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.     

Study of a Crack at a Fastener Hole by Digital Image Correlation

In this work the efficacy of using digital image correlation to determine stress intensity factors for a crack emanating from a fastener hole has been investigated. To this end a fatigue crack was grown in pure mode I from a 50 mm diameter hole in an Al 7010 alloy plate test-piece. This crack was then loaded elastically under several combinations of mixed mode (I + II) displacements. In each case, images of the sample surface before and after the deformation were recorded using a high resolution digital camera. The surface preparation consisted only of scratching the surface lightly with silicon carbide abrasive paper. The crack location and resulting displacements were then calculated using digital image correlation. The analytical displacement fields for a traction free crack under arbitrary loading conditions based on the Muskhelishvili’s complex function approach were fitted to the experimentally measured displacement fields and the mixed mode stress intensity factor was determined in each case. Good agreement with the nominal applied values was obtained. The uncertainty of the crack tip position has a major influence on the accuracy of the stress intensity factors and so the Sobel edge finding filter was successfully applied to experimental displacement fields to establish precisely the crack tip location. This paper was presented at the 2007 SEM Congress held in Springfield, Massachusetts, USA     

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.     

Tuning Mechanism‐Based Inactivators of Neuraminidases: Mechanistic and Structural Insights

3‐Fluorosialosyl fluorides are inhibitors of sialidases that function by the formation of a long‐lived covalent active‐site adduct and have potential as therapeutics if made specific for the pathogen sialidase. Surprisingly, human Neu2 and the Trypanosoma cruzi trans‐sialidase are inactivated more rapidly by the reagent with an equatorial fluorine at C3 than by its axial epimer, with reactivation following the same pattern. To explore a possible stereoelectronic basis for this, rate constants for spontaneous hydrolysis of the full series of four 3‐fluorosialosyl fluorides were measured, and ground‐state energies for each computed. The alpha (equatorial) anomeric fluorides hydrolyze more rapidly than their beta anomers, consistent with their higher ground‐state energies. However ground‐state energies do not explain the relative spontaneous reactivities of the 3‐fluoro‐epimers. The three‐dimensional structures of the two 3‐fluoro‐sialosyl enzyme intermediates of human Neu2 were solved, revealing key stabilizing interactions between Arg21 and the equatorial, but not the axial, fluorine. Because of changes in geometry these interactions will increase at the transition state, likely explaining the difference in reaction rates.     

Predicting the onset of rafting of γ′ precipitates by channel deformation in a Ni superalloy

The growth or shrinkage, normal to {001}, of the interfaces between the γ matrix and cuboidal γ′ precipitates is examined for a Ni-base superalloy, by considering the force acting on the interfaces. The force is produced by the precipitate coherency misfit and the stress produced by plastic deformation in channels of the γ matrix. A simple expression, which directly addresses the origin of the surface force, is given. The plastic deformation within the initially active γ matrix channels exerts the force to cause rafting. The subsequent activation of other types of channels also promotes the rafting in the same direction as the first active channels, when the plastic strain of the former channels increases. These issues are also discussed in terms of analysis based on those dislocations caused by the precipitate misfit and those produced by the plastic deformation.     

Facile Formation of β‐thioGlcNAc Linkages to Thiol‐Containing Sugars,Peptides, and Proteins using a Mutant GH20 Hexosaminidase

Thioglycosides are hydrolase‐resistant mimics of O‐linked glycosides that can serve as valuable probes for studying the role of glycosides in biological processes. The development of an efficient, enzyme‐mediated synthesis of thioglycosides, including S‐GlcNAcylated proteins, is reported, using a thioglycoligase derived from a GH20 hexosaminidase from Streptomyces plicatus in which the catalytic acid/base glutamate has been mutated to an alanine (SpHex E314A). This robust, easily‐prepared, engineered enzyme uses GlcNAc and GalNAc donors and couples them to a remarkably diverse set of thiol acceptors. Thioglycoligation using 3‐, 4‐, and 6‐thiosugar acceptors from a variety of sugar families produces S‐linked disaccharides in nearly quantitative yields. The set of possible thiol acceptors also includes cysteine‐containing peptides and proteins, rendering this mutant enzyme a promising catalyst for the production of thio analogues of biologically important GlcNAcylated peptides and proteins.     

Improved tomographic reconstructions using adaptive time‐dependent intensity normalization

The first processing step in synchrotron‐based micro‐tomography is the normalization of the projection images against the background, also referred to as a white field. Owing to time‐dependent variations in illumination and defects in detection sensitivity, the white field is different from the projection background. In this case standard normalization methods introduce ring and wave artefacts into the resulting three‐dimensional reconstruction. In this paper the authors propose a new adaptive technique accounting for these variations and allowing one to obtain cleaner normalized data and to suppress ring and wave artefacts. The background is modelled by the product of two time‐dependent terms representing the illumination and detection stages. These terms are written as unknown functions, one scaled and shifted along a fixed direction (describing the illumination term) and one translated by an unknown two‐dimensional vector (describing the detection term). The proposed method is applied to two sets (a stem Salix variegata and a zebrafish Danio rerio) acquired at the parallel beam of the micro‐tomography station 2‐BM at the Advanced Photon Source showing significant reductions in both ring and wave artefacts. In principle the method could be used to correct for time‐dependent phenomena that affect other tomographic imaging geometries such as cone beam laboratory X‐ray computed tomography.     

Some conditional expectation identities for the multivariate normal

We give formulas for the conditional expectations of a product of multivariate Hermite polynomials with multivariate normal arguments. These results are extended to include conditional expectations of a product of linear combination of multivariate normals. A unified approach is given that covers both Hermite and modified Hermite polynomials, as well as polynomials associated with a matrix whose eigenvalues may be both positive and negative.     

Tilted Edgeworth expansions for asymptotically normal vectors

We obtain the Edgeworth expansion for P(n^1/2([^(q)]-q) < x){P(n^{1/2}(\hat{\theta}-\theta) < x)} and its derivatives, and the tilted Edgeworth (or saddlepoint or small sample) expansion for P([^(q)] < x){P(\hat{\theta} < x)} and its derivatives where [^(q)]{\hat{\theta}} is any vector estimate having the standard cumulant expansions in powers of n^-1{n^{-1}} .