A TEM study of Ni ordering in the Ni6Se5−xTex, 0<x<∼1.7, system

The Ni₆Se_5−xTe_x, 0<x<∼1.7, system has been carefully investigated via electron diffraction and TEM imaging. They reveal a somewhat disordered modulated superstructure phase arising from Ni ion ordering within an essentially well-defined chalcogen sub-structure. As x, and the underlying parent substructure cell dimensions increase, the incommensurate primary modulation wave-vector q characteristic of this Ni ion ordering quickly swings from close to for x=0 towards for x?0.5. A lock-in to would formally transform the underlying parent Bmmb (a_p, b_p, c_p) structure into a P1a1 (a_s=2a_p, b_s=b_p, c_s=a_p+c_p) superstructure phase.     

On the microstructure and symmetry of apparently hexagonal BaAl2O4

The P6₃ (a=2a_p, b=2b_p, c=c_p) crystal structure reported for BaAl₂O₄ at room temperature has been carefully re-investigated by a combined transmission electron microscopy and neutron powder diffraction study. It is shown that the poor fit of this P6₃ (a=2a_p, b=2b_p, c=c_p) structure model for BaAl₂O₄ to neutron powder diffraction data is primarily due to the failure to take into account coherent scattering between different domains related by enantiomorphic twinning of the P6₃22 parent sub-structure. Fast Fourier transformation of [0 0 1] lattice images from small localized real space regions (∼10 nm in diameter) are used to show that the P6₃ (a=2a_p, b=2b_p, c=c_p) crystal structure reported for BaAl₂O₄ is not correct on the local scale. The correct local symmetry of the very small nano-domains is most likely orthorhombic or monoclinic.     

Glycosynthase-based synthesis of xylo-oligosaccharides using an engineered retaining xylanase from Cellulomonas fimi

Glycosynthases are synthetic enzymes derived from retaining glycosidases in which the catalytic nucleophile has been replaced. The mutation allows irreversible glycosylation of sugar acceptors using glycosyl fluoride donors to afford oligosaccharides without any enzymatic hydrolysis. Glycosynthase technology has proven fruitful for the facile synthesis of useful oligosaccharides, therefore the expansion of the glycosynthase repertoire is of the utmost importance. Herein, we describe for the first time a glycosynthase, derived from a retaining xylanase, that synthesizes a range of xylo-oligosaccharides. The catalytic domain of the retaining endo-1,4-beta-xylanase from Cellulomonas fimi (CFXcd) was successfully converted to the corresponding glycosynthase by mutation of the catalytic nucleophile to a glycine residue. The mutant enzyme (CFXcd-E235G) was found to catalyze the transfer of a xylobiosyl moiety from alpha-xylobiosyl fluoride to either p-nitrophenyl beta-xylobioside or benzylthio beta-xylobioside to afford oligosaccharides ranging in length from tetra- to dodecasaccharides. These products were purified by high performance liquid chromatography in greater than 60% combined yield. 1H and 13C NMR spectroscopic analyses of the isolated p-nitrophenyl xylotetraoside and p-nitrophenyl xylohexaoside revealed that CFXcd-E235G catalyzes both the regio- and stereo-selective synthesis of xylo-oligosaccharides containing, exclusively, beta-(1 --> 4) linkages.     

Polymerization of Methane and Olefin Mixtures over Fluosulfonic Acid and Antimony Pentafluoride

Mixtures of methane and olefins (ethylene, propylene, butenes, butadiene, and styrene) have been polymerized over HSO₃ F-SbF₃ to yield an oily oligomer with a molecular weight ranging from 100 to 700. The NMR spectra of each polymer showed a sharp peak at or near 1.25 &, suggesting the presence of block methylene in the polymer. The formation of block methylene is surprising considering the fact that the polymerization reaction is carbonium ion in nature. A primary cation has been invoked to explain the results. The formation of this primary cation must involve some extraordinary stabilization by some component in the acid.     

Graft Copolymerizations of Polysaccharides. II. Acrylamide Grafting to Yellow Dextrin

Graft copolymers of acrylamide and yellow dextrin were prepared using cerium(IV) as initiator. The yellow dextrin had a very broad molecular weight distribution but was fractionated utilizing dialysis and ultrafiltration membranes. Initiator efficiencies were determined using size exclusion chromatography and were found to be between 2.4 and 34%. Initiator efficiency increased with acrylamide concentration at constant cerium (IV) and yellow dextrin concentrations, and decreased with increasing cerium(IV) concentration at constant acrylamide and yellow dextrin concentrations. Plots of acrylamide conversion and intrinsic viscosity vs initial acrylamide concentration at constant yellow dextrin and ceric ion concentrations showed a maximum at about 2.0 M.     

