Pretreatment of sugarcane bagasse hemicellulose hydrolysate for xylitol production byCandida guilliermondii

Applied Biochemistry and Biotechnology - In order to remove or reduce the concentrations of toxic substances present in the sugarcane bagasse hemicellulose hydrolysate for xyloseto-xylitol...     

Theoretical study of ribonucleotide reductase mechanism-based inhibition by 2'-azido-2'-deoxyribonucleoside 5'-diphosphates

2'-azido-2'-deoxyribonucleoside 5'-diphosphates are mechanism-based inhibitors of Ribonucleotide Reductase. Considerable effort has been made to elucidate their mechanism of inhibition, which is still controversial and not fully understood. Previous studies have detected the formation of a radical intermediate when the inhibitors interact with the enzyme, and several authors have proposed possible structures for this radical. We have conducted a theoretical study of the possible reactions involved, which allowed us to identify the structure of the new radical among the several proposals. A new reactional path is also proposed that is the most kinetically favored to yield this radical and ultimately inactivate the enzyme. The energetic involved in this mechanism, both for radical formation and radical decay, as well as the calculated Hyperfine Coupling Constants for the radical intermediate, are in agreement with the correspondent experimental values. This mechanistic alternative is fully coherent with remaining experimental data.     

Chemical, structural, and thermal properties of Zn(II)-Cr(III) layered double hydroxides intercalated with sulfated and sulfonated surfactants

Zn(II)-Cr(III)-LDHs (layered double hydroxides) containing sulfated or sulfonated surfactants as the interlamellar anion were synthesized by the coprecipitation method. The syntheses were conducted under various different experimental conditions, such as the Zn : Cr ratio, pH, and aging time. In each of the prepared materials, unlike previously reported data, the interlayer anion arrangement did not change, being consistent with a perpendicular monolayer. The thermal decomposition process of the prepared materials was studied by a set of analysis methods, such as TG/DTA, TG/MS, PXRD, and FT-IR. From the results obtained it was possible to conclude that, in an air atmosphere, the anions decomposed by a partial combustion, leading to the formation of sulfide. The results also showed that sulfonated surfactants containing LDHs are much more stable than those containing sulfated surfactants. A mechanism was proposed for the thermal decomposition of such LDHs based on the experimental results.     

Water adsorption in disordered mesoporous silica (Vycor) at 300 K and 650 K: a Grand Canonical Monte Carlo simulation study of hysteresis

This numerical simulation paper focuses on the adsorption/desorption of water in disordered mesoporous silica glasses (Vycor-like). The numerical adsorbent was previously obtained by off lattice method, and was shown to reproduce quite well the micro- and mesotextural properties of real Vycor, as well as morphological (pore size distribution) and topological (pore interconnections) disorder. The water-water interactions are described by the SPC model while water-silica interactions are calculated in the framework of the PN-TrAZ model. The water adsorption/desorption isotherms and the configurational energies are calculated by the Grand Canonical Monte Carlo simulation method. The low pressure results compare well with experiments, showing the good transferability of the intermolecular potential. It is shown that if the hysteresis loop observed in the adsorption/desorption isotherm is considered as a true phase transition (which is actually still an open question in the case of disordered porous materials), then it is possible to calculate the grand potential by applying the thermodynamic integration scheme. The grand potential is shown to be multivalued for low (subcritical) temperature, and continuous for high (supercritical) temperature. A coexistence point is found within the hysteresis loop, actually close to the vertical desorption line. Below the equilibrium chemical potential, the gaslike branch is stable whereas the liquidlike branch is metastable. The situation is reversed above the coexistence point.     

Thermal decomposition kinetics of titanium hydride and Al alloy melt foaming process

Metal foams, now one of research foci, are a newclass of materials with low densities and novel physi-cal, mechanical, thermal, electrical and acoustic prop-erties[1—8]. Demands from high-tech make Al alloyfoam, which has much higher specific strength than ofpure Al foam, the new development focus[9—11]. Melt foaming process is one of the approaches tofabricate Al foam and Al alloy foam and their porestructure (pore diameter and porosity) has close rela-tionship with the thermal decompo…     

S1P1-selective in vivo-active agonists from high-throughput screening: off-the-shelf chemical probes of receptor interactions, signaling, and fate

The essential role of the sphingosine 1-phosphate (S1P) receptor S1P(1) in regulating lymphocyte trafficking was demonstrated with the S1P(1)-selective nanomolar agonist, SEW2871. Despite its lack of charged headgroup, the tetraaromatic compound SEW2871 binds and activates S1P(1) through a combination of hydrophobic and ion-dipole interactions. Both S1P and SEW2871 activated ERK, Akt, and Rac signaling pathways and induced S1P(1) internalization and recycling, unlike FTY720-phosphate, which induces receptor degradation. Agonism with receptor recycling is sufficient for alteration of lymphocyte trafficking by S1P and SEW2871. S1P(1) modeling and mutagenesis studies revealed that residues binding the S1P headgroup are required for kinase activation by both S1P and SEW2871. Therefore, SEW2871 recapitulates the action of S1P in all the signaling pathways examined and overlaps in interactions with key headgroup binding receptor residues, presumably replacing salt-bridge interactions with ion-dipole interactions.     

Formation of adenine-N3/guanine-N7 cross-link in the reaction of trans-oriented platinum substrates with dinucleotides

The reactions of the anticancer complex trans-[PtCl2{(E)-HN=C(OMe)Me}2] (trans-EE) with a series of ribo and deoxyribodinucleotides have been studied by HPLC and 2D [1H, 15N] HMQC NMR spectroscopy and compared with those of the inactive trans isomer of cisplatin, trans-[PtCl2(NH3)2] (trans-DDP). Reactions of trans-EE with r(ApG) and d(ApG) take place through solvolysis of the starting substrate and subsequent formation of trans G-N7/monochloro and G-N7/monoaqua adducts. Slowly, the monofunctional adducts evolve to a bifunctional adduct forming an unprecedented and unexpected A-N3/G-N7 platinum cross-link spanning two trans positions. For stereochemical reasons, trans platinum complexes cannot form N7/N7 cross-links between adjacent purines in di- or polynucleotides. For the reverse sequence r(GpA), no chelate structure was formed even after a two-week reaction. The reaction of trans-DDP with r(ApG) produces many more products than the analogous reaction with trans-EE. One of these products was identified as the A-N3/G-N7 trans-chelate.     

Solid-phase synthesis of amidines by the reduction of amidoximes

Amidines can be prepared on a solid support by reducing polymer-bound amidoximes with SnCl₂·2H₂O. The method has proved to be straightforward and highly efficient. Amidoximes attached to the solid support are readily available by treating resin-bound nitriles with hydroxylamine.     

X-ray diffraction and solid-state NMR studies of a germanium binuclear complex

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).     

A new,simple, and selective palladium-catalyzed cleavage of triethylsilyl ethers

[reaction: see text] A simple procedure for the cleavage of triethylsilyl (TES) ethers in the presence of 10 wt % Pd/C in methanol or 95% ethanol is reported. This method allows selective removal of alkyl TES ethers in the presence of aromatic TES ethers or tert-butyldimethylsilyl (TBS) protecting groups.