6,251 research outputs found
Influence of supercoiling on the disruption of dsDNA
We propose that supercoiling energizes double-stranded DNA (dsDNA) so as to
facilitate thermal fluctuations to an unzipped state. We support this with a
model of two elastic rods coupled via forces that represent base pair
interactions. Supercoiling is shown to lead to a spatially localized higher
energy state in a small region of dsDNA consisting of a few base pairs. This
causes the distance between specific base pairs to be extended, enhancing the
thermal probability for their disruption. Our theory permits the development of
an analogy between this unzipping transition and a second order phase
transition, for which the possibility of a new set of critical exponents is
identified
Methanesulfonic Acid in Coastal Antarctic Snow Related to Sea‐ice Extent
Proxy records of biogenic sulfur gas obtained from ice cores suggest that variability in marine biogenic sulfur emissions may reflect changes in climate [Saigne and Legrand, 1987; Legrand et al., 1988, Legrand et al., 1991; Anderson and Charlson, 1991]. Increased sea‐ice extent has previously been proposed as one cause of relatively high methanesulfonic acid (MSA) in glacial‐age ice core samples [Gibson et al., 1990]. We have analyzed MSA, one of the oxidation products of the biogenic sulfur gas dimethylsulfide [Hatakeyama et al., 1985], from snowpit samples recovered from a coastal site in Southern Victoria Land, Antarctica. Time series of MSA correlate significantly with the longest continuous record available of Southern Ocean sea‐ice extent (two decades) [Jacka, 1990]
The Community Authorization Service: Status and Future
Virtual organizations (VOs) are communities of resource providers and users
distributed over multiple policy domains. These VOs often wish to define and
enforce consistent policies in addition to the policies of their underlying
domains. This is challenging, not only because of the problems in distributing
the policy to the domains, but also because of the fact that those domains may
each have different capabilities for enforcing the policy. The Community
Authorization Service (CAS) solves this problem by allowing resource providers
to delegate some policy authority to the VO while maintaining ultimate control
over their resources. In this paper we describe CAS and our past and current
implementations of CAS, and we discuss our plans for CAS-related research.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003. 9 Pages, PD
Nonlinear Magneto-Optical Rotation of Elliptically Polarized Light
We predict theoretically and demonstrate experimentally an ellipticity-dependent nonlinear magneto-optic rotation of elliptically-polarized light propagating in a coherent atomic medium. We show that this effect results from a hexadecapole and higher order momenta of atomic coherence, and is associated with an enhancement of Kerr and higher orders nonlinearities accompanied by suppression of the other linear and nonlinear susceptibility terms of the medium. These nonlinearities might be useful for quantum signal processing. In particular, we report an observation of an enhancement the polarization rotation of elliptically polarized light resonant with the 5S_{1/2} F=2 -> 5P_{1/2} F=1 transition of Rb87
Effect of Magnetization Inhomogeneity on Magnetic Microtraps for Atoms
We report on the origin of fragmentation of ultracold atoms observed on a
permanent magnetic film atom chip. A novel technique is used to characterize
small spatial variations of the magnetic field near the film surface using
radio frequency spectroscopy of the trapped atoms. Direct observations indicate
the fragmentation is due to a corrugation of the magnetic potential caused by
long range inhomogeneity in the film magnetization. A model which takes into
account two-dimensional variations of the film magnetization is consistent with
the observations.Comment: 4 pages, 4 figure
Enhancement of Kerr nonlinearity via multi-photon coherence
We propose a new method of resonant enhancement of optical Kerr nonlinearity
using multi-level atomic coherence. The enhancement is accompanied by
suppression of the other linear and nonlinear susceptibility terms of the
medium. We show that the effect results in a modification of the nonlinear
Faraday rotation of light propagating in an Rb87 vapor cell by changing the
ellipticity of the light.Comment: 4 pages, 3 figures Submitted to Optics Letter
[18F]Fluoro-2-Deoxy-D-Glucose Incorporation by MCF-7 Breast Tumour Cells In Vitro Is Modulated by Treatment with Tamoxifen, Doxorubicin, and Docetaxel: Relationship to Chemotherapy-Induced Changes in ATP Content, Hexokinase Activity, and Glucose Transport
Breast tumours responding to chemotherapy exhibit decreased [18F]fluoro-2-deoxy-D-glucose ([18F]FDG) incorporation. Underlying mechanisms of these changes is poorly understood. Here, in MCF-7 cells, responding to chemotherapy drugs commonly utilised in the treatment of breast cancer, [18F]FDG incorporation and several pivotal factors associated with [18F]FDG incorporation investigated. Methods. IC50 and subclinical doxorubicin, docetaxel, and tamoxifen doses determined using MTT assay. [18F]FDG incorporation by cells treated with IC50 drug doses for 48 hours and 72 hours were determined and FDG dephosphorylation estimated by measuring loss of 18F from [18F]FDG-preincubated cells (pulse-chase). Glucose transport determined by measuring initial uptake rate of non-metabolised glucose analogue omethylglucose; hexokinase activity and ATP content measured in cell homogenates; Cell cycle distribution determined using flow cytometry of propidium iodide stained nuclei. Results. [18F]FDG incorporation and ATP content decreased in cells after 72 hours treatment with IC50 doses of tamoxifen, doxorubicin, and docetaxel compared with untreated controls. Decreased glucose transport and/or hexokinase activity accompanied decreased [18F]FDG incorporation by MCF-7 cells treated with tamoxifen or doxorubicin but not docetaxel. Conclusions. Tumour cell [18F]FDG incorporation along with ATP content decreased by treatment with tamoxifen, doxorubicin and docetaxel paralleling clinical observations for solid tumours. Effect of each treatment on glucose transport and hexokinase activity was chemotherapy-drug dependent
An Ice-Core-Based, Late Holocene History for the Transantarctic Mountains, Antarctica
Ice core records (major anions and cations, MSA, oxygen isotopes and particles) developed from two shallow (~200 m depth) sites in the Transantarctic Mountains provide documentation of much of the Holocene paleoenvironmental history of this region. From the more southerly site, Dominion Range, an ~7000-year-long record reveals change in the influence of tropospheric transport to the region. At this site, milder conditions and increased tropospheric inflow prior to ~1500 yr BP are characterized by increased seasalt (ss), terrestrial and marine biogenic inputs. Increased persistence and/or extent of polar stratospheric clouds accompanying generally cooler conditions characterize much of the period since ~1500 yr BP. From the more northerly site, Newall Glacier, the dramatic influence of the retreat of grounded ice from McMurdo Sound dated at[Denton et al., 1989] dominates much of the ice core record. This regional environmental change is documented by massive influxes to the core site of evaporitic salts from areas exposed during low lake level stands. During the past ~150 yr, both Dominion Range and Newall Glacier appear to be experiencing an overall increase in the exposure of ice-free terrain
Buffer-gas induced absorption resonances in Rb vapor
We observe transformation of the electromagnetically induced transparency
(EIT) resonance into the absorption resonance in a interaction
configuration in a cell filled with Rb and a buffer gas. This
transformation occurs as a one-photon detuning of the coupling fields is varied
from the atomic transition. No such absorption resonance is found in the
absence of a buffer gas. The width of the absorption resonance is several times
smaller than the width of the EIT resonance, and the changes of absorption near
these resonances are about the same. Similar absorption resonances are detected
in the Hanle configuration in a buffered cell.Comment: 11 pages, 15 figures; 13 pages, 17 figures, added numerical
simulatio
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