) and 23 binary (As _m Te _r ^+/– ) species for As₂Te₃ material. The fragmentation of chalcogenide materials was diminished using some polymers and in this way 45 new, higher mass clusters have been detected. This novel approach opens a new possibility for laser desorption ionization mass spectrometry analysis of chalcogenides as well as other materials.

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Nitrogen heteroaromatic cations by [2+2+2] cycloaddition

A modular approach to the construction of monocationic quaternary N-heteroaromatic frameworks was developed capitalizing on a direct pyridine-type nitrogen quaternization followed by metal-catalyzed [2+2+2] cycloaddition with gaseous acetylene. The flexibility of the route is demonstrated on 12 diverse scaffolds based on pyridinium, quinolinium, thiazolium, benzothiazolium, imidazolium, and pyrimidinium. Electrochemical study revealed a quinolinium redox system with two electrochemically distinct forms that are interconverted by a homogeneous chemical reaction triggered by fast electron transfers (reduction at -0.7 V and oxidation at -0.05 V).     

Molecular mechanisms of signalling specificity via phosphorelay pathways in Arabidopsis

Multistep phosphorelay (MSP) pathways mediate a wide spectrum of adaptive responses in plants, including hormonal and abiotic stress regulations. Recent genetic evidence suggests both partial redundancy and possible functional cross-talk on the one hand and a certain level of specificity on the other. Here, we discuss recent achievements improving our understanding of possible molecular mechanisms of specificity in MSP. We consider a certain evolutionary conservation of ancestral two-component signalling systems from bacteria in a process of molecular recognition that, as we have recently shown, could be applied also to a certain extent in the case of plant MSP. Furthermore, we discuss possible roles of kinase and phosphatase activities, kinetics of both these enzymatic reactions, and phosphorylation lifetime. We include also recent findings on the expression specificity of individual members of MSP pathways and, finally, based on our recent findings, we speculate about a possible role of magnesium in regulation of MSP pathways in plants. All these mechanisms could significantly influence specificity and signalling output of the MSP pathways.     

Extended viologen as a source of electric oscillations

A long organic molecule 1 with five bipyridinium functions separated by benzene rings (extended viologen) undergoes a reversible multi-step electron transfer. Here we show that this decacation accepts electrons at the heterogeneous interface with the occurrence of the periodically changing electric reduction currents. According to the applied bias voltage the observed current-time dependence changes from chaotic through periodic and irregular to sinusoidal and finally to monotonous. A careful choice of the controlling parameters yields the sustained periodic sinusoidal currents lasting for a prolonged time. Oscillations stem from a mutual interplay of the heterogeneous supply of electrons and the homogeneous redox reactions (disproportionation) between the transient redox forms. In difference to many other electrochemical oscillating systems the described oscillations do not require any additional external impedance. The principle of these oscillatory currents may serve as a model of a truly 'molecular oscillator'.     

Structure of reduced and oxidized manganese superoxide dismutase: a combined computational and experimental approach

Manganese superoxide dismutases catalyze the disproportionation of the superoxide radical anion to molecular oxygen and hydrogen peroxide. Recently, atomic-resolution crystal structures of the reduced and oxidized enzymes have been reported. They show an active site with the manganese ion bound to one aspartate, three histidine residues, and a solvent molecule. In this paper, we combine crystallographic refinement with quantum mechanical methods to show that the solvent ligand is undoubtedly a water molecule in the reduced state. However, the putative oxidized structure is to a large extent reduced during data collection, so that it contains a mixture of the Mn2+ and Mn3+ structure. The crystal structures show that the Mn-bound solvent molecule accepts a hydrogen bond from the side chain of the conserved Gln-146 residue. If the solvent ligand is water, then this could lead to a steric clash, but it is avoided by the plane of water molecule forming an angle of 72 degrees to the Mn-O bond. Such a conformation is also found outside the enzyme, giving a minimal destabilization of the reduced state. We show by molecular dynamics simulations that the suggested Mn2+-H2O and Mn3+-OH- structures are stable. Moreover, we show that the superoxide substrate may bind both in the first coordination sphere of the Mn ion, opposite to the aspartate ligand, or in the second sphere, close to the conserved Tyr-34 and His-30 residues, approximately 5 A from Mn. However, the second-sphere structures are not stable in long molecular dynamics simulations. We see no difference in the coordination between the reduced and the oxidized states of the enzyme.     

Evaluation of the intramolecular basis set superposition error in the calculations of larger molecules: [n]helicenes and Phe-Gly-Phe tripeptide

Correlated ab initio calculations on large systems, such as the popular MP2 (or RI-MP2) method, suffer from the intramolecular basis set superposition error (BSSE). This error is typically manifested in molecules with folded structures, characterized by intramolecular dispersion interactions. It can dramatically affect the energy differences between various conformers as well as intramolecular stabilities, and it can even impair the accuracy of the predictions of the equilibrium molecular structures. In this study, we will present two extreme cases of intramolecular BSSE, the internal stability of [n]helicene molecules and the relative energies of various conformers of phenylalanyl-glycyl-phenylalanine tripeptide (Phe-Gly-Phe), and compare the calculated data with benchmark values (experimental or high-level theoretical data). As a practical and cheap solution to the accurate treatment of the systems with large anticipated value of intramolecular BSSE, the recently developed density functional method augmented with an empirical dispersion term (DFT-D) is proposed and shown to provide very good results in both of the above described representative cases.     

Electrochemical conversion of dinitrogen to ammonia mediated by a complex of fullerene C60 and gamma-cyclodextrin

We report on an electrochemical conversion of N2 to NH3 at ambient pressure and 60 degrees C, which is mediated by reduced C(60) inside the molecular cavity of gamma-cyclodextrin.