Silicone polycarbonates from new aryl-terminated siloxane precursors

Several synthetic routes to silicon polycarbonate copolymers utilizing aryl-terminated siloxanes have been examined. Anhydride capped siloxanes with varying polydimethylsiloxane units were prepared by the acid equilibration of 1,3-bis(4,4′-phthalicanhydride)tetramethyldisiloxane. These siloxanes were then imidized with aminophenol to the corresponding phenol capped siloxane. Reaction of these materials with 4,4′-isopropylidenediphenol (BPA) and phosgene (interfacially and non-interfacially) produced silicone polycarbonate copolymers. Similar copolymers were obtained by reaction with bis-chloroformate derivatives of 4,4′-isopropylidenediphenol (monomer and oligomers). The different synthetic routes significantly affect the thermal and mechanical properties as well as the composition of the new polymers. Materials synthesized by the bis-chloroformate route exhibit good hydrolytic and melt stabilities. Those made interfacially underwent phase separation in the melt.     

Dielectric-loaded wideband gyro-TWT

The bandwidth of a gyro-TWT (traveling-wave tube) can be widened by employing a dielectric-loaded waveguide to reduce the circuit's dispersion. Fast wave interaction allows the requirements on the beam's quality to be relaxed compared with slow wave interaction. A low-α (≡ν_⊥/ν_z) electron beam is chosen to avoid the absolute instability and minimize the possibility of dielectric charging. This device is investigated using a self-consistent single-mode, large-signal simulation based on a slow time scale formulation. Simulation results show that a constant drive bandwidth of 20% can be achieved for a 100 kV, 5 A electron beam with a velocity ratio of α=0.59 and an axial velocity spread of 2.0%. The growth rate is relatively low because of the low α of the electron beam. The design of a proof-of-principle experiment is described. The tube is expected to deliver a power of 80 kW from 9 to 11 GHz with 15% efficiency and a saturated gain of 30 dB. The performance of a single-anode magnetron injection gun designed to produce the required high-quality electron beam has been studied through simulation     

Enantioselective synthesis of the farnesyltransferase inhibitor, A-345665.0

The stereoselective synthesis of A-345665.0 1, a novel farnesyl transferase inhibitor, is described. The key step involves a stereoselective addition of an imidazolyl Grignard reagent to aldehyde 8 in the presence of an external chiral auxiliary. Crystallization of the product as the dimeric zinc complex 12 facilitates the isolation of product in >98:2 er. The biaryl linkage is formed by the use of a Suzuki coupling, employing boronic acid 4 prepared by the directed ortho-lithiation of benzonitrile 6. The overall yield for the six step sequence is 21%.     

Pitch ranking ability of cochlear implant recipients: a comparison of sound-processing strategies

Pitch ranking of sung vowel stimuli, separated in fundamental frequency (F0) by half an octave, was measured with a group of eleven Nucleus 24 cochlear implant recipients using different sound coding strategies. In three consecutive studies, either two or three different sound coding strategies were compared to the Advanced Combinational Encoder (ACE) strategy. These strategies included Continuous Interleaved Sampling (CIS), Peak Derived Timing (PDT), Modulation Depth Enhancement (MDE), F0 Synchronized ACE (FOSync), and Multi-channel Envelope Modulation (MEM), the last four being experimental strategies. While pitch ranking results on average were poor compared to those expected for most normal hearing listeners, significantly higher scores were obtained using the MEM, MDE, and FOSync strategies compared to ACE. These strategies enhanced coding of temporal F0 cues by providing deeper modulation cues to F0 coincidentally in time across all activated electrodes. In the final study, speech recognition tests were also conducted using ACE, CIS, MDE, and MEM. Similar results among all strategies were obtained for word tests in quiet and between ACE and MEM for sentence tests in noise. These findings demonstrate that strategies such as MEM may aid perception of pitch and still adequately code segmental speech features as per existing coding strategies.     

Uniformly more powerful tests for hypotheses about linear inequalities when the variance is unknown

Let X be a -dimensional normal random vector with unknown mean and covariance matrix , where is a known matrix and an unknown parameter. This paper gives a test for the null hypothesis that lies either on the boundary or in the exterior of a closed, convex polyhedral cone versus the alternative hypothesis that lies in the interior of the cone. Our test is uniformly more powerful than the likelihood ratio test.

     

Cytochrome P450 BM-3 in complex with its substrate: Temperature-dependent spin state equilibria in the oxidized and reduced states

In structural studies of cytochrome P450 enzymes, substrates have been seen to bind in a variety of modes; it is important to identify those with the closest resemblance to the configurations adopted during selective oxidation. We attempt here to identify conditions in which the catalytic binding mode of cytochrome P450 BM-3 saturated with N-palmitoylglycine is highly populated. When the substrate binds directly atop the heme, primed for oxidation, displacement of the water ligand is necessary, and thereby the ferric heme is generally converted from low-spin to high-spin. Using both optical spectroscopy and solid-state nuclear magnetic resonance, studying both the full-length enzyme and the isolated heme domain, we show that a high population of the high-spin form is seen at room temperature and above, but not at reduced temperatures. In contrast, the reduced state exhibits high spin throughout the temperature range. The isotropic chemical shift of deuterons in the substrate bound to the oxidized and reduced forms of the enzyme was temperature-dependent, consistent with the presence of a nearby paramagnetic center, but temperature-independent for the diamagnetic CO-bound form, and for the free form of the compound. The reduced (ferrous heme) species shows Curie law dependence of the²H substrate chemical shift with respect to temperature from ?54 to +35 °C, but the oxidized (ferric heme) species showed a pronounced non-Curie dependence in both the²H and the¹³C shift of the substrate’s methyl group, with the effect of the paramagnetic heme at low temperatures being much reduced. These data are consistent with a mixture of at least two binding modes in rapid equilibrium wherein the heme is high-spin at room temperature but low-spin at cryogenic temperatures.     

Estimating the maximum coordination number for spheres of different sizes

Eduljee, G.H. and McDermott, C., 1984. Estimating the maximum coordination number for spheres of different sizes. Fluid Phase Equilibria, 18: 103–108.The maximum coordination number is determined from a physical model in which the coordinatng spheres are packed on the surface of the central sphere to form a series of triangles. The efficiency of packing is given by a parameter α, which is 60° for the closest possible packing and greater than 60° for looser packing. The model is applied to computer-simulated coordination number data using the principles of spherical trigonometry to obtain a value of α that provides the best match. The results compare favourably with those of empirical fitting equations.