Synthesis of some new lignans and the mechanism of intramolecular nonphenolic oxidative coupling of aromatic compounds

Treatment of cis-1,4-diaryl-2,3-epoxy butane with 2,3-dichloro-5,6-dicyano-1,4-benzo-quinone(DDQ) in trifluoroacetic acid(TFA) gave dibenzocycloheptadienes by pinacol rearrangement and intramolecular oxidative coupling of the aryl groups. Meanwhile, the benzofuran analogues were also formed by the same method. These new lignans have potent CNS inhibitory activities. It has been proven that the method is particularly useful when applied to the substrates in which the ring substituents are electron donating. The structures of these products were identified by the MS, UV, IR, NMR spectra, and elemental and X-ray crystallographic analyses.     

Electronic excitation of sulfur-organic compounds – performance of time-dependent density functional theory

 To define the scope and limitations of the time-dependent density functional theory (TDDFT) method, spectral absorption data of a series of about 100 neutral or charged sulfur-organic compounds with up to 24 non-hydrogen atoms and up to four sulfur atoms were calculated in the near-UV, visible and IR regions. Although the theoretical vertical transition energies correspond only approximately to experimental absorption band maxima, the mean absolute deviation was calculated to be 0.21 eV (1600 cm⁻¹). The main absorption features of various compounds with monocoordinated or dicoordinated sulfur atoms are well reproduced. As far as possible TDDFT results were compared with those of semiempirical Zerner's intermediate neglect of differential overlap (ZINDO/S) and of Pariser–Parr–Pople (PPP) calculations. TDDFT also works well in cases where the semiempirical methods fail. Limitations of TDDFT were encountered with calculations of spectral absorptions of dye molecules. The “vinylene shift” of polymethine dyes is not reproduced by TDDFT. Whereas electronic excitation energies delocalized polar and betainic chromophores are reasonably well reproduced, excitation energies of charge-transfer-type and charge-resonance-type transitions of weakly interacting composite chromophores are significantly underestimated. Received: 30 October 2000 / Accepted: 29 November 2000 / Published online: 22 May 2001     

Aromatase — its mechanism and mode of action of some inhibitors — a molecular modelling perspective

The mechanism for the final step of Aromatase (AR) and the mode of action of some steroidal and non-steroidal inhibitors are studied with a molecular modelling approach involving consideration of the positioning of the P-450 heme with respect to the substrate backbone — the “substrate-heme complex”.This study suggests that Aminoglutethimide (AG) type compounds mimic the steroid C(17) carbonyl and not the C(3) carbonyl group as previously suggested by other workers. It also shows that inhibitors based on pyridine ligands such as 3-(4′-pyridyl)-3-ethyl piperidine-2,6-dione (PYG) utilise the steroid C(3) carbonyl binding region, thereby agreeing with other workers. Consideration of the orientation of the R and S enantiomers of PYG shows a reversal of results previously reported. By using inhibitors bound to the “substrate-heme complex”, and data from previous studies of derivatives of androstenedione, reasons for the differences in activity of enantiomers of AG, PYG, N-octyl-3-(4′-pyridyl)-3-ethyl piperidine-2,6-dione and 10-thiiranylestr-4-ene-3,17-dione, as well as other potent and less potent inhibitors, are suggested.The study also considers the three mechanisms of aromatisation, as proposed by Wright and Akhtar, from a geometric point of view and concludes that only a ferroxy radical is involved in all three steps of aromatisation, and not the mixture of both ferroxy and peroxy as previously postulated. An alternative theoretical mechanism, which circumvents the production of the CHO radical for the final step of aromatisation, is suggested.     

