High-resolution TEM and the application of direct and indirect aberration correction.

Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes. Correction of the aberrations has been achieved electron-optically through a hexapole-based corrector and also indirectly by computational analysis of a focal or tilt series of images. These direct and indirect methods are complementary, and a combination of the two offers further advantages. Materials characterization has benefitted from the reduced delocalization and higher resolution in the corrected images. It is now possible, for example, to locate atomic columns at surfaces to higher accuracy and reliability. This article describes the JEM-2200FS in Oxford, which is equipped with correctors for both the image-forming and probe-forming lenses. Examples of the use of this instrument in the characterization of nanocrystalline catalysts are given together with initial results combining direct and indirect methods. The double corrector configuration enables direct imaging of the corrected probe, and a potential confocal imaging mode is described. Finally, modifications to a second generation instrument are outlined.     

The synthesis and structure of terpyridine-N-oxide complexes of copper(II) perchlorate

The co-ordination of a series of terpyridine-N-oxide ligands to the Cu(ii) ion is reported. Addition of two equivalents of ligand results in the formation of the expected 2 : 1 six co-ordinate product, while the addition of one equivalent of ligand allows the isolation of 1 : 1 species if the ligand has a meridonal binding mode. The five complexes isolated were characterised in the solid state by X-ray crystallography while they are studied in solution using EPR, UV-vis spectroscopy and IR spectroscopy.     

Asymmetric synthesis of an N-acylpyrrolidine for inhibition of HCV polymerase

A practical asymmetric synthesis of a highly substituted N-acylpyrrolidine on multi-kilogram scale is described. The key step in the construction of the three stereocenters is a [3+2] cycloaddition of methyl acrylate and an imino ester prepared from l-leucine t-butyl ester hydrochloride and 2-thiazolecarboxaldehyde. The cycloaddition features novel asymmetric catalysis via a complex of silver acetate and a cinchona alkaloid, particularly hydroquinine, with complete diastereomeric control and up to 87% enantiomeric control. The alkaloid serves as a ligand as well as a base for the formation of the azomethine ylide or 1,3-dipole. Experiments have shown that the hydroxyl group of hydroquinine is a critical element for the enantioselectivities observed. The cycloaddition methodology is also applicable to methylvinyl ketone, providing access to either alpha- or beta-epimers of 4-acetylpyrrolidine depending on the reaction conditions utilized. The synthesis also highlights an efficient N-acylation, selective O- versus N-methylation, and a unique ester reduction with NaBH4-MeOH catalyzed by NaB(OAc)3H that not only achieves excellent chemoselectivity but also avoids formation of the undesired but thermodynamically favored epimer. The highly functionalized target is synthesized in seven linear steps from l-leucine t-butyl ester hydrochloride with all three isolated intermediates being highly crystalline.     

Counting and coding identity trees with fixed diameter and bounded degree

Identity trees with bounded maximum degree play a fundamental role in applications-oriented problems, especially when the trees are classified by their diameters. This paper offers results related to enumeration of such tree classes obtained by extending the methods of Gordon and Kennedy [The counting and coding of trees of fixed diameter, SIAM J. Appl. Math. 28 376–398 (1975)]. We set our results into the context of other enumerative work on identity trees.We derive formulae for the numbers of identity trees of various types, with fixed diameter and maximum degree. This then leads to asymptotic formulae (for large diameter). By combining these with formulae derived by Gordon and Kennedy [loc. cit.] we obtain the asymptotic fractions of identity trees among trees in various classes. These fractions are juxtaposed with asymptotic results that have appeared elsewhere.Our final section derives algorithms for integer coding and decoding identity trees in a way that is highly convenient for computer applications.     

High-performance polymer electrolyte membranes incorporated with 2D silica nanosheets in high-temperature proton exchange membrane fuel cells

Silica nanosheets(SN)derived from natural vermiculite(Verm)were successfully incorporated into polyethersulfone-polyvinylpyrrolidone(PES-PVP)polymer to fabricate high-temperature proton exchange membranes(HT-PEMs).The content of SN filler was varied(0.1-0.75 wt%)to study its influence on proton conductivity,power density and durability.Benefiting from the hydroxyl groups of SN that enable the formation of additional proton-transferring pathways,the inorganic-organic membrane displayed enhanced proton conductivity of 48.2 mS/cm and power density of 495 mW/cm² at 150℃ without humidification when the content of SN is 0.25 wt%.Furthermore,exfoliated SN(E-SN)and sulfonated SN(S-SN),which were fabricated by a liquid-phase exfoliation method and silane condensation,respectively,were embedded in PES-PVP polymer matrix by a simple blending method.Due to the significant contribution from sulfonic groups in S-SN,the membrane with 0.25 wt%S-SN reached the highest proton conductivity of51.5 mS/cm and peak power density of 546 mW/cm² at150℃,48%higher than the pristine PES-PVP membranes.Compared to unaltered PES-PVP membrane,SN added hybrid composite membrane demonstrated excellent durability for the fuel cell at 150℃.Using a facile method to prepare 2D SN from natural clay minerals,the strategy of exfoliation and functionalization of SN can be potentially used in the production of HT-PEMs.     

Loss Behavior of Single-mode Optical Fiber Microbend Sensors

Periodic microbending has been studied to develop an insight into the loss behavior of single-mode microbend sensors by using the finite-difference beam propagation method. Loss sensitivity variations with wavelength are examined, as well as the optical loss dependence on the number of microbends and the modal field perturbations caused by the microbending.     

A Functional Role for Aβ in Metal Homeostasis? N‐Truncation and High‐Affinity Copper Binding

Accumulation of the β‐amyloid (Aβ) peptide in extracellular senile plaques rich in copper and zinc is a defining pathological feature of Alzheimer′s disease (AD). The Aβ1–x (x=16/28/40/42) peptides have been the primary focus of Cu^II binding studies for more than 15 years; however, the N‐truncated Aβ4–42 peptide is a major Aβ isoform detected in both healthy and diseased brains, and it contains a novel N‐terminal FRH sequence. Proteins with His at the third position are known to bind Cu^II avidly, with conditional log K values at pH 7.4 in the range of 11.0–14.6, which is much higher than that determined for Aβ1–x peptides. By using Aβ4–16 as a model, it was demonstrated that its FRH sequence stoichiometrically binds Cu^II with a conditional K_d value of 3×10⁻¹⁴ M at pH 7.4, and that both Aβ4–16 and Aβ4–42 possess negligible redox activity. Combined with the predominance of Aβ4–42 in the brain, our results suggest a physiological role for this isoform in metal homeostasis within the central nervous system.     

Correlated electromigration of H in the switchable mirror YH(3-delta)

Electromigration of hydrogen in YH(3-delta) is studied by exploiting the H concentration dependence of the optical transmission of YH(3-delta). We find the effective valence Z* of H in YH(3-delta) to be negative. Its value is dominated by a huge wind-force-like term, i.e., Z* approximately K/rho, with K approximately -60 mOmega cm. This value is 3 orders of magnitude larger than typical for H in metals. In an Arrhenius plot, the ratio of hydrogen and electron fluxes extrapolates to unity at infinite temperature, suggesting a one-to-one correlation of hydrogen and electron hopping. We discuss our results in the light of strong electron correlation theories which predict each proton to bind two electrons in a sort of Zhang-Rice singlet.