Synthesis under Pressure of Potential Precursors of CN x Materials Based on Melamine and Phenolic Resins

The cross-linking under pressure of a mixture of novolak and melamine resins with a weight ratio of 1 has been investigated by means of several techniques, including DSC, TGA, XRD, and IR. Four samples were prepared under varying pressure and temperature. The application of pressure to the reaction vessel favored the formation of a more structured cross-linked resin. The sample cross-linked at both the highest temperature and pressure levels exhibited higher structuring effects than the others. Moreover, this sample exhibited much higher thermal stability than the others. This achievement may allow us to open the pathway toward new precursors of structured CN _x materials.     

Sparse representation of vectors in lattices and semigroups

Mathematical Programming - We study the sparsity of the solutions to systems of linear Diophantine equations with and without non-negativity constraints. The sparsity of a solution vector is the...     

Oxidation of 1,4-disubstituted-1,2,3-triazoles with H2O2–CF3CO2H: efficient synthesis of 1,2,3-triazole 3-oxides

A collection of 1,2,3-triazole-3-oxides was obtained from oxidation of the corresponding 1,4-disubstituted-1,2,3-triazoles mediated by a H₂O₂–CF₃CO₂H system through a simple protocol in good yields showing high efficiency.     

Detailed Investigation of the Immunodominant Role of O‐Antigen Stoichiometric O‐Acetylation as Revealed by Chemical Synthesis,Immunochemistry, Solution Conformation and STD‐NMR Spectroscopy for Shigella flexneri 3a

Shigella flexneri 3a causes bacillary dysentery. Its O‐antigen has the {2)‐[α‐d ‐Glcp‐(1→3)]‐α‐l ‐Rhap‐(1→2)‐α‐l ‐Rhap‐(1→3)‐[Ac→2]‐α‐l ‐Rhap‐(1→3)‐[Ac→6]_{≈40 %}‐β‐d ‐GlcpNAc‐(1→} ([(E)AB_AcC_AcD]) repeating unit, and the non‐O‐acetylated equivalent defines S. flexneri X. Propyl hepta‐, octa‐, and decasaccharides sharing the (E′)A′B_AcCD(E)A sequence, and their non‐O‐acetylated analogues were synthesized from a fully protected B_AcCD(E)A allyl glycoside. The stepwise introduction of orthogonally protected mono‐ and disaccharide imidate donors was followed by a two‐step deprotection process. Monoclonal antibody binding to twenty‐six S. flexneri types 3a and X di‐ to decasaccharides was studied by an inhibition enzyme‐linked immunosorbent assay (ELISA) and STD‐NMR spectroscopy. Epitope mapping revealed that the 2_C‐acetate dominated the recognition by monoclonal IgG and IgM antibodies and that the B_AcCD segment was essential for binding. The glucosyl side chain contributed to a lesser extent, albeit increasingly with the chain length. Moreover, tr‐NOESY analysis also showed interaction but did not reveal any meaningful conformational change upon antibody binding.     

Effect of the Substituents of the Neighboring Ring in the Conformational Equilibrium of Iduronate in Heparin‐like Trisaccharides

Based on the structure of the regular heparin, we have prepared a smart library of heparin‐like trisaccharides by incorporating some sulfate groups in the sequence α‐D ‐GlcNS‐ (1‐4)‐α‐L ‐Ido2S‐(1‐4)‐α‐D ‐GlcN. According to the 3D structure of heparin, which features one helix turn every four residues, this fragment corresponds to the minimum binding motif. We have performed a complete NMR study and found that the trisaccharides have a similar 3D structure to regular heparin itself, but their spectral properties are such that allow to extract very detailed information about distances and coupling constants as they are isotropic molecules. The characteristic conformational equilibrium of the central iduronate ring has been analyzed combining NMR and molecular dynamics and the populations of the conformers of the central iduronate ring have been calculated. We have found that in those compounds lacking the sulfate group at position 6 of the reducing end glucosamine, the population of ²S₀ of the central iduronate residue is sensitive to the temperature decreasing to 19 % at 278 K. On the contrary, the trisaccharides with 6‐O‐sulfate in the reducing end glucosamine keep the level of population constant with temperature circa 40 % of ²S₀ similar to that observed at room temperature. Another structural feature that has been revealed through this analysis is the larger flexibility of the L ‐IdoAS‐ D ‐GlcN glycosidic linkage, compared with the D ‐GlcNS‐L ‐IdoA. We propose that this is the point where the heparin chain is bended to form structures far from the regular helix known as kink that have been proposed to play an important role in the specificity of the heparin–protein interaction.     

In Vitro Production of Plant Peroxidases—A Review

In the latest two decades, the interest received by plant enzymes has increased significantly. Plant enzymes such as peroxidases are widely used in medicine as diagnostic tools and in the bioremediation and biobleaching industries, among others. Traditionally, these enzymes have been obtained from a natural source, a process that is sometimes laborious and affected by weather conditions and low yields. To overcome this hurdle, some efforts have been made to establish plant cell cultures in vitro to use the system as a continuous source of plant enzymes. The focus of this review will be the production of plant peroxidases in vitro, including novel approaches such as the use of bioreactors and genetically transformed tissues to enhance the yield of desired enzymes.     

Supramolecular polymeric hydrogels

The supramolecular crosslinking of polymer chains in water by specific, directional and dynamic non-covalent interactions has led to the development of novel supramolecular polymeric hydrogels. These aqueous polymeric networks constitute an interesting class of soft materials exhibiting attractive properties such as stimuli-responsiveness and self-healing arising from their dynamic behaviour and that are crucial for a wide variety of emerging applications. We present here a critical review summarising the formation of dynamic polymeric networks through specific non-covalent interactions, with a particular emphasis on those systems based on host-guest complex formation, as well as the characterisation of their physical characteristics. Aqueous supramolecular chemistry has unlocked a versatile toolbox for the design and fine-tuning of the material properties of these hydrogels (264 references).     

α-Hydroxy ketones as useful templates in asymmetric reactions

Approaching the Nature's efficiency in controlling both reactivity and stereoselectivity of organic reactions by means of a catalyst species remains a formidable challenge for chemists to face. Despite impressive advances in the design of novel catalysts and activation modes, current catalytic and asymmetric methodologies rarely meet desirable standards of robustness, substrate scope, and selectivity altogether. One trick to improve catalyst behaviour is to identify adequate substrate template-catalyst combinations so that optimum performance of the reaction system could be achieved. During the last couple of years α-hydroxy ketones, and most particularly α'-hydroxy enones, have emerged as useful templates with applications in a number of metal-catalyzed as well as organocatalyzed C-C and C-X bond-forming stereoselective reactions. The first review of these accomplishments is presented here along with a brief historical introduction.