Kinetics and mechanism of base hydrolysis of Reinecke's salt revisited – specific salt effects

The kinetics of base hydrolysis of the trans-[Cr(NH₃)₂(NCS)₄]^– anion follows the rate law: -d[complex]/dt = k ₀ + k ₁[OH^–] (50–70 °C, [OH^–] = 0.1–1.9 M and = 2.0 M). The specific salt effect has been investigated for eight aqueous media: NaCl, NaBr, NaI, NaClO₄, KCl, KBr, CsCl and CsBr. The alkali-independent path (k ₀) does not show any specific effect of inert electrolyte ions, the activation parameters: H = 113.5 ± 0.4 kJ mol^–1 and S = 24.1 ± 1.3 J mol^–1 K^–1 are interpreted in the frame of a dissociative interchange mechanism (I _d). For the alkali-dependent path (k ₁) the specific salt effect is observed for cations of the inert electrolyte, showing an important role for ion-pair formation between the cations and reagent complex anion in the activation process. A linear correlation between lnk ₁ and lnK ₀ (K ₀ – ion-pair formation constant) has been found for the cations studied. The dissociative, via conjugate base, mechanism (D _CB) has been proposed for the alkali-dependent path.     

Studies of the atomization mechanism in a graphite furnace—I. The contents of the gaseous impurities in the furnace and their variations

Using theoretical calculations of the partial pressures of gaseous impurities in the atmosphere in a GFA as a basis, the authors arrived at conclusions which are at variance with those of L'vov and frech and their co-workers. Accordingly, the partial pressure of free oxygen (p_o) in a GFA originates predominantly from the thermal decomposition of the oxides of the analyte and impurities in the matrix during atomization when the temperature is higher than 1000°C; then, p_o depends on the nature of the sample. The total amount of oxygen in a GFA depends on the initial amount of it in the argon gas and the amount released from the sample when the temperature is lower than 1000°C.The variation of the partial pressure of CO is for the first time characterized as follows: the content of CO, which is mainly dependent on the sample and the total amounts of gaseous impurities in the argon purge gas, does not vary markedly with the temperature and other factors during the whole process of atomization. The gaseous impurities exist mainly in the form of CO.     

Molecular mechanism of the protective effect of monomer polyvinylpyrrolidone on antioxidants – experimental and computational studies

We previously developed a lutein–polyvinylpyrrolidone (PVP) complex with improved aqueous saturation solubility and stability, though the conjugation mechanism is still unclear. In this paper, experiments with astaxanthin–PVP complex and curcumin–PVP complex were carried out, which indicated that PVP could improve the solubility and stability of astaxanthin and curcumin. We aimed to construct a computational model capable of understanding the protective effect in complexes formed between PVP and antioxidants, through which the binding mode of PVP and antioxidants was investigated with molecular modelling in order to obtain the interactions, binding energy, binding site and surface area between PVP and antioxidants. Solubility enhancement was attributed to the H-bonds between PVP and antioxidants, and the saturation solubility was curcumin > lutein > astaxanthin. Binding energy, binding site and surface area were beneficial for the stability of complex, and the stability enhancement was lutein > astaxanthin > curcumin. The experimental results were in agreement with the computational results. Furthermore, we established a method for the exploration of a similar system with other polymer complexes. Additionally, the proposed PVP model could predict the interactions between PVP and various ligands, such as antioxidants and drugs.     

Characterization of the drag-reducing nature of long-chain polymer–water solutions through atomization

Inclusion of minute concentrations of elongated long chain polymers is known to exhibit drastic reduction in frictional losses for turbulent fluid flows. The polymer chains interact with solvent molecules to act as eddy stress absorbers causing reduction in drag and inducing the viscoelastic nature in the fluid layer. In this work, we propose a characterization method for viscoelastic behavior of polymer-water solutions using a spray atomization experiment. The methodology is demonstrated for polyacrylamide and polyethylene oxide-based aqueous solutions. The experimental technique assesses the nature of the fluid using a predefined parameter, impact ratio (R). Computational fluid dynamics simulations of spray atomization are performed to validate the experimental values of the impact ratio. The simulation results indicate coherence with the experimental data, and are concordant with the reported behavior of such solutions in the literature.     