Synthesis and Evaluation as Irreversible Glycosidase Inhibitors of Mono- and Oligo(glycosylthio)benzoquinones

The mono(glucosylthio)hydroquinone 2 was prepared by S-glycosidation of 2-mercaptobenzene-1,4-diol and by addition of the acetylated 1-thioglucose 3 to benzo-1,4-quinone (Scheme 1). The second, higher yielding procedure was adopted for the preparation of a range of (glucosylthio)hydroquinones. Addition of 3 to 2-chlorobenzo-1,4-quinone, followed by oxidation gave the 1-thioglucosides 7 and 12 (1.3:1), while addition of HCl to the (glucosylthio)quinone 4 and oxidation gave mainly 12 (Scheme 1). Similarly, the bis(glucosylthio)hydroquinone 33 was obtained from 3 and 4 (Scheme 4), and the (cellobiosylthio)hydroquinone 18 from the thiol 16 and benzo-1,4-quinone (Scheme 2). Addition of the 4-thioglucoside 21 to benzo-1,4-quinone (→ 22 ) and to 4 was followed by oxidation to yield the mono(glucosylthio)quinone 23 and the disubstituted quinones 24 and 25 , respectively (Scheme 3). A mixture 24 / 25 was also obtained from the addition of 3 to 23 . The tris(glucosylthio)hydroquinone 36 was obtained by addition/elimination to the dichloroquinone 29 or the dimesylate 31 , which was prepared in a simplified way (Scheme 4). The tetrakis(glucosylthio)hydroquinone 37 was obtained from 3 and chloranil, followed by reduction. The acylated hydroquinones were deprotected (→ 5, 9, 14, 19, 27, 34 , and 38 ), and oxidized to the corresponding quinones ( 6, 10, 15, 20, 28, 35 , and 40 ). The (glucosylthio)quinones 6, 15, 20, 28 , and 35 were tested as time-dependent inactivators of a retaining β-1,4-glucosidase from Agrobacterium faecalis (Abg), which has a strong exo-glucosidase action (Table 1). Similarly, compounds 20, 28 , and 35 were tested with a cellulase from Cellulomonas fimi (Cex) which degrades cellulose and cellooligosaccharides by hydrolysis of a cellobiose unit from the nonreducing terminus. The most effective inactivators for Abg were 6, 15 , and 35 , which inactivated this enzyme with similar second-order rate constants. (Glycosylthio)quinone 28 was the worst inactivator and did not show normal saturation behaviour. Inactivation of Cex by the (glycosylthio)quinones was 3–500 times slower than that of Abg. The three inactivators 20, 28 , and 35 had approximately the same efficacy with Cex, suggesting that they bind to this enzyme in a similar mode. Further, the K_i values observed are very similar to K_m values measured for aryl cellobiosides, implying that they bind at the active site.     

N-Alkylated Derivatives of 1,5-Dideoxy-1,5-iminoxylitol as β-Xylosidaseand β-Glucosidase Inhibitors

Summary.  A range of lipophilic derivatives of the D-xylosidase inhibitor 1,5-dideoxy-1,5-iminoxylitol was synthesized by N-alkylation of the parent compound with different alkyl halides. Inhibitory activities of the products obtained were measured with β-glucosidase from Agrobacterium sp., a family 1 enzyme, as well as with β-xylosidase from Thermoanaerobacterium saccharolyticum belonging to family 39 of the glycohydrolases. Whereas the former enzyme, which has low β-xylosidase activity, was only moderately inhibited, with K _i values in the millimolar range, good inhibition was observed with the latter one. Inhibitors with terminally functionalized N-alkyl chains open up opportunities for novel applications as affinity ligands as well as for various types of tagging for diagnostic purposes. Received October 25, 2001. Accepted November 19, 2001     

Dinucleotide junction cleavage versatility of 8-17 deoxyribozyme

We conducted 16 parallel in vitro selection experiments to isolate catalytic DNAs from a common DNA library for the cleavage of all 16 possible dinucleotide junctions of RNA incorporated into a common DNA/RNA chimeric substrate sequence. We discovered hundreds of sequence variations of the 8-17 deoxyribozyme--an RNA-cleaving catalytic DNA motif previously reported--from nearly all 16 final pools. Sequence analyses identified four absolutely conserved nucleotides in 8-17. Five representative 8-17 variants were tested for substrate cleavage in trans, and together they were able to cleave 14 dinucleotide junctions. New 8-17 variants required Mn2+ to support their broad dinucleotide cleavage capabilities. We hypothesize that 8-17 has a tertiary structure composed of an enzymatic core executing catalysis and a structural facilitator providing structural fine tuning when different dinucleotide junctions are given as cleavage sites.     

Synthesis and evaluation of potential inhibitors of chondroitin AC lyase from Flavobacterium heparinum

Chondroitin AC lyase from Flavobacterium heparinum degrades chondroitin sulfate glycosaminoglycans via an elimination mechanism, resulting in disaccharides or oligosaccharides with Delta4,5-unsaturated uronic acid residues at their nonreducing end. The syntheses and testing of two potential inhibitors of this lyase are described. Methyl O-(2-acetamido-2-deoxy-beta-D-galactopyranosyl)-(1-->4)-alpha-L-threo-hex-4-enopyranoside, 1, has the trigonal geometry at C5 of the uronic acid moiety expected at the transition state, yet retains the "leaving group" sugar moiety. Surprisingly, compound 1 showed no inhibition of the enzyme. The novel 5-nitro sugar, phenyl (5S)-5-nitro-beta-D-xylopyranoside, 2, is a monosaccharide nitro analogue of the natural substrate, with C5 being a carbon acid of pK(a) 8.8. The rate of reprotonation of the anion generated at this center is sufficiently low that the anion of 2 can be used directly in initial steady-state velocity measurements without significant interference from the conjugate carbon acid. The anion of compound 2 was found to be a competitive inhibitor with a K(i) value of 5 mM, whereas the conjugate acid had a K(i) value of 35 mM.     

On the Application of Xe+ Plasma FIB for Micro-fabrication of Small-scale Tensile Specimens

Experimental Mechanics - In order to perform site-specific mechanical studies to examine the contribution or interaction of a material constituent, a reliable methodology for the production of...