Diagram,valence-to-core,and hypersatellite Kβ X-ray transitions in metallic chromium

We report on measurements of the Kβ diagram, valence-to-core (VtC), and hypersatellite X-ray spectra induced in metallic Cr by photon single and double K-shell ionization. The experiment was carried out at the Stanford Synchrotron Radiation Lightsource using the seven-crystal Johann-type hard X-ray spectrometer of the beamline 6-2. For the Kβ diagram and VtC transitions, the present study confirms the line shape features observed in previous works, whereas the K^hβ hypersatellite transition was found to exhibit a complex spectral line shape and a characteristic low-energy shoulder. The energy shift of the hypersatellite relative to the parent diagram line was deduced from the measurements and compared with the result of extensive multiconfiguration Dirac–Fock (MCDF) calculations. A very good agreement between experiment and theory was found. The MCDF calculations were also used to compute the theoretical line shape of the hypersatellite. A satisfactory agreement was obtained between the overall shapes of the experimental and theoretical spectra, but deviations were observed on the low- and high-energy flanks of the hypersatellite line. The discrepancies were explained by chemical effects, which were not considered in the MCDF calculations performed for isolated atoms.     

Bi-Lipschitz homeomorphic subanalytic sets have bi-Lipschitz homeomorphic tangent cones

We prove that if there exists a bi-Lipschitz homeomorphism (not necessarily subanalytic) between two subanalytic sets, then their tangent cones are bi-Lipschitz homeomorphic. As a consequence of this result, we show that any Lipschitz regular complex analytic set, i.e., any complex analytic set which is locally bi-Lipschitz homeomorphic to an Euclidean ball must be smooth. Finally, we give an alternative proof of S. Koike and L. Paunescu’s result about the bi-Lipschitz invariance of directional dimensions of subanalytic sets.     

Brasiliensic and isobrasiliensic acids: isolation from Calophyllum brasiliense Cambess. and anti-Helicobacter pylori activity

The occurrence of chromanone derivatives has been noticed as a distinctive feature of the genus Calophyllum (Calophyllaceae). Previous studies have demonstrated that the extract of the stem bark of Calophyllum brasiliense and its chromanone-rich fractions show anti-ulcer activity in murine gastric ulcer models. In this work, brasiliensic and isobrasiliensic acids, the two main compounds of the n-hexane extract of the stem bark extract of C. brasiliense, were isolated by flash chromatography using silica gel impregnated with silver nitrate and their structures were elucidated by NMR techniques and mass spectrometry. ¹³C NMR data is available for the first time for both compounds. Brasiliensic and isobrasiliensic acids showed good in vitro bacteriostatic activity against Helicobacter pylori, and are responsible, at least in part, for the bacteriostatic anti-H. pylori activity of the n-hexane extract of the stem bark of C. brasiliense.     

Synthesis and characterization of nanostructured iron compounds prepared from the decomposition of iron pentacarbonyl dispersed into carbon materials with varying porosities

This work describes the production and characterization of carbon-iron nanocomposites obtained from the decomposition of iron pentacarbonyl (Fe(CO)₅) mixed with different carbon materials: a high surface area activated carbon (AC), powdered graphite (G), milled graphite (MG), and carbon black (CB). The nanocomposites were prepared either under argon or in ambient atmosphere, with a fixed ratio of Fe(CO)₅ (4.0 mL) to carbon precursor (2.0 g). The images of scanning electron microscopy and the analysis of textural properties indicated the presence of nanostructured Fe compounds homogeneously dispersed into the different classes of pores of the carbon matrices. The elemental Fe content was always larger for samples prepared in ambient atmosphere, reaching values in the range of 20–32 wt%. On the other hand, samples prepared under argon showed reduced Fe content, with values in the range 5–10 wt% for samples prepared from precursors with low surface area (G, MG, and CB) and a much higher value (~19 wt%) for samples prepared from the precursor of high surface area (AC). Mössbauer spectroscopy and X-ray diffractometry showed that the nanoparticles were mostly composed of iron oxides in the case of the samples prepared in oxygen-rich ambient atmosphere and also for the AC-derived nanocomposite prepared under argon, which is consistent with the large oxygen content of this precursor. For the other precursors, with reduced or no oxygen content, metallic iron and iron carbides were found to be the dominant phases in samples prepared under oxygen-free atmosphere. The samples prepared in ambient atmosphere and the AC-derived sample prepared under argon exhibited superparamagnetic behavior at room temperature, as revealed by temperature-dependent magnetization curves and Mössbauer spectroscopy.     

Comparative investigation of the cleavage step in the synthesis of model peptide resins: implications for Nalpha-9-fluorenylmethyloxycarbonyl-solid phase peptide synthesis

Based on our studies of the stability of model peptide-resin linkage in acid media, we previously proposed a rule for resin selection and a final cleavage protocol applicable to the Nalpha-tert-butyloxycarbonyl (Boc)-peptide synthesis strategy. We found that incorrect choices resulted in decreases in the final synthesis yield, which is highly dependent on the peptide sequence, of as high as 30%. The present paper continues along this line of research but examines the Nalpha-9-fluorenylmethyloxycarbonyl (Fmoc)-synthesis strategy. The vasoactive peptide angiotensin II (AII, DRVYIHPF) and its [Gly8]-AII analogue were selected as model peptide resins. Variations in parameters such as the type of spacer group (linker) between the peptide backbone and the resin, as well as in the final acid cleavage protocol, were evaluated. The same methodology employed for the Boc strategy was used in order to establish rules for selection of the most appropriate linker-resin conjugate or of the peptide cleavage method, depending on the sequence to be assembled. The results obtained after treatment with four cleavage solutions and with four types of linker groups indicate that, irrespective of the circumstance, it is not possible to achieve complete removal of the peptide chains from the resin. Moreover, the Phe-attaching peptide at the C-terminal yielded far less cleavage (50-60%) than that observed with the Gly-bearing sequences at the same position (70-90%). Lastly, the fastest cleavage occurred with reagent K acid treatment and when the peptide was attached to the Wang resin.     

Rigidity of critical circle mappings I

We prove that two C ³ critical circle maps with the same rotation number in a special set ? are C ^1+α conjugate for some α>0 provided their successive renormalizations converge together at an exponential rate in the C ⁰ sense. The set ? has full Lebesgue measure and contains all rotation numbers of bounded type. By contrast, we also give examples of C ^∞ critical circle maps with the same rotation number that are not C ^1+β conjugate for any β>0. The class of rotation numbers for which such examples exist contains Diophantine numbers. Received November 1, 1998 / final version received July 7, 1999     

Fermi acceleration with memory-dependent excitation

Some scaling properties for a classical particle confined to bounce between two walls, where one wall is fixed and the other one moves in time according to a random signal with a memory length are studied. We have considered two different kinds of collisions of the particle with the moving wall namely: (i) elastic and (ii) inelastic. The dynamics of the model is described in terms of a two-dimensional nonlinear mapping. For the case of elastic collisions, we show that the memory of the stochastic signal affects directly the behaviour of the average velocity of the particle. It then exhibits different slopes for the average velocity at different stages of the series with β≅3/4 for a short time, β≅1 for the average stage and β≅1/2 for a long time, as predicted by the Central Limit Theorem, therefore leading to the Fermi acceleration. The situation where inelastic collisions are taken into account yields a more drastic change, particularly suppressing the Fermi acceleration.