Enhancing the enantioselectivity of an epoxide hydrolase by directed evolution

[reaction: see text] The epoxide hydrolase (EH) from Aspergillus niger, which shows a selectivity factor of only E = 4.6 in the hydrolytic kinetic resolution of glycidyl phenyl ether, has been subjected to directed evolution for the purpose of enhancing enantioselectivity. After only one round of error-prone polymerase chain reaction (epPCR), enantioselectivity was more than doubled (E = 10.8). The improved mutant enzyme contains three amino acid exchanges, two of which are spatially far from the catalytically active center.     

Stacking in sediments of colloidal hard spheres

We use computer simulations to investigate the crystallization dynamics of sedimenting hard spheres in large systems (hundreds of thousands of particles). We show that slow sedimentation results primarily in face-centered cubic (fcc) stacked crystals, instead of random hexagonal close packed or hexagonal close packed (hcp) crystals. We also find slanted stacking faults, in the fcc regions. However, we attribute the formation of fcc to the free energy difference between fcc and hcp and not to the presence of these slanted stacking faults. Although the free energy difference between hcp and fcc per particle is small (only 10(-3) times the thermal energy), it can become considerable, when multiplied by the number of particles in each domain. The ratio of fcc to hcp obtained from dynamic simulations is in excellent agreement with well-equilibrated Monte Carlo simulations, in which no slanted stacking faults were found. Our results explain a range of experiments on colloids, in which the amount of fcc increases upon lowering the sedimentation rate or decreasing the initial volume fraction.     

A search for μ-e conversion in muonic gold

We report on a search for μ-e conversion in muonic gold performed with the SINDRUM II spectrometer at PSI. The measurement resulted in Γ(μ^-Au→e^-Au_g.s.)/Γ_capture(μ^-Au)<7×10^-13 (90% C.L.). PACS 13.35.Bv; 13.60.-r; 36.10.-k; 36.10.Dr     

Polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3

We report polarized Raman scattering in single crystals of Nd_0.7Sr_0.3MnO₃. The temperature dependence of the MnO₆ octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.     

Structure and properties of Ce3Pd3Bi4, CePdBi, and CePd2Zn3

The intermetallic cerium compounds Ce₃-Pd₃Bi₄, CePdBi, and CePd₂Zn₃ were synthesized from the elements in sealed tantalum ampoules in an induction furnace. The compounds were characterized by X-ray powder and single crystal diffraction: CeCo₃B₂ type (ordered version of CaCu₅), P6/mmm, a = 538.4(4), c = 427.7(4) pm, wR2 = 0.0540, 115 F ² values, 9 variables for CePd₂Zn₃ and Y₃Au₃Sb₄ type, I [`4]{\bar 4} 3d, a = 1005.2(2) pm, w R2 = 0.0402, 264 F ² values, 9 variables for Ce₃Pd₃Bi₄, and MgAgAs type, a = 681.8(1) pm for CePdBi. The bismuthide structures are build up from three-dimensional networks of corner-sharing PdBi₄ tetrahedra with Pd–Bi distances of 281 (Ce₃Pd₃Bi₄) and 296 pm (CePdBi), respectively. The cerium atoms are located in larger voids of coordination number 12 (Ce₃Pd₃Bi₄) and 10 (CePdBi). In CePd₂Zn₃ the cerium atoms fill larger channels within the three-dimensional [Pd₂Zn₃] network with 18 (6 Pd + 12 Zn) nearest neighbors. The three compounds contain stable trivalent cerium with experimental magnetic moments of μ_eff = 2.70(2), 2.48(1), and 2.49(1) μ_B/Ce atom for CePd₂Zn₃, Ce₃Pd₃Bi₄, and CePdBi, respectively. Susceptibility and specific heat data gave no hint for magnetic ordering down to 2.1 K.     

In situ fluorescence probing of the chemical and structural changes during formation of hexagonal phase cetyltrimethylammonium bromide and lamellar phase CTAB/Poly(dodecylmethacrylate) sol–gel silica thin films

Surfactant-templated mesostructured sol–gel films formed by evaporation induced self assembly (EISA) exhibit highly-ordered hexagonal, lamellar, and cubic structures. The steady-state dip-coating configuration allows both the chemistry and the dynamics of the EISA process to be traced in real time because the steps involved in the formation of the mesostructured material are separated both spatially and temporally in the dip-coating direction. The dynamic processes occurring during film formation can be conveniently monitored by the combination of interferometry and fluorescence spectroscopy of incorporated molecular probes. The selected probes respond to changes in their rotational mobility and the surrounding solvent composition and report these changes through their fluorescence characteristics. By taking in situ fluorescence spectra at various positions within the progressively thinning film, changes in the solvent composition, onset of micelle formation and further organization to the final mesophase structure can be followed. The luminescence of the probe molecule is measured with a spatial resolution of 100 μm. Two categories of surfactant-templated mesostructured sol–gel films were examined. Cetyltrimethylammonium bromide (CTAB) systems assemble into a 2-D hexagonal surfactant/silica mesophase with the surfactant concentration used in this study. CTAB dodecylmethacrylate systems assemble into a lamellar mesophase, which can be further polymerized to form a poly(dodecylmethacrylate)/silica hybrid nanocomposite that mimics nacre. X-ray diffraction patterns, transmission electron microscopy images, and other techniques are used to characterize the final films.