Carbon molecular sieves for air separation from Nomex aramid fibers

Activated carbon fibers prepared from aramid fibers have proved to possess outstanding homogeneity in pore size, most of all when Nomex aramid fiber is used as precursor. Taking advantage of this feature, microporous carbon molecular sieves for air separation have been prepared through carbon vapor deposition of benzene on Nomex-derived carbon fibers activated to two different burnoff degrees. Carbon molecular sieves with good selectivity for this separation and showing acceptable adsorption capacities were obtained from ACFs activated to the two burnoff degrees chosen.     

Synthesis,solid-state crystal structure,and reactivity of a monomeric copper(I) anilido complex

Synthesis and isolation of the Cu(I) amido complex (dtbpe)Cu(NHPh) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) is accomplished upon reaction of [(dtbpe)Cu(mu-Cl)](2) with LiNHPh. The anilido complex has been fully characterized by IR spectroscopy and multinuclear NMR spectroscopy as well as by single-crystal X-ray diffraction study. Salient features of the solid-state structure include an amido orientation that allows pi-interaction of the nitrogen-based lone pair with both the empty copper p-orbital and the pi-system of the phenyl substituent. A solid-state X-ray diffraction study of [(dtbpe)Cu(NH(2)Ph)][BF(4)] has allowed a direct comparison of the structural features upon conversion of the amine ligand to an amido. The reactivity of the amido ligand of (dtbpe)Cu(NHPh) is consistent with nucleophilic character. For example, the formation of Ph(3)CNHPh is observed upon treatment with [Ph(3)C][BF(4)], and reaction at room temperature with EtX (X = Br or I) yields N-ethylaniline. The reactivity of (dtbpe)Cu(NHPh) is compared to that of the octahedral and d(6) complex TpRu(PMe(3))(2)(NHPh) (Tp = hydridotris(pyrazolyl)borate).     

Self-recognition and hydrogen bonding by polycyclic bridgehead monoalcohols

Our interest in the relationship between the hydrogen bonding motifs displayed by monoalcohols and the properties of the solids which contain these motifs has led us to determine the crystal structures of three polycyclic bridgehead monoalcohols. One C10H16O isomer crystallises in the space group P2(1)2(1)2(1) but the three molecules which comprise the asymmetric unit are related approximately by the operations of a 3(1) screw axis. They are linked by hydrogen bonds to form an infinite helix. A second C10H16O isomer forms rings containing four molecules joined by cooperative hydrogen bonds. The chiral space group P4(1)2(1)2 accommodates molecules of the S,S and R,R enantiomers in the molar ratio 92:8 (ee 84%) owing to disorder. A related C9H14O2 keto-alcohol forms infinite chains by C-OH...O = C hydrogen bonding. These hydrogen bond motifs are shown to be typical for 45 tertiary monoalcohols, CmHnOH, present in the Cambridge Structural Database. Tertiary monoalcohols display in a more pronounced form the strong preferences for trigonal and tetragonal space groups and for asymmetric units containing several molecules which are established features of the crystallochemistry of monoalcohols.     

Chiral recognition in aqueous solutions: On the role of urea in hydrophilic and hydrophobic interactions of unsubstituted α-amino acids

Calorimetric and nuclear relaxation time measurements were carried out at 25°C on concentrated aqueous solutions of urea containing the L and D forms of the following -aminoacids: alanine, -aminobutyric acid, norvaline, and norleucine. Glycine was also studied under the same experimental conditions. The enthalpic interaction coefficients were rationalized according to the preferential configuration model. The trends of the homochiral coefficients at increasing concentration of urea underline an interaction mechanism not different from that occurring in water. Chiral recognition is the quantity more affected by the nature of the solvent. Among the amino acids studied, only norleucine presents this effect, which vanishes at the highest concentrations of urea because the attenuated electrostatic interactions cannot impose preferential configurations.     

