Anatomy of glycosynthesis: structure and kinetics of the Humicola insolens Cel7B E197A and E197S glycosynthase mutants

The formation of glycoconjugates and oligosaccharides remains one of the most challenging chemical syntheses. Chemo-enzymatic routes using retaining glycosidases have been successfully harnessed but require tight kinetic or thermodynamic control. "Glycosynthases," specifically engineered glycosidases that catalyze the formation of glycosidic bonds from glycosyl donor and acceptor alcohol, are an emerging range of synthetic tools in which catalytic nucleophile mutants are harnessed together with glycosyl fluoride donors to generate powerful and versatile catalysts. Here we present the structural and kinetic dissection of the Humicola insolens Cel7B glycosynthases in which the nucleophile of the wild-type enzyme is mutated to alanine and serine (E197A and E197S). 3-D structures reveal the acceptor and donor subsites and the basis for substrate inhibition. Kinetic analysis shows that the E197S mutant is considerably more active than the corresponding alanine mutant due to a 40-fold increase in k(cat).     

Solid phase microextraction/isothermal GC for rapid analysis of complex organic samples

Solid phase microextraction (SPME) was used as the sample introduction technique for high-speed isothermal GC. An injector dedicated for SPME fiber injection was designed and built. The injector was operated in two modes, continuously heated and flash heated. The latter mode proved to be better for high-speed separations. The injector was then used for sample introduction in separation of BTEX. When sampling directly from water with a fiber having a 56 μm thick poly(dimethylsiloxane) coating, the BTEX components were separated under isothermal conditions in ca. 18 s. A fiber with a thinner coating (15 μm) enabled the separation to be completed in ca. 12 s when sampling from headspace. In both cases the results were highly reproducible, as measured by the estimated values of the relative standard deviation.     

Uniaxial alignment of the columnar super-structure of a hexa (alkyl) hexa-peri-hexabenzocoronene on untreated glass by simple solution processing

Uniaxially aligned, thin films of a discotic columnar thermotropic liquid crystalline material can be prepared by a simple solution zone-casting method, without the need for modified surfaces or traditional alignment techniques. Atomic force microscopy together with X-ray diffraction reveals single-crystalline-like order over several square centimeters, far exceeding the requirements for application of such films in organic molecular electronic devices such as field-effect transistors.     

Thermodesorption studies of energetic properties of Ni/MgO-Al2O3 catalysts. Determination of adsorption energy distribution functions

The thermodesorption spectra of hydrogen from coprecipitated catalysts (70-x)NiO-xMgO-30Al(2)O(3) (x = 0-50%(wt)) are reported. The catalysts were calcined at 400 degrees C and reduced with H(2) at 20-800 degrees C and for 3 h at 800 degrees C. NiO reduction degree was between 49.3 and 92.1%. The active surface areas changed from 8.4 to 32.4 m(2)/g whereas mean size of nickel crystallites was between 3.7 and 9.7 nm. The TPD spectra were next analyzed in order to determine the adsorption energy distributions functions. To obtain these functions a theoretical model of adsorption/desorption kinetics based on the statistical rate theory (SRT) was applied. This approach allows for determination of the adsorption energy at nonequilibrium conditions as well as at quasiequilibrium conditions. The resulting distribution functions reveal the presence of two main bands of adsorption energy. Some correlation is found between the determined distributions of adsorption energy and the size of nickel crystallites determined using the XRD method. The presence of MgO favors creation of high energy adsorption sites on Ni crystallites.     

A new application of the Mitsunobu reaction in the synthesis of phosphonium salts

The condensation of methanol or primary alcohols with triphenylphosphonium tetrafluoroborate in the presence of ethyl azodicarboxylate and triphenylphosphine in THF at room temperature gives the respective alkyltriphenylphosphonium salts in good yields. The reaction also worked for the conversion of N-acyl-2-hydroxyglycinates into N-acyl-2-triphenylphosphonioglycinates.     

meso-tetrakis[2.2]paracyclophanylporphyrin: Electronic structure and electrochemical behavior

Replacement of all phenyl groups in meso-tetraphenylporphyrin, TPP, by [2.2]paracyclophane, PCP, enhances the increase of energy of the HOMO and HOMO-1 of porphine, P, already noticed for the mono-[2.2]paracyclophanyl-substituted TPP, and fills the energy gap by the occupied MOs of the PCP units. The first oxidation half-wave potential is respectively decreased to 0.52 V. The CNDO/S-CIS calculations agree with the experimental bathochromic shifts of all bands in the electronic spectra of the considered atropoisomers of the title compound, TPCPP, as compared to TPP. In the excited B states the interactions between the PCP and porphine units are represented mainly by the charge transfer of 0.44 e from PCP to P, according to transition density matrix calculations. While electroreduction of the title compound results in a successive formation of the anion radical and dianion, oxidation represents a four-step process involving one electron transfer per step, and resulting in the oxidation of two PCP units. Formation of the conductive polymeric film on the electrode seems to be connected with the transient formation of a quinoid system of bonds.     

Simulation of polymer--polymer interdiffusion using the dynamic lattice liquid model

In this paper, we present computer simulation results concerning interdiffusion of fully compatible components in symmetric binary (AB) polymer mixtures in solutions. The simulation is performed in two dimensions using the algorithm based on the dynamic lattice liquid model. The solvent molecules are taken into account explicitly. The evolution of the concentration profiles in time at an interface is studied for chain lengths N=2,4,8,16 for three polymer concentrations phi=0.1,0.5,0.9. The tracer diffusion coefficients for polymer chains and for the solvent are obtained by monitoring the mean square displacements of their center of mass. The relationships between coefficients of interdiffusion and self-diffusion are tested.     

The structure of binuclear molybdenum(VI) oxocomplexes with dianionic tridentate Schiff bases

Summary The new Mo^VI complex [Mo(O) (-O) (SAE)]₂ was prepared and characterized by means of spectroscopic (i.r.,¹Hn.m.r.) and crystallographic measurements. Crystals of the [Mo(C₉H₈NO₂)(H₂O)]₂, M=582.3, orthorhombic, space group Pbca,a=13.108(4),b=8.982(2),c=16.842(4) Å, v=1982.9 ų, z=4, D_m=1.93(2), D_c=1.95 g cm^–3, MoK =0.71069 Å, =1.3 mm^–1, F(000)=1152, T=295K, R=0.035 for 2413 reflections.The crystal structure consists of [Mo(O)(-O)(L)]₂, in dimeric units. The coordination geometry around each molybdenum atom is distorted octahedral with Mo–O (terminal) and Mo–O (bridge) distances of 1.694(3) and 1.760(3) Å, respectively, and with the angle O(1)–Mo–O(2) of 105.6(1)^o, typical for manycis-dioxocomplexes of Mo^VI. The asymmetric bridge is characterized by the following parameters: Mo–O(2)ⁱ and Mo–Moⁱ distances equal 2.408(3) and 3.319(3) Å, respectively, and the Mo–O(2)–Moⁱ and O(2)–Mo–O(2)ⁱ angles equal 104.5(1)^o and 75.5(1)^o, respectively.The i. r. and Raman spectra of molybdenum complexes with an asymmetric double oxygen bridge display a strong absorption the 800–850 cm^–1 range.     

The isoxazole heterocyclic system: synthesis of a new model structure in immunosuppressing agents

A new lead structure in the isoxazole heterocyclic system showing immunosuppressing activity is described. Synthesis, spectral and X-ray data giving evidence of the structures of these compounds are presented. X-ray studies were carried out on compounds I C₁₈H₂₂N₄O₄ with a = 9.606(3), b = 9.813(3), c = 9.849(3)Å, = 87.28(3), = 79.06(3). = 80.89(3)° Z = 2, and space group ; and II C₁₄H₁₅N₄O₃Cl with a = 7.090(3), b = 16.807(3), c = 12.925(3)Å, = 104.41(3)°, Z = 4 and space group P2₁/n.