Analogue simulation of stochastic parametric oscillators

The problem of a harmonic oscillator subject to both additive and multiplicative gaussian noise is solved analytically by means of an iteration procedure. Our solution agrees very closely with earlier predictions available in the literature. The reliability of perturbation approaches to stochastic systems that do not verify the detailed balance conditions is criticized by comparison with exact results worked out for this peculiar model. The effects due to finite correlation times of the fluctuations are also accounted for analytically. All of our predictions have been checked by means of analogue simulation.     

Cis-UROCANIC ACID SYNERGIZES WITH HISTAMINE FOR INCREASED PGE2 PRODUCTION BY HUMAN KERATINOCYTES: LINK TO INDOMETHACIN-INHIBITABLE UVB-INDUCED IMMUNOSUPPRESSION

Abstract— There is considerable evidence that suppression of the immune system by UVB (280–320 nm UV) irradiation is initiated by UVB-dependent isomerization of a specific skin photoreceptor, urocanic acid (UCA), from the trans to the cis form. Previous studies have confirmed that cis -UCA administration to mice 3–5 days prior to hapten sensitization at a distant site, suppresses the contact hypersensitivity (CHS) response upon challenge. This study demonstrates in mice that cis -UCA, like UVB, suppresses CHS to trinitrochlorobenzene by a mechanism partly dependent on prostanoid production. In vitro experimentation showed that human keratinocytes, isolated from neonatal foreskin, increased prostaglandin E₂ (PGE₂) production in response to histamine but not UCA alone. However, cis -UCA synergized with histamine for increased PGE₂ production by keratinocytes. cis -urocanic acid also increased the sensitivity of keratinocytes for PGE₂ production in response to histamine. Prostaglandin E₂ from keratinocytes exposed to cis -UCA and histamine may contribute directly, or indirectly, to the regulation of CHS responses by UVB irradiation.     

Core edge energy loss studies of Pd clusters on graphite

In order to verify that the d-band of Pd clusters moves towards higher binding energies when the cluster size decreases, we have carried out energy loss measurements of the M_4,5 core edge of Pd clusters with different diameters evaporated on polycristalline graphite. Generally the shifts have been attributed in the past both to initial and or final state effects. In the last case one could not observe variations with our technique which probes the partial unfilled density of states. Our experimental results show an enhancement of the absorption and a narrowing of the lineshape with increasing cluster size. We attribute the observed effects to a genuine change in the electronic structure which occurs in the initial state of the cluster. Comparative studies of this effect have been performed following the evolution of the M_4,5 VV Auger levels and the collective excitations in the range 0–30 eV far from the elastic peak.     

Catalytic and spectroscopic properties of cytochrome-c, horseradish peroxidase, and ascorbate oxidase embedded in a sol-gel silica matrix as a function of gelation time

In this study, we investigated the optical features of the redox metal-dependent proteins cytochrome-c, horseradish peroxidase (HRP), and ascorbate oxidase embedded in a sol-gel-processed silica matrix as a function of gelation time. Circular dichroism, absorbance, and fluorescence spectroscopies revealed that the sol-gel process affects the complex structure of the dimeric ascorbate oxidase (although the prosthetic coppers still remain bound to the enzyme) but not that of monomeric cytochrome-c and HRP. Any modifications in ascorbate oxidase occurred in the initial gelation phase; the drying process induced no further alterations and the enzyme remained stable for months. Unfolding-refolding experiments on cytochrome-c revealed severely restricted motility in the protein moiety in the xerogel, the concentrated matrix that forms after drying. The diffusion time of the solvent within the matrix, which regulated the enzyme-substrate reaction rate, depended on the thickness of the monolith, not on the dryness of the specimen.     

Study of the dynamics of water/aerosol OT/n-heptane system by dielectric relaxation measurements

Most of the previous dielectric studies on microemulsions and related systems have been conducted at relatively low frequencies where the dielectric response is sensitive to dynamic percolation phenomena. There is a lack of experimental studies at microwave frequencies where water plays a central role in dielectric relaxation. In this paper the dielectric complex permittivity of water/Aerosol OT/n-heptane microemulsions has been measured by a frequency domain coaxial technique in the range 0.02–20 GHz as a function of molar ratioW = [water]/[AOT] at low volume fraction of the dispersed phase (=0.1 and =0.2). The data show two dielectric dispersions: The first located in the 100 MHz frequency region and the second at frequencies higher than 20 GHz. The evolution of the dielectric parameters of these relaxations has been studied as a function of the molar ratioW in the range 0<W<28.     

Ozone adsorption on carbon nanotubes: the role of Stone-Wales defects

First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.     

Soft-x-ray photoemission spectroscopy and ab initio studies on the adsorption of NO2 molecules on defective multiwalled carbon nanotubes

The adsorption of NO(2) molecules on defective multiwalled carbon nanotubes has been studied by soft-x-ray photoemission. The valence band and carbon core-level spectra have been acquired before, during, and after NO(2) exposure. The spectra show a reversible decrease of the density of states at the top of the valence band when NO(2) molecules are adsorbed on the (carbon nanotubes) CNTs. No shift of the C 1s spectra has been observed. Theoretical calculations, using density-functional theory, have been performed on the CNT + NO(2) system, considering semiconducting nanotubes with different diameters and introducing a Stone-Wales [Chem. Phys. Lett. 128, 501 (1986)] defect. The calculation confirms the decrease of the density of states at the top of the valence band in the CNT + NO(2) system, while close to the adsorption site new states appear very close to the Fermi level.