1,129 research outputs found
Nucleon Quark Distribution Functions from the Dyson-Schwinger Equations
We present results for the nucleon's leading-twist spin-independent valence
parton distribution functions obtained from a theoretical framework based on
the Dyson-Schwinger equations (DSEs) of QCD that previously gave an excellent
description of nucleon electromagnetic form factors. We employ the
rainbow-ladder truncation of the DSEs and utilize nucleon bound state
amplitudes from the Poincar\'e-covariant Faddeev equation, where the dominant
scalar and axial-vector quark-quark correlations are included. This DSE
framework is used to numerically evaluate the first 20 moments of the valence
and quark distribution functions, from which the -dependence of the
distributions is found to be well constrained. We find good agreement with
empirical parameterizations of experimental data and make the prediction that
the ratio in the limit, invariant under scale evolution, takes
the value . We find that this ratio is rather
sensitive to the strength of axial-vector diquark correlations. However,
contrary to a naive expectation, our result for the ratio in the
limit does not vanish when only scalar diquark correlations are present,
although it is an order of magnitude smaller than our result that also
includes axial-vector diquarks. The valence quark distribution results are set
in a broader context via a simple pion cloud model estimate of sea-quark
light-cone momenta and gluon light-cone momentum.Comment: 8 pages, 5 figure
Torsional Directed Walks, Entropic Elasticity, and DNA Twist Stiffness
DNA and other biopolymers differ from classical polymers due to their
torsional stiffness. This property changes the statistical character of their
conformations under tension from a classical random walk to a problem we call
the `torsional directed walk'. Motivated by a recent experiment on single
lambda-DNA molecules [Strick et al., Science 271 (1996) 1835], we formulate the
torsional directed walk problem and solve it analytically in the appropriate
force regime. Our technique affords a direct physical determination of the
microscopic twist stiffness C and twist-stretch coupling D relevant for DNA
functionality. The theory quantitatively fits existing experimental data for
relative extension as a function of overtwist over a wide range of applied
force; fitting to the experimental data yields the numerical values C=120nm and
D=50nm. Future experiments will refine these values. We also predict that the
phenomenon of reduction of effective twist stiffness by bend fluctuations
should be testable in future single-molecule experiments, and we give its
analytic form.Comment: Plain TeX, harvmac, epsf; postscript available at
http://dept.physics.upenn.edu/~nelson/index.shtm
Sequence Effects on DNA Entropic Elasticity
DNA stretching experiments are usually interpreted using the worm-like chain
model; the persistence length A appearing in the model is then interpreted as
the elastic stiffness of the double helix. In fact the persistence length
obtained by this method is a combination of bend stiffness and intrinsic bend
effects reflecting sequence information, just as at zero stretching force. This
observation resolves the discrepancy between the value of A measured in these
experiments and the larger ``dynamic persistence length'' measured by other
means. On the other hand, the twist persistence length deduced from
torsionally-constrained stretching experiments suffers no such correction. Our
calculation is very simple and analytic; it applies to DNA and other polymers
with weak intrinsic disorder.Comment: LaTeX; postscript available at
http://dept.physics.upenn.edu/~nelson/index.shtm
Fluctuating Filaments I: Statistical Mechanics of Helices
We examine the effects of thermal fluctuations on thin elastic filaments with
non-circular cross-section and arbitrary spontaneous curvature and torsion.
Analytical expressions for orientational correlation functions and for the
persistence length of helices are derived, and it is found that this length
varies non-monotonically with the strength of thermal fluctuations. In the weak
fluctuation regime, the local helical structure is preserved and the
statistical properties are dominated by long wavelength bending and torsion
modes. As the amplitude of fluctuations is increased, the helix ``melts'' and
all memory of intrinsic helical structure is lost. Spontaneous twist of the
cross--section leads to resonant dependence of the persistence length on the
twist rate.Comment: 5 figure
Enhancement of Magneto-Optic Effects via Large Atomic Coherence
We utilize the generation of large atomic coherence to enhance the resonant
nonlinear magneto-optic effect by several orders of magnitude, thereby
eliminating power broadening and improving the fundamental signal-to-noise
ratio. A proof-of-principle experiment is carried out in a dense vapor of Rb
atoms. Detailed numerical calculations are in good agreement with the
experimental results. Applications such as optical magