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Calorimetric insight into coupling between functionalized primary alkyl halideand vinylic organocuprate reagent
Zdenko Časar, Marko Tramšek, Andreja Goršek, 2010, original scientific article

Abstract: The first calorimetric study of coupling between organocuprate, derived from Grignard reagent (vinyl magnesium chloride), and primary alkyl halide (e.g. (S)-ethyl 3-(tert-butyldimethylsilyloxy)-4-iodobutanoate) has been conducted. This transformation is paramountly important for efficient preparation of (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate – a key lactonized statin side chain precursor. The results obtained give thorough calorimetric insight into this complex low-temperature synthesis as well as a new understanding of the suggested reductive elimination of the final intermediates in the coupling reaction. Namely, the surprising unexpected spontaneous three-step exothermal event has been observed during controlled progressive heating of the mixture of the final intermediates to the room temperature. This phenomenon confirms that coupling between functionalized primary alkyl halide and vinylic organocuprate reagent is not a simple $S_N2$ substitution reaction. The presented study provides among others the first reported values of reaction enthalpies and corresponding adiabatic temperature rises of reaction mixture for all exothermic events that occurred in the (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate synthesis. The obtained results ensure consequential thermal process safety knowledge which can be incorporated into safe process scale-up as well as design of reactor system with sufficient cooling capacity for industrial production of (R)-ethyl 3-(tert-butyldimethylsilyloxy)hex-5-enoate. Moreover, the results provide a basic guidance for other organocuprate coupling reaction systems.
Keywords: Grignard reagents, organocuprates, coupling reaction, reaction calorimetry, statins
Published: 17.08.2017; Views: 585; Downloads: 54
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Gap junctions and epileptic seizures - two sides of the same coin?
Vladislav Volman, Matjaž Perc, Maxim Bazhenov, 2011, original scientific article

Abstract: Electrical synapses (gap junctions) play a pivotal role in the synchronization of neuronal ensembles which also makes them likely agonists of pathological brain activity. Although large body of experimental data and theoretical considerations indicate that coupling neurons by electrical synapses promotes synchronous activity (and thus is potentially epileptogenic), some recent evidence questions the hypothesis of gap junctions being among purely epileptogenic factors. In particular, an expression of inter-neuronal gap junctions is often found to be higher after the experimentally induced seizures than before. Here we used a computational modeling approach to address the role of neuronal gap junctions in shaping the stability of a network to perturbations that are often associated with the onset of epileptic seizures. We show that under some circumstances, the addition of gap junctions can increase the dynamical stability of a network and thus suppress the collective electrical activity associated with seizures. This implies that the experimentally observed post-seizure additions of gap junctions could serve to prevent further escalations, suggesting furthermore that they are a consequence of an adaptive response of the neuronal network to the pathological activity. However, if the seizures are strong and persistent, our model predicts the existence of a critical tipping point after which additional gap junctions no longer suppress but strongly facilitate the escalation of epileptic seizures. Our results thus reveal a complex role of electrical coupling in relation to epileptiform events. Which dynamic scenario (seizure suppression or seizure escalation) is ultimately adopted by the network depends critically on the strength and duration of seizures, in turn emphasizing the importance of temporal and causal aspects when linking gap junctions with epilepsy.
Keywords: epilepsy, nonlinear dynamics, electrical synapses, coupling, synchronization
Published: 19.06.2017; Views: 500; Downloads: 315
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Pentaquarks and the mass spectrum of the elementary particles of the standard model
Leila Marek-Crnjac, 2006, original scientific article

Abstract: Predstavljene so nove izrazitve matematičnega upanja mase pentakvarka. Možna povezava med Higgsovimi masami in maso pentakvarka je bila raziskana (El Naschie M. S.: Pentaqkvarkovo masno pravilo vsote in Higgsovi delci). Higgsove mase in pentakvarkove mase se lahko izrazijo s supersimetrično konstanto, z nesupersimetrično konstanto in tudi z napovedanim številom elementarnih delcev standardnega modela ▫$N(MS) approx 69$▫. Teoretične napovedi masnega spektra elementarnih delcev standardnega modela so prikazane v tabeli skupaj z razcepitvenimi faktorji.
Keywords: pentakvark, Higgsov delec, masni spekter, elementarni delci standardnega modela, super simetrična konstanta, pentaquark, Higgs particle, mass spectrum, elementary particles of the standard model, super symmetric coupling constant
Published: 10.07.2015; Views: 633; Downloads: 70
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Tamilselvan Mohan, 2012, doctoral dissertation

Abstract: The aim of the thesis is to investigate the applicability of nanometric amorphous cellulose model films for the immobilization of functional DNA molecules and to apply this knowledge in the field of DNA microarray preparation. To achieve this aim, the whole thesis work is divided into three major parts, namely part I (partly and fully regenerated cellulose film preparation and its characterization), part II (functional polysaccharide conjugates preparation) and part III (DNA microarray preparation from polysaccharide functional conjugates). The first part of the work mainly focuses on the preparation of cellulose model films from spin coated trimethylsilyl cellulose (TMSC) using an in-situ and ex-situ regeneration methods and its characterization. In the in-situ method, the conversion of TMSC to pure cellulose via acid vapor hydrolysis is investigated at the gas-solid interface in real time and at ambient conditions employing quartz crystal microbalance with dissipation (QCM-D). For this purpose, a permanent flow of gaseous HCl is employed which reacts with TMSC coated surface to form pure cellulose. The kinetics behind this reaction is elucidated and reveals first order. Moreover, the influence of the acid concentration on the kinetics and on changes in mass and film thickness of TMSC is studied. In the case of an ex-situ method, partly and fully regenerated cellulose model films are prepared from spin coated TMSC films through acid vapor phase hydrolysis. This is done by exposing the TMSC films placed in a closed container to vapors of HCl. The regeneration is carried out on one hand by exposing the films to different time intervals and on the other hand to different volume of HCl. The changes in surface morphology, structure, surface composition and film thickness in the course of regeneration (i.e. desilylation) is studied by using various surface analytical techniques like atomic force microscopy (AFM), attenuated total reflectance infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) and ‘Sarfus’-technique. In order to gain detailed insight into the desilylation reaction of the films the results from ATR-IR, XPS and thickness measurements are compared with data from static contact angle (SCA) and surface free energy (SFE) determination. Besides, to verify the completion of regeneration of cellulose from TMSC the partly and fully regenerated films prepared using ex-situ method are interacted with cellulase enzymes from Trichoderma viride using QCM-D technique. The changes in mass and energy dissipation due to the interaction of the enzymes with the substrates are correlated with the surface wettability and elemental composition of the regenerated films. The enzymatic degradation rate correlated well to the rate of regeneration. It is demonstrated that capillary zone electrophoresis (CZE) can be used to support QCM-D data via the detection of enzyme hydrolysis products in the eluates of the QCM-D cells. It is also shown that a combination of QCM-D together with enzymatic digestion is a reliable method to monitor the time dependent regeneration of TMSC to pure cellulose. Furthermore, the effect of heat treatment on partly and fully regenerated cellulose films prepared using ex-situ method is investigated by exposing the films to elevated temperature (105 °C) for a prolonged time (6 hours). Upon heating, a structural rearrangement in the films from a featureless to a fibrillar-like structure is observed as evidenced by AFM. Several analytical methods, namely GIXRD, ATR-IR, Sarfus, XPS and SCA data are employed to analyze the alteration in the structure, surface composition, film thickness, wettability and SFE of the time dependent regenerated films before and after heat treatment. Results from solvent (D2O/H2O) exchange studies proved that water content and water uptake capacity of heat treated cellulose films are significantly reduced compared to non heated films.Additionally, a new method is developed to structure nanometric cellulose films via vapor phase acid hydrolysis and enzymat
Keywords: Cellulose model films, carboxymethyl cellulose, trimethylsilyl cellulose, microarrays, quartz crystal microbalance, carbodiimide coupling, aminofluorescein, DNA
Published: 28.11.2012; Views: 1959; Downloads: 131
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Tyrosinase-catalysed coating of wool fibres with different protein-based biomaterials
Suzana Jus, Vanja Kokol, Georg M. Gübitz, 2009, original scientific article

Abstract: The potential of tyrosinases to activate tyrosine residues of wool protein fibres for cross-linking with different materials like collagen, elastin and gelatine was assessed. Natural fibres like wool offer an excellent environment for the growth of micro-organisms when the conditions like moisture, oxygen and temperature are appropriate. Coating with collagen, a very useful biomaterial with bactericidal and fungicidal properties, could be used to improve the properties of wool-based materials, especially when applied in hygienically sensitive applications like in hospitals. Tyrosinases were shown to catalyse the oxidation of tyrosine residues in wool and wool hydrolysates as model substrates, as determined by UV-Vis spectroscopy. Structural differences of the surface were evident from the increase of the intensity in the NH bending and stretching regions in the spectra of NIR FT Raman analysis of the enzyme treated and grafted wool fibres. The durability of the coating was also shown by using FITC-labelled collagen that was bound to the wool fibres, even after severe washing. Additionally, antimicrobial properties were successfully imparted due to the collagen grafted on the wool fibres. The functional and mechanical properties of the treated wool fibres showed no significant changes.
Keywords: textile fibres, wool, protein substrates, tyrosinase, enzymatic coupling, antimicrobial properties, coating
Published: 01.06.2012; Views: 1215; Downloads: 73
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PolyHIPE supports in batch and flow-through Suzuki cross-coupling reactions
Jane F. Brown, Peter Krajnc, Neil R. Cameron, 2005, original scientific article

Abstract: As part of ongoing research efforts to discover alternative support materials to polymer beads for use in polymer-supported synthesis, particularly under flow-through conditions, this work involves the synthesis of PolyHIPE (High Internal Phase Emulsion) polymer monoliths. PolyHIPEs containing high loadings of chloromethyl groups were efficiently prepared by the direct copolymerization of 4-vinylbenzyl chloride and divinylbenzene monomers. The 'Merrifield' PolyHIPE proved to be an excellent support for batch and flow-through Suzuki cross-coupling reactions. A remarkably high yield of pure biaryl product was obtained using the PolyHIPE support in cubic form and utilizing an electron-rich boronic acid. In comparison to polymer beads, this material was found to be a much more efficient support in both batch and continuous flow modes. PolyHIPE converted a greater amount of chloromethyl groups into biaryl product under identical reaction conditions. It is suggested that the absence of channelling with PolyHIPE monoliths gives better performance under flow-through conditions than permanently porous beads.
Keywords: polymer supports, polymer monoliths, emulsion polymerisation, solid-phase synthesis, cross-coupling reactions
Published: 01.06.2012; Views: 1548; Downloads: 69
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