6,874 research outputs found
Fat transforms ascorbic acid from inhibiting to promoting acid-catalysed N-nitrosation
<b>Background</b>: The major potential site of acid nitrosation is the proximal stomach, an anatomical site prone to
a rising incidence of metaplasia and adenocarcinoma. Nitrite, a pre-carcinogen present in saliva, can be
converted to nitrosating species and N-nitroso compounds by acidification at low gastric pH in the presence
of thiocyanate.
<b>Aims</b>: To assess the effect of lipid and ascorbic acid on the nitrosative chemistry under conditions simulating
the human proximal stomach.
<b>Methods</b>: The nitrosative chemistry was modelled in vitro by measuring the nitrosation of four secondary
amines under conditions simulating the proximal stomach. The N-nitrosamines formed were measured by gas
chromatography–ion-trap tandem mass spectrometry, while nitric oxide and oxygen levels were measured
amperometrically.
<b>Results</b>: In absence of lipid, nitrosative stress was inhibited by ascorbic acid through conversion of nitrosating
species to nitric oxide. Addition of ascorbic acid reduced the amount of N-nitrosodimethylamine formed by
fivefold, N-nitrosomorpholine by .1000-fold, and totally prevented the formation of N-nitrosodiethylamine
and N-nitrosopiperidine. In contrast, when 10% lipid was present, ascorbic acid increased the amount of Nnitrosodimethylamine,
N-nitrosodiethylamine and N-nitrosopiperidine formed by approximately 8-, 60- and
140-fold, respectively, compared with absence of ascorbic acid.
<b>Conclusion</b>: The presence of lipid converts ascorbic acid from inhibiting to promoting acid nitrosation. This
may be explained by nitric oxide, formed by ascorbic acid in the aqueous phase, being able to regenerate
nitrosating species by reacting with oxygen in the lipid phase
Different mechanics of snap-trapping in the two closely related carnivorous plants Dionaea muscipula and Aldrovanda vesiculosa
The carnivorous aquatic Waterwheel Plant (Aldrovanda vesiculosa L.) and the
closely related terrestrial Venus Flytrap (Dionaea muscipula SOL. EX J. ELLIS)
both feature elaborate snap-traps, which shut after reception of an external
mechanical stimulus by prey animals. Traditionally, Aldrovanda is considered as
a miniature, aquatic Dionaea, an assumption which was already established by
Charles Darwin. However, videos of snapping traps from both species suggest
completely different closure mechanisms. Indeed, the well-described snapping
mechanism in Dionaea comprises abrupt curvature inversion of the two trap
lobes, while the closing movement in Aldrovanda involves deformation of the
trap midrib but not of the lobes, which do not change curvature. In this paper,
we present the first detailed mechanical models for these plants, which are
based on the theory of thin solid membranes and explain this difference by
showing that the fast snapping of Aldrovanda is due to kinematic amplification
of the bending deformation of the midrib, while that of Dionaea unambiguously
relies on the buckling instability that affects the two lobes.Comment: accepted in Physical Review
Semiconductor-enriched single wall carbon nanotube networks applied to field effect transistors
Substantial progress on field effect transistors "FETs" consisting of
semiconducting single wall carbon nanotubes "s-SWNTs" without detectable traces
of metallic nanotubes and impurities is reported. Nearly perfect removal of
metallic nanotubes is confirmed by optical absorption, Raman measurements, and
electrical measurements. This outstanding result was made possible in
particular by ultracentrifugation (150 000 g) of solutions prepared from SWNT
powders using polyfluorene as an extracting agent in toluene. Such s-SWNTs
processable solutions were applied to realize FET, embodying randomly or
preferentially oriented nanotube networks prepared by spin coating or
dielectrophoresis. Devices exhibit stable p-type semiconductor behavior in air
with very promising characteristics. The on-off current ratio is 10^5, the
on-current level is around 10 A, and the estimated hole mobility is larger
than 2 cm2 / V s
Pressure-Induced Interlinking of Carbon Nanotubes
We predict new forms of carbon consisting of one and two dimensional networks
of interlinked single wall carbon nanotubes, some of which are energetically
more stable than van der Waals packing of the nanotubes on a hexagonal lattice.
These interlinked nanotubes are further transformed with higher applied
external pressures to more dense and complicated stable structures, in which
curvature-induced carbon sp re-hybridizations are formed. We also discuss
the energetics of the bond formation between nanotubes and the electronic
properties of these predicted novel structures.Comment: 4 pages, 4 postscript figures; To be appear in PR
Quantum Conductance Steps in Solutions of Multiwalled Carbon Nanotubes
We have prepared solutions of multiwalled carbon nanotubes in Aroclor 1254, a
mixture of polychlorinated biphenyls. The solutions are stable at room
temperature. Transport measurements were performed using a scanning--tunneling
probe on a sample prepared by spin--coating of the solution on gold substrates.
Conductance steps were clearly seen. An histogram of a high number of traces
shows maximum peaks at integer values of the conductance quantum , demonstrating ballistic transport at room temperature along the carbon
nanotube over distances longer than .Comment: 4 pages and 2 figure
Metal-to-semiconductor transition in squashed armchair carbon nanotubes
We investigate electronic transport properties of the squashed armchair
carbon nanotubes, using tight-binding molecular dynamics and Green's function
method. We demonstrate a metal-to-semiconductor transistion while squashing the
nanotubes and a general mechanism for such transistion. It is the distinction
of the two sublattices in the nanotube that opens an energy gap near the Fermi
energy. We show that the transition has to be achieved by a combined effect of
breaking of mirror symmetry and bond formation between the flattened faces in
the squashed nanotubes.Comment: 4 papges, 4 figures, to appear in Phys. Rev. Let
A Monolithic Time Stretcher for Precision Time Recording
Identifying light mesons which contain only up/down quarks (pions) from those
containing a strange quark (kaons) over the typical meter length scales of a
particle physics detector requires instrumentation capable of measuring flight
times with a resolution on the order of 20ps. In the last few years a large
number of inexpensive, multi-channel Time-to-Digital Converter (TDC) chips have
become available. These devices typically have timing resolution performance in
the hundreds of ps regime. A technique is presented that is a monolithic
version of ``time stretcher'' solution adopted for the Belle Time-Of-Flight
system to address this gap between resolution need and intrinsic multi-hit TDC
performance.Comment: 9 pages, 15 figures, minor corrections made, to appear as JINST_008
Pressure dependence of the thermoelectric power of single-walled carbon nanotubes
We have measured the thermoelectric power (S) of high purity single-walled
carbon nanotube mats as a function of temperature at various hydrostatic
pressures up to 2.0 GPa. The thermoelectric power is positive, and it increases
in a monotonic way with increasing temperature for all pressures. The low
temperature (T < 40 K) linear thermoelectric power is pressure independent and
is characteristic for metallic nanotubes. At higher temperatures it is enhanced
and though S(T) is linear again above about 100 K it has a nonzero intercept.
This enhancement is strongly pressure dependent and is related to the change of
the phonon population with hydrostatic pressure.Comment: 4 pages, 3 figure
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