Determination by high-frequency and -field EPR of zero-field splitting in iron(IV) oxo complexes: implications for intermediates in nonheme iron enzymes

[Fe(IV)O](2+) species have been implicated as the active form of many nonheme iron enzymes. The electronic structures of iron(IV) oxo complexes are thus of great interest. High-frequency and -field electron paramagnetic resonance is employed to determine accurately the spin Hamiltonian parameters of two stable complexes that contain the FeO unit: [FeO(TMC)(CH 3CN)](CF 3SO 3) 2, where TMC = tetramethylcyclam and [FeO(N4py)](CF 3SO 3) 2, where N4Py = bis(2-pyridylmethyl)bis(2-pyridyl)methylamine. Both complexes exhibit zero-field splittings that are positive, almost perfectly axial, and of very large magnitude: D = +26.95(5) and +22.05(5) cm (-1), respectively. These definitive experimental values can serve as the basis for further computational studies to unravel the electronic structures of such complexes.     

High energy gain of trapped electrons in a tapered, diffraction-dominated inverse-free-electron laser

Energy gain of trapped electrons in excess of 20 MeV has been demonstrated in an inverse-free-electron-laser (IFEL) accelerator experiment. A 14.5 MeV electron beam is copropagated with a 400 GW CO2 laser beam in a 50 cm long undulator strongly tapered in period and field amplitude. The Rayleigh range of the laser, approximately 1.8 cm, is much shorter than the undulator length yielding a diffraction-dominated interaction. Experimental results on the dependence of the acceleration on injection energy, laser focus position, and laser power are discussed. Simulations, in good agreement with the experimental data, show that most of the energy gain occurs in the first half of the undulator at a gradient of 70 MeV/m and that the structure in the measured energy spectrum arises because of higher harmonic IFEL interaction in the second half of the undulator.     

The effects of distance and neighborhood union conditions on hamiltonian properties in graphs

We prove that a 2-connected graph G of order p is hamiltonian if for all distinct vertices u and v, dist(u,v) = 2 implies that |N(u) U N(v)| ? (2p - 1)/3. We also demonstrate hamiltonian-connected and traceability properties in graphs under similar conditions.     

Emission of α-particles in deep inelastic reactions induced by 148 MeV 14N

Energy and angular correlations between α-particles and heavy ejectiles have been measured for 148 MeV ¹⁴N incident on ¹²C, ²⁷Al and ⁵⁸Ni targets. The coincidence cross sections for ejectiles with Z < 6 may be parametrised as a product of singles cross sections for the detection of α-particles and heavy ions. For ²⁷Al and ⁵⁸Ni targets, this indicates that the α-particles are emitted at an early stage of the reaction prior to the formation of the deep inelastic fragments. For the ¹²C target, however, kinematic effects are found to dominate the observed correlations. For $\text{Z ≧ 6}$ ejectiles the observed correlations appear consistent with the emission of α-particles from the recoiling nucleus produed in the reaction.     

Crystallographic shear in the Nb2O5WO3 and Ta2O5WO3 systems

Crystallographic shear (CS) phases occurring in the Nb₂O₅WO₃ and Ta₂O₅WO₃ systems near to WO₃ were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The Nb₂O₅WO₃ samples were heated at 1600K. They contained ordered {104} and {001} CS planes and wavy CS which were composed of intergrowths of {104} and {001} CS segments. The composition range over which the {104} CS series extended was from (Nb,W)O_2.954 i.e., (Nb,W)₆₅O₁₉₂, to (Nb,W)O_2.942, i.e., (Nb,W)₅₂O₁₅₃. The composition range over which the {001} CS series extended was from (Nb,W)O_2.9375, i.e., (Nb,W)₁₆O₄₇ to (Nb,W)O_2.875, i.e., (Nb,W)₈O₂₃. The Ta₂O₅WO₃ samples were prepared at 1593, 1623, and 1672K. At lower temperatures ordered {103} CS phases were found, with a composition range extending between (Ta,W)O_2.960, i.e., (Ta,W)₅₀O₁₄₈, to (Ta,W)O_2.944, i.e., (Ta,W)₃₆O₁₀₆. At 1673K ordered {103} CS phases occurred, as did wavy CS composed of intergrowths of {103} and {104} CS segments.     

Cyclopentannulation of 3-alkylindoles: a synthesis of a tetracyclic subunit of the kopsane alkaloids.

Indoles which bear an alkyl substituent in the 3-position undergo a [3 + 2] annulation reaction when treated with 1,1-cyclopropane diesters in the presence of Yb(OTf)(3) resulting in 2,3-cyclopentanoindolines. Typically, the reactions are performed at elevated temperatures or at ultrahigh pressures. In cases where steric crowding is an issue, ultrahigh pressures are required. In reactions involving substituted cyclopropanes, significant regio- and diastereocontrol was observed. When the substituent was aromatic or olefinic, the reactions took place at ambient temperature and pressure. The applicability of this methodology to the preparation of a key tetracyclic subunit of the kopsane alkaloids was demonstrated.     

A fundamental study of organofunctionalised PDMS membranes for the pervaporative recovery of phenolic compounds from aqueous streams

Modified polydimethylsiloxane (PDMS) composite membranes in which a known mol% of the SiMe groups were replaced by a short methylene spacer group terminated by one of the four side-arm functional groups, acetate, ---CO₂Me; ethylether, OEt; dimethylamino, ---NMe₂; and pyridyl, ---py; were fabricated and tested in short term trials for the pervaporative recovery of cresols from aqueous solution. The influences of functional group type, functional group loading, pH, temperature and purity of the feed solution were investigated. Three commercially available hydrophobic membranes were also examined for comparison. It was found that significant performance enhancements, compared with an unfunctionalised PDMS membrane, were realised for all functionalised PDMS membranes, with dimethylamino and ethylether functionalities proving the most effective. A functional group loading level close to 20% was found to be optimal for this application, The influence of feed pH in the range of 5–8.5 was small on total flux but significant on selectivity for amine loaded membranes. By contrast, the effect of feed temperature was significant on total flux but negligible on selectivity. Low levels of phenolic impurities in a p-cresol feed solution were found to have negligible effect on the separation. Compared with commercially available zeolite filled PDMS and PEBA membranes, the functionalised PDMS membranes showed a better overall performance. The PV performance of all new membranes followed the trend: o-cresol>p-cresol>phenol.     

Prion protein self-peptides modulate prion interactions and conversion

Background  

Molecular mechanisms underlying prion agent replication, converting host-encoded cellular prion protein (PrP^C) into the scrapie associated isoform (PrP^Sc), are poorly understood. Selective self-interaction between PrP molecules forms a basis underlying the observed differences of the PrP^C into PrP^Sc conversion process (agent replication). The importance of previously peptide-scanning mapped ovine PrP self-interaction domains on this conversion was investigated by studying the ability of six of these ovine PrP based peptides to modulate two processes; PrP self-interaction and conversion.