Nikolsky’s ion exchange theory versus Baucke’s dissociation mechanism of the glass electrode

The discovery of the glass electrode by Max Cremer was possible because of the advances made in the nineteenth century in understanding the electrical properties of glass, and because of the studies of electrical potential drops at the interface of phases. The discovery of the Leiden jar by E. G. von Kleist and the follow-up studies of the properties of that capacitor disclosed that glass is a dielectric. Much later, the ionic conductivity of glass was noticed and studied by J. H. Buff, W. von Beetz, W. Thomson (Baron Kelvin of Largs), W. Giese, H. L. F. von Helmholtz, E. Warburg, etc. It needed also the discovery of electromotive forces due to the partition of mobile ions (charge separation) by W. Nernst and E. H. Riesenfeld to pave the way for the idea that ion partition also occurs at solid–solution interfaces producing electromotive forces (emf). At the beginning of the twentieth century, the ground was laid to expect that a very thin glass membrane may produce an electromotive force because the glass has a finite ionic conductivity and ion partition may cause an emf. It obviously needed a physiologist like Max Cremer who desired to mimic a cell membrane (a semipermeable membrane), to use a glass membrane for that purpose. Cremer’s congenial choice of a thin glass bulb was rooted in a thorough understanding of the origin of electromotive forces, and it was not initiated directly by the Giese-Helmholtz cell, as some later reviews suggested. Later Cremer realized that an emf builds up when aqueous solutions are separated by a thin glass membrane. Cremer’s discovery was picked up by F. Haber who developed the glass electrode together with his PhD student Z. Klemensiewicz as an analytical tool. The following decades have brought improvements of the glasses and measuring techniques, and a deeper insight into the functioning of the glass electrode. Here, it will be shown that full credit for the discovery of the glass electrode effect must be given to Max Cremer. Unfortunately, his role has not been adequately described so far, mainly because Haber dominated the literature.     

Micro-electromembrane extraction using multiple free liquid membranes and acceptor solutions – Towards selective extractions of analytes based on their acid-base strength

This work investigated selective micro-electromembrane extractions (μ-EMEs) of the colored indicators metanil yellow and congo red (visual proof-of-principle) and the small drug substances nortriptyline, papaverine, mianserin, and citalopram (model analytes) based on their acid-base strength. With two free liquid membranes (FLMs), the target analytes were extracted from aqueous donor solution, across FLM 1 (1-pentanol, 1-ethyl-2-nitrobenzene (ENB) or 4-nitrocumene (4-NC)), into aqueous acceptor solution 1, further across FLM 2 (1-pentanol, ENB or 4-NC), and finally into aqueous acceptor solution 2. All phases had volumes between 1.0 and 1.5 μL and extractions were promoted by 200–300 V d.c. applied across the five-phase μ-EME system formed in a perfluoroalkoxy capillary tubing. The anode was located in acceptor solution 2 and the cathode was located in donor solution for μ-EMEs of acidic analytes, and locations of the electrodes were vice versa for μ-EMEs of basic analytes. After μ-EME, donor solution and acceptor solution 1 and 2 were analyzed by capillary electrophoresis or liquid chromatography-mass spectrometry. The model analytes migrated efficiently in the proposed μ-EME system, their migration behavior was controlled by pH in aqueous solutions and their selective fractionation into acceptor solution 1 and 2 was demonstrated based on their acid-base strength. Under optimal conditions, acceptor solution 2 contained 60% nortriptyline (pK_a = 10.5) and less than 1% papaverine (pK_a = 6.0) and acceptor solution 1 contained 17% nortriptyline and 27% papaverine after 15 min of μ-EME. The five-phase μ-EME system was also compatible with human plasma samples. Work is in progress to further increase the fractionation capability, and to implement the concept into microfluidic platforms.     

