Coherent single pion production by antineutrino charged current interactions and test of PCAC

The cross section for coherent production of a single π^? meson in charged current antineutrino interactions on neon nuclei has been measured in BEBC to be (175±25) 10^?40 cm²/neon nucleus, averaged over the energy spectrum of the antineutrino wide band beam at the CERN SPS; this corresponds to (0.9±0.1) % of the total charged current \(\bar v_\mu \) cross section. The distributions of kinematical variables are in agreement with theoretical predictions based on the PCAC hypothesis and the meson dominance model; in particular, theQ ² dependence is well described by a propagator containing a massm=(1.35±0.18) GeV. The absolute value of the cross section is also in agreement with the model. This analysis thus provides a test of the PCAC hypothesis in the antineutrino energy range 5–150 GeV.     

Oxidation of nitrided si(100) by gaseous atomic and molecular oxygen

The nitridation of Si(100) by ammonia and the subsequent oxidation of the nitrided surface by both gaseous atomic and molecular oxygen was investigated under ultrahigh vacuum (UHV) conditions using X-ray photoelectron spectroscopy (XPS). Nitridation of Si(100) by the thermal decomposition of NH3 results in the formation of a subsurface nitride and a decrease in the concentration of surface dangling bond sites. On the basis of changes in the N1s spectra obtained after NH3 adsorption and decomposition, we estimate that the nitride resides about four to five layers below the vacuum-solid interface and that the concentration of surface dangling bonds after nitridation is only 59% of its value on Si(100)-(2 x 1). Oxidation of the nitrided surface is found to produce an oxide phase that remains in the outer layers of the solid and interacts only weakly with the underlying nitride for oxygen coverages up to 2.5 ML. Slight changes in the N1s spectra observed after oxidizing at 300 K are suggested to arise primarily from the introduction of strain within the nitride, and by the formation of a small amount of Si2=N-O species near the nitride-oxide interface. The nitrogen bonding environment changes negligibly after oxidizing at 800 K, which is indicative of greater phase separation at elevated surface temperature. Nitridation is also found to significantly reduce the reactivity of the Si(100) surface toward both atomic and molecular oxygen. A comparison of the oxygen uptake on the clean and nitrided surfaces shows quantitatively that the decrease in dangling bond concentration is responsible for the reduced activity of the nitrided surface toward oxidation, and therefore that dangling bonds are the initial adsorption site for both gaseous oxygen atoms and molecules. Increasing the surface temperature is found to promote the uptake of oxygen when O2 is used as the oxidant, but brings about only a small enhancement in the uptake of gaseous O-atoms. The different effects of surface temperature on the uptake of O versus O2 are interpreted in terms of the efficiency at which dangling bond pairs are regenerated on the surface at elevated temperature and the different site requirements for the adsorption of O and O2.     

Pincer-porphyrin hybrids

[reaction: see text] Starting from tetrakis(3,5-bis(bromomethyl)phenyl)porphyrin, pincer-porphyrin hybrid molecules (tetrakis(ECE-pincer)porphyrin; E = N, P, S) based on a tetraphenylporphyrin skeleton have been prepared in high yields. These multi-ligand site compounds could be selectively metalated at their peripheries, which was shown by X-ray crystallography.     

Dual quartz crystal microbalance compensation using a submerged reference crystal. Effect of surface roughness and liquid properties

Absolute resonant frequency measurements were made on gold-coated AT-cut quartz crystals with one face in contact with a series of liquids. The effect of surface roughness and liquid properties (viscosity and density) was analyzed in terms of a “trapped liquid” model. In this model, liquid present in surface imperfections is viewed as rigidly coupled mass. In some of the literature this density-dependent, but not viscosity-dependent, term is viewed as being additive to the hydrodynamic shift seen for a smooth surface. Data obtained using 1 μm and 5 μm surface finish crystals are inconsistent with the predictions of the trapped liquid model. This suggests hydrodynamic coupling between liquid internal and external to the crevices. Despite the lack of a theoretical model for the liquid motion, it is possible to compensate for frequency variations resulting from changing liquid properties and for roughness effects by making direct measurements of the resonant frequency difference between two crystals exposed to the same solution. This novel procedure works to the extent that the two crystals have similar surface topographies. Compensation is excellent for 1 μm finish crystals and good for 5 μm finish crystals.     

Mapping of possible prion protein self-interaction domains using peptide arrays

Background  

The common event in transmissible spongiform encephalopathies (TSEs) or prion diseases is the conversion of host-encoded protease sensitive cellular prion protein (PrP^C) into strain dependent isoforms of scrapie associated protease resistant isoform (PrP^Sc) of prion protein (PrP). These processes are determined by similarities as well as strain dependent variations in the PrP structure. Selective self-interaction between PrP molecules is the most probable basis for initiation of these processes, potentially influenced by chaperone molecules, however the mechanisms behind these processes are far from understood. We previously determined that polymorphisms do not affect initial PrP^C to PrP^Sc binding but rather modulate a subsequent step in the conversion process. Determining possible sites of self-interaction could elucidate which amino acid(s) or amino acid sequences contribute to binding and further conversion into other isoforms. To this end, ovine – and bovine PrP peptide-arrays consisting of 15-mer overlapping peptides were probed with recombinant sheep PrP^C fused to maltose binding protein (MBP-PrP).     

OPTICAL AND THERMAL CHARACTERIZATION OF NATURAL (Sepia officinalis) MELANIN

Abstract The optical properties and the thermal diffusivity of natural cuttlefish ( Sepia officinalis ) melanin have been measured. The optical absorption and scattering properties of melanin particles were determined at 580 nm and 633 nm, using photometric and photothermal techniques. For the photometric studies, the absorption and the transport scattering coefficients were determined from the measurements of diffuse reflectance and transmittance. The scattering anisotropy was obtained from an additional measurement of the total attenuation coefficient and independently obtained by goniometry. For photothermal studies, pulsed photothermal radiometry was used to deduce the absorption and transport scattering coefficients via a model based on optical diffusion theory. Pulsed photothermal radiometry was also used to provide the thermal diffusivity of solid melanin pressed pellets.     

One‐Dimensional,Cofacial Porphyrin Polymers Formed by Self‐Assembly of meso‐Tetrakis(ERE Donor) Zinc(II) Porphyrins

A series of meso‐tetrakis‐(ERE donor) zinc(II) porphyrins n Zn (ERE donor=4‐R‐3,5‐bis[(E)‐methyl]phenyl; 1 Zn: E=NMe₂, R=Br; 2 Zn: E=NMe₂, R=H; 3 Zn: E=OMe, R=Br; 4 Zn: E=OMe, R=H) have been synthesized in excellent yields. As a result of the combination of a Lewis acidic site and eight Lewis basic sites within one molecule, monomeric molecules of n Zn self‐assemble to form one‐dimensional porphyrin polymers [ n Zn]_∞ in the solid state, as confirmed for 1 Zn and 3 Zn by X‐ray crystallography. The coordination environment around the zinc(II) ions in these polymers is octahedral. They are ligated by four equatorial nitrogen atoms of the porphyrin and two apical E atoms (E=N, O) provided by the EBrE donor groups of adjacent n Zn molecules. Complexes 2 Zn and 4 Zn did not form single crystals, but solid‐state UV/Vis analysis points to the formation of similar structures. Solution UV/Vis and ¹H NMR spectroscopy indicated that interactions between 1 Zn and 2 Zn monomers in the polymers are stronger than between 3 Zn and 4 Zn monomers. Interestingly, they also revealed that the presence of a neighboring bromine atom in the EBrE donor groups has a considerable influence on the coordination properties of the benzylic N or O atoms. The zinc(II) ions of the porphyrins most likely adopt only hexacoordination in the solid state, owing to the unique predisposition of Lewis acidic and basic sites in the n Zn molecules. Several parameters of the aggregates, for example, the interplanar separation between porphyrins and the zinc–zinc distances, change as a function of the coordinating E groups. The high degree of modularity in their synthesis makes these zinc(II) porphyrins an interesting new entry in noncovalent multiporphyrin assemblies.     

Lithium bromide in acetonitrile: Thermodynamics,theory, and simulation

A variety of methods has been used for the study of lithium bromide solutions in acetonitrile yielding by their combination reliable information on different levels of approximation. Osmotic coefficients based on precise vapor pressure measurements are reproduced by CM (chemical model) and HNC (hypernetted chain) calculations and by BD (brownian dynamics) simulations. The results of neutron scattering experiments are treated with the help of HNC and BD methods. Hartree-Fock calculations on isolated LiBr pairs and solvated lithium ions yield reliable particle distances and reveal the geometry of the lithium solvation sphere.     

Solid-phase synthesis of dihydrovirginiamycin S1, a streptogramin B antibiotic

We describe the first solid-phase synthesis of dihydrovirginiamycin S(1), a member of the streptogramin B family of antibiotics, which are nonribosomal-peptide natural products produced by Streptomyces. These compounds, along with the synergistic group A components, are "last line of defense" antimicrobial agents for the treatment of life-threatening infections such as vancomycin-resistant enterococci. The synthesis features an on-resin cyclization and is designed to allow production of streptogramin B analogues with diversification at positions 1', 1, 2, 3, 4, and 6. Several synthetic challenges known to hinder the synthesis of this class of compounds were solved, including sensitivity to acids and bases, and epimerization and rearrangements, through the judicious choice of deprotection conditions, coupling conditions, and synthetic strategy. This work should enable a better understanding of structure-activity relationships in the streptogramin B compounds, possible identification of analogues that bypass known resistance mechanisms, and perhaps the identification of analogues with novel biological activities.     

Evidence for a trigonal twist mechanism involving a phosphite ligand of Os3(CO)7,[P(OMe)3]5

The fluxional molecule Os₃(CO)₇[P(OMe)₃]₅ has been prepared from OS₃(CO)₁₂ and P(OMe)₃ by a combination of thermal and UV irradiation synthetic methods. An investigation by ³¹P and ¹³C NMR spectroscopy indicated that the mechanism of fluxionality in this compound probably involves the P(OMe)₃ ligand of the Os(CO)₃[P(OMe)₃] unit moving from one equatorial site to the other via a trigonal twist mechanism.