8,846 research outputs found
Sulfation and glucuronidation as competing pathways in the metabolism of hydroxamic acids: the role of N,O-sulfonation in chemical carcinogenesis of aromatic amines.
Aromatic amines can be metabolized by N-acetylation and N-hydroxylation to hydroxamic acids; these subsequently are conjugated to form the N,O-sulfonate and N,O-glucuronide conjugates. The N,O-sulfonates are highly labile metabolites that generate reactive intermediates involved in the covalent binding of the parent compound to protein, RNA and DNA, as well as to low molecular compounds like glutathione. This paper discusses methods used to decrease sulfation in vivo, and thereby to enhance the formation of the more stable N,O-glucuronides from N-hydroxy-2-acetylaminofluorene and N-hydroxy-4-acetylamino-4'-fluorobiphenyl. Acetaminophen pretreatment decreases the sulfate availability, but results in many side effects that complicate the analysis of the results. An 8% casein diet reduces the sulfate availability in the rat to approximately 20% of control and thus offers an effective approach to decrease sulfation. The most effective selective inhibition of sulfation is by pentachlorophenol, which very strongly reduces N,O-sulfonation of both hydroxamic acids, and selectively inhibits the formation of DNA adducts that have retained the N-acetyl group. This inhibitor and the related 2,6-dichloro-4-nitrophenol can be employed to study the role of sulfation of hydroxamic acids in initiation and promotion of tumor formation by aromatic amines
Resonant diaphragm pressure measurement system with ZnO on Si excitation
The principle of measuring pressure by means of a resonant diaphragm has been studied. An oscillator consisting of an integrated amplifier with a piezoelectrically driven diaphragm in its feedback loop has been built. The oscillator frequency is accurately proportional to the square of the pressure in the range of 60 to 130 Torr.\ud
The frequency range is 1324 to 1336 Hz (this range being limited by a spurious mode which could be suppressed by better processing) for a 25 mm diameter diaphragm made of a silicon wafer and with PZT ceramics as driver and receptor. We have made an integrated version (1 × 1 mm2) of a square resonant diaphragm pressure guage by selective etching of (1 0 0) planes with ethylenediamine. The piezoelectric driving materials was sputtered zinc oxide. A driver was deposited midway between the bending point and the point of greatest curvature.\ud
A receptor was located at a symmetrical position to give a optimum transfer condition.\ud
The integrated current amplifier had a low impedance differential input stage, two gain cells and a high impedance output stage. These electrical conditions ensured maximum elastic freedom of the diaphragm. A digital circuit in I2L technology has been designed and made with eight-bit parallel read out of the frequency. This circuit may be directly connected to a microprocessor. The whole system contains the sensor chip, the analog amplifier chip and the digital chip, all in compatible technology.\ud
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Spiral order by disorder and lattice nematic order in a frustrated Heisenberg antiferromagnet on the honeycomb lattice
Motivated by recent experiments on BiMnO(NO), we study a
frustrated - Heisenberg model on the two dimensional (2D) honeycomb
lattice. The classical - Heisenberg model on the two dimensional (2D)
honeycomb lattice has N\'eel order for , it
exhibits a one-parameter family of degenerate incommensurate spin spiral ground
states where the spiral wave vector can point in any direction. Spin wave
fluctuations at leading order lift this accidental degeneracy in favor of
specific wave vectors, leading to spiral order by disorder. For spin ,
quantum fluctuations are, however, likely to be strong enough to melt the
spiral order parameter over a wide range of . Over a part of this
range, we argue that the resulting state is a valence bond solid (VBS) with
staggered dimer order - this VBS is a nematic which breaks lattice rotational
symmetry. Our arguments are supported by comparing the spin wave energy with
the energy of the dimer solid obtained using a bond operator formalism. Turning
to the effect of thermal fluctuations on the spiral ordered state, any nonzero
temperature destroys the magnetic order, but the discrete rotational symmetry
of the lattice remains broken resulting in a thermal analogue of the nematic
VBS. We present arguments, supported by classical Monte Carlo simulations, that
this nematic transforms into the high temperature symmetric paramagnet via a
thermal phase transition which is in the universality class of the classical
3-state Potts (clock) model in 2D. We discuss the possible relevance of our