Kinetics and mechanism of oxidation of some nickel(II)–imine–oxime complexes by peroxodisulfate in aqueous acidic solutions

The kinetics of the oxidation of [Ni(II)(H₂L¹)](ClO₄)₂, (H₂L¹ = 3,8-dimethyl-4,7-diaza-3,7-decadiene-2,9-dione dioxime) and [Ni(II)(HL²)]ClO₄, (H₂L² = 3,9-dimethyl-4,8-diaza-3,8-undecadiene-2,10-dione dioxime) by peroxodisulfate anion (PDS) in aqueous media at 298.0 K have been studied. The kinetics of oxidation of both Ni(II) complexes was found to be first order in the complex concentration. The dependence of the pseudo-first-order rate constant, k _obs, for both complexes showed first-order dependence on PDS concentration. The kinetics of oxidation of [Ni(II)(H₂L¹)]²⁺ complex showed a complex dependence on [H⁺] over the pH range of 4.98–7.50, whereas that of [Ni(II)(HL²)]⁺ is independent of pH over the pH range of 5.02–7.76. The value of k _obs, for both complexes, decreased with increasing ionic strength consistent with the involvement of oppositely charged ions in the rate-determining step. The effect of ionic strength is more pronounced for [Ni(II)(H₂L¹)]²⁺–PDS reaction than for [Ni(II)(HL²)]⁺–PDS reaction, confirming the higher charges of the latter.     

Insilico validation and comparison of antifungal competence and druglikeness of some natural xanthones – A step towards antimycotic therapeutics

In the current work, the xanthones (from the plant source) that possess strong experimental evidences for their antifungal properties are chosen from the literatures and docking studies were employed with a fungal protein for computational validation to simplify the process of electing the top hits among a large set of naturally occurring substituted xanthones. The results indicate that computationally 86% of xanthones under study exhibits good binding affinity towards (B.E ≥ ?7 kcal/mol) the target protein (GlcN-6-P). The current docking analysis established the Garcinone D (?10.25 kcal/mol: Garcinia Mangostana), Caledol (?10.14 kcal/mol: Calophyllum Caledonicum), Jacareubin (?9.82 kcal/mol: Calophyllum Brasiliensis), γ-mangostin (?9.79 kcal/mol: Garcinia Mangostana) and 2-deprenylrheedia xanthone (?9.64 kcal/mol: Hypericum Roperanum) as good inhibitors of GlcN-6-P among a large set of natural xanthones. And also, most of xanthones have greater binding ability than the compared antifungal drugs. The effect of substituents on binding energy have also been investigated. A detailed analysis of drug likeness for 50 xanthones were also performed and found that most of the xanthone derivatives falls under the category of drug templates.     

Synthesis of some α-fluoroacryl esters

Russian Journal of Applied Chemistry - A possibility of producing α-fluoroacrylic esters by dehydrochlorination of the products of reaction of α-fluoro-β-chloropropanoic acid or...     

Viscosity and thermal conductivity of some refrigerants at moderate and high densities on the basis of Rainwater–Friend theory and the corresponding states principle

The initial density dependence of viscosity and thermal conductivity was formulated on the basis of Rainwater–Friend (RF) theory. In this work, we have first focused on the calculation of viscosity and thermal conductivity of moderately dense argon by using RF theory and an accurate ab initio potential function. This theory which was originally presented for spherical potentials have been adapted for calculation of viscosity and translational contribution of thermal conductivity of some refrigerants by introducing the corresponding states correlations for the second transport virial coefficients. Then the internal states contribution for the thermal conductivity has been determined based on the Mason–Monchick and modified Enskog theories. So, we have calculated the viscosity and thermal conductivity of some refrigerants, R32, R14, R12, R13, R22, R134a, R143a, R125, R123, R142b, at moderate densities up to about 2 mol dm⁻³. At high densities, beyond the validity range of RF theory, we have applied correlation expressions for the viscosity and thermal conductivity residual functions to calculate the viscosity and thermal conductivity of supercritical refrigerants and then compared with the available experimental data. In conclusion, we have shown that the RF theory in conjunction with the corresponding states residual functions present the reliable model for calculation of viscosity and thermal conductivity of refrigerants over a comparatively wide temperature and pressure range up to 65 MPa within the experimental errors.