The use of microcalorimetry to assess the size exclusion properties of carbon molecular sieves

Carbon molecular sieves (CMSs) for separating gaseous mixtures have been prepared through chemical vapour deposition (CVD) of benzene on activated carbon fibres (ACFs) obtained from Nomex aramid fibres. The effect of the CVD treatment on the porous texture of the ACFs has been followed by immersion calorimetry into liquids of different molecular sizes (dichloromethane, benzene and cyclohexane). Both the kinetics of the immersion process and the equilibrium immersion enthalpies have been studied. Besides, the derived specific surface areas accessible to the different liquids employed have been calculated. The results show that the carbon deposition has taken place mainly at the pore entrances and the CVD treatment has succeeded to introduce selectivity without a significant loss of capacity.     

A concise approach to structurally diverse beta-amino acids

We have demonstrated that the high yields and selectivities of 1,3-dipolar cycloadditions can be translated into facile stereoselective syntheses of a diverse array of beta-amino acids, key components of bioactive natural products, beta-lactams, and peptidomimetics. Simply by selecting different combinations of three readily available starting materials (an oxime, a chiral allylic alcohol, and a nucleophile), we used the reaction sequence to prepare four different beta-amino acid structural types with a variety of substitution patterns in good overall yield. Of particular note is the use of this approach to prepare highly substituted beta-amino acids not readily accessible by previously reported methodologies. This will pave the way for future studies of the structure and function of this important class of molecules.     

Succinct synthesis of beta-amino acids via chiral isoxazolines

beta-Amino acids are important synthetic targets due to their presence in a wide variety of natural products, pharmaceutical agents, and mimics of protein structural motifs. While beta-amino acids containing geminal substitution patterns have enormous potential for application in these contexts, synthetic challenges to the stereoselective preparation of this class of compound have thus far limited more complete studies. We present here a straightforward method employing chiral isoxazolines as key intermediates to access five different beta-amino acid structural types with excellent selectivity. Of particular note is the use of this approach to prepare highly substituted cis-beta-proline analogues. The ready access to these diversely substituted compounds is expected to facilitate future studies of the structure and function of this important class of molecules.     

Extraction and characterization of cellulose nanowhiskers from <Emphasis Type="Italic">Mandacaru</Emphasis> (<Emphasis Type="Italic">Cereus jamacaru</Emphasis> DC.) spines

Cellulose nanowhiskers were extracted from the spines of Mandacaru (Cereus jamacaru DC.), a cactus native to the Caatinga biome of northeastern Brazil, using sulfuric acid hydrolysis preceded by alkaline treatment and bleaching. Results showed that three bleaching steps were required to remove most of the non-cellulosic constituents of the spines that yielded 77.4% cellulose. Nanowhiskers size decreased from about 400 to 260 nm when extraction time varied from 60 to 120 min, this was also evidenced by X-ray diffraction. Alkaline treated and bleached samples had lower thermal stability as compared to untreated spines due to removal of lignin and increased surface area. The amount of time samples were treated with sulfuric acid influenced the thermal stability and consequently the degree of crystallinity of the nanowhiskers. Cellulose nanowhiskers were obtained from Mandacaru spines providing a new renewable source of reinforcement with potential applications in nanocomposites.     

ANCUT2, a Thermo-alkaline Cutinase from <Emphasis Type="Italic">Aspergillus nidulans</Emphasis> and Its Potential Applications

Biochemical characterization of purified ANCUT2 cutinase from Aspergillus nidulans is described. The identified amino acid sequence differs from that predicted in Aspergillus genomic databases in amino acids not relevant for catalysis. The enzyme is thermo-alkaline, showing its maximum activity at pH 9 and 60 °C, and it retains more than 60% of its initial activity after incubation for 1 h at 60 °C for pH values between 6 and 10. ANCUT2 is more active towards long-chain esters and it hydrolyzes cutin; however, it also hydrolyzes short-chain esters. Cutinase is inhibited by metal ions, PMSF, SDS, and EDTA (10 mM). It retains 50% of its activity in most of the solvents tested, although it is more stable in hydrophobic solvents. According to its found biochemical properties, preliminary assays demonstrate its ability to synthesize methyl esters from sesame oil and the most likely application of this enzyme remains in detergent formulations.