1,721,022 research outputs found
Sugar-based ionic liquids: Multifaceted challenges and intriguing potential
Full text linkCarbohydrates represent a promising option in transitioning from oil-based chemical resources to renewable ones, with the goal of developing chemistries for a sustainable future. Cellulose, hemicellulose, and largely available monosaccharides already provide useful chemical building blocks, so-called platform chemicals, such as levulinic acid and hydroxymethyl furfural, as well as solvents like cyrene or gamma-valerolactone. Therefore, there is great anticipation for novel applications involving materials and chemicals derived from sugars. In the field of ionic liquids (ILs), sugar-based ILs have been overlooked for a long time, mainly on account of their multistep demanding preparation. However, exploring new strategies for accessing sugar-based ILs, their study, and their exploitation, are attracting increasing interest. This is due to the growing concerns about the negative (eco)toxicity profile of most ILs in conjunction with their non-sustainable nature. In the present review, a literature survey concerning the development of sugar-based ILs since 2011 is presented. Their preparation strategies and thermal behavior analyses, sorted by sugar type, make up the first two sections with the intention to provide the reader with a useful guide. A final overview of the potential applications of sugar-based ILs and their future perspectives complement the present analysis
History repeats itself again: Will the mistakes of the past for ILs be repeated for DESs? From being considered ionic liquids to becoming their alternative: the unbalanced turn of deep eutectic solvents
No full textThe establishment of the place of Deep Eutectic Solvents (DESs) as a new class of green solvents was essentially grounded on naïve comparisons with Ionic Liquids (ILs), since both are composed of charged compounds. The easiness of DESs’ preparation afforded the quick preparation and utilization of a massive number of solvents and their use in wide variety of applications with minimal fundamental knowledge of their thermophysical properties and phase equilibria studies. As time went on, the need to define DESs from the thermodynamic point of view and to differentiate them from other classes of solvents was imperative. This perspective review aims at dispelling some myths about DESs through the use of experimental data and computational chemical calculations and establishing fair comparisons with other classes of solvents, ILs, eutectic solvents (ESs) and volatile organic compounds (VOCs), so that clear and sound conclusions can be drawn. Several important parameters typically used to characterize solvents and that have been much used to justify DESs’ wide range of applications, such as vapor pressure, thermal stability, polarity, toxicity and water miscibility, were accessed for these different solvents and comparisons were established. Moreover, a comparative analysis in a selected research area, biopolymer dissolution and treatment, was chosen to illustrate the unique potential of ILs and DESs and the challenges that still need to be addressed. Literature available for the diverse polysaccharides selected (cellulose, hemicellulose and chitin) and lignin highlighted pros and cons and the different level of knowledge gained to date for both ILs and DESs. This part is complemented by recycling and techno-economic considerations for the two classes of solvents, which are additional key aspects to consider for the development of an effective integrated biorefinery process. The conclusion is obvious: DESs are a new class of solvents, with distinct properties from other classes of solvents which are essentially dependent on the properties of their constituent compounds. Therefore, when starting compounds are wisely selected, DESs become an additional and promising pathway in the pursuit of environmentally friendly solvents to replace traditional VOCs for a given application. However, some fundamental studies are still needed to fully understand these systems and define their most effective areas of application
Preparation of 1,6-di-deoxy-D-galacto and 1,6-di-deoxy-L-altro nojirimycin derivatives by aminocyclization of a 1,5-dicarbonyl derivative
Full text linkIminosugars are known glycosidase inhibitors which are the subject of drug development efforts against several diseases. The access to structurally-related families of iminosugars is of primary importance for running structure-activity relationship studies. In this work, the double reductive amination (aminocyclization) reaction of a dicarbonyl derivative of the L-arabino series, in turn obtained from lactose, is reported. Different ratios of 1,6-di-deoxy-D-galacto and 1,6-di-deoxy-L-altro nojirimycin derivatives were obtained depending on the amine employed in this transformation which provided an insight into the effects of their structure on the outcome of the reaction. Of particular interest were the results obtained when two enantiomeric amino acids (D-Phe-OMe and L-Phe-OMe) were used, which resulted in the inversion of the reaction stereoselectivity