results for honeycomb magnets, such as BiMO(NO) (with
M=Mn,V,Cr), and bilayer triangular lattice magnets.Comment: Slightly revise
Adsorbent filled membranes for gas separation. Part 1. Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents
The effect of the introduction of specific adsorbents on the gas separation properties of polymeric membranes has been studied. For this purpose both carbon molecular sieves and zeolites are considered. The results show that zeolites such as silicate-1, 13X and KY improve to a large extent the separation properties of poorly selective rubbery polymers towards a mixture of carbon dioxide/methane. Some of the filled rubbery polymers achieve intrinsic separation properties comparable to cellulose acetate, polysulfone or polyethersulfone. However, zeolite 5A leads to a decrease in permeability and an unchanged selectivity. This is due to the impermeable character of these particles, i.e. carbon dioxide molecules cannot diffuse through the porous structure under the conditions applied. Using silicate-1 also results in an improvement of the oxygen/nitrogen separation properties which is mainly due to a kinetic effect. Carbon molecular sieves do not improve the separation performances or only to a very small extent. This is caused by a mainly dead-end (not interconnected) porous structure which is inherent to their manufacturing process
A model for the orientational ordering of the plant microtubule cortical array
The plant microtubule cortical array is a striking feature of all growing
plant cells. It consists of a more or less homogeneously distributed array of
highly aligned microtubules connected to the inner side of the plasma membrane
and oriented transversely to the cell growth axis. Here we formulate a
continuum model to describe the origin of orientational order in such confined
arrays of dynamical microtubules. The model is based on recent experimental
observations that show that a growing cortical microtubule can interact through
angle dependent collisions with pre-existing microtubules that can lead either
to co-alignment of the growth, retraction through catastrophe induction or
crossing over the encountered microtubule. We identify a single control
parameter, which is fully determined by the nucleation rate and intrinsic
dynamics of individual microtubules. We solve the model analytically in the
stationary isotropic phase, discuss the limits of stability of this isotropic
phase, and explicitly solve for the ordered stationary states in a simplified
version of the model.Comment: 15 pages, 5 figure
Preparation of zeolite filled glassy polymer membranes
The incorporation of zeolite particles in the micrometer range into polymeric matrices was investigated as a way to improve the gas separation properties of the polymer materials used in the form of membranes. The adhesion between the polymer phase and the external surface of the particles appeared to be a major problem in the preparation of such membranes when the polymer is in the glassy state at room temperature. Various methods were investigated to improve the internal membrane structure, that is, surface modification of the zeolite external surface, preparation above the glass-transition temperature, and heat treatment. Improved structures were obtained as observed by scanning electron microscopy, but the influence on the gas separation properties was not in agreement with the observed structural improvements
On the size and shape of excluded volume polymers confined between parallel plates
A number of recent experiments have provided detailed observations of the
configurations of long DNA strands under nano-to-micrometer sized confinement.
We therefore revisit the problem of an excluded volume polymer chain confined
between two parallel plates with varying plate separation. We show that the
non-monotonic behavior of the overall size of the chain as a function of
plate-separation, seen in computer simulations and reproduced by earlier
theories, can already be predicted on the basis of scaling arguments. However,
the behavior of the size in a plane parallel to the plates, a quantity observed
in recent experiments, is predicted to be monotonic, in contrast to the
experimental findings. We analyze this problem in depth with a mean-field
approach that maps the confined polymer onto an anisotropic Gaussian chain,
which allows the size of the polymer to be determined separately in the
confined and unconfined directions. The theory allows the analytical
construction of a smooth cross-over between the small plate-separation de
Gennes regime and the large plate-separation Flory regime. The results show
good agreement with Langevin dynamics simulations, and confirm the scaling
predictions.Comment: 15 pages, 3 figure
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