Thermal behavior analysis as a valuable tool for comparing ionic liquids of different classes
No full textIonic liquids (ILs) are neoteric solvents characterized by unique physicochemical properties and considerable structural variability. The thermal behavior of 24 ILs, belonging to two structurally related families with either bromide or Tf2N as counteranion, was analyzed. For bromide mono- and dicationic ionic liquids (DILs), thermal gravimetric analysis showed similar decomposition events, with only small gain in stability for a few members of the latter class. Conversely, all Tf2N DILs displayed higher stabilities (up to 34 K) than the corresponding monocations, thus highlighting the different role played by the two counteranions. Mono- and dicationic ILs bearing a reactive group on the imidazolium substituent resulted instead the least stable ILs studied. Differential scanning calorimetry analyses of most of the (D)ILs only showed glass transition temperatures, a behavior in agreement with the broad liquid range of ILs. The impact of the cationic structure and/or of the type of anion on the above-mentioned transition temperatures has been studied. The apparent activation energy (Ea) and the fragility index (m) for some (D)ILs have also been obtained. Finally, a few bromide (D)ILs presented peculiar thermal events
Radical-mediated thiol-ene ‘click’ reactions in deep eutectic solvents for bioconjugation
Full text linkHerein, we report the first application of deep eutectic solvents (DESs) in radical-mediated hydrothiolation reactions. Under UV and atmospheric-oxygen mediated conditions, the thiol-ene reaction was applied to amino acid and peptide ligation in DESs. The conditions facilitate highly-efficient synthesis of biomolecular targets using a ‘green’ methodology, with complete recycling of the reaction medium. Fluorescent labelling and glycosylation of a minimal sequence mimetic of angiotensin-converting enzyme 2 capable of binding the SARS-CoV-2 spike protein is demonstrated for applications in the study of inhibition of COVID-19 infectivity
Synthesis of fucose derivatives with thiol motifs towards suicide inhibition of helicobacter pylori
Full text linkThe syntheses of six thiol-exhibiting monosaccharides towards suicide inhibition of Helicobacter pylori are reported. Blood group Antigen Binding Adhesin (BabA), a bacterial membrane-bound lectin, binds to human ABO and Lewis b blood group structures displayed on the surface of host epithelial cells. Crystal structures of the carbohydrate-recognition domain revealed a conserved disulfide bonded loop that anchors a critical fucose residue in these blood group structures. Disruption of this loop by N-acetylcysteine results in reduced BabA-mediated adherence to human gastric tissue sections and attenuated virulence in Lewis b-expressing transgenic mice. With a view of creating specific inhibitors of the lectin, we designed and successfully synthesised six fucose-derived compounds with thiol motifs to engage in a thiol-disulfide exchange with this disulfide bond of BabA and form a glycan-lectin disulfide linkage. Branching and extending the fucose backbone with 2- and 3-carbon thiol motifs delivered a range of candidates to be tested for biological activity against BabA
Betaine and L-carnitine ester bromides: Synthesis and comparative study of their thermal behaviour and surface activity
No full textSix esters of both betaine and L-carnitine bromides, featuring alkyl groups ranging from C8 to C18 in length, have been synthesized. The thermal behaviour of these twelve bio-based salts has been analyzed and compared by thermal gravimetric analysis and differential scanning calorimetry. The L-carnitine alkyl ester bromides melted below 100 °C and can hence be considered ionic liquids (ILs) with full rights. Conversely, the betaine alkyl ester bromides, with the exception of the shortest member of the series, melted slightly above this upper limit. Also, they resulted less thermally stable when compared to their L-carnitine analogues. Moreover, the self-aggregation behavior of these structurally related betaine and L-carnitine alkyl ester bromides, inspected through surface tension, conductivity measurements and isothermal titration calorimetry, varied significantly. The critical micelle concentration (CMC) values for the L-carnitine series were lower than those of the betaine series, and the former ILs were also capable of adsorbing more efficiently at the air/water interface as well as of reducing surface tension. A potential use of these organic salts as components of deep eutectic solvents is envisaged, thus showcasing the applicability of this natural trimethylammonium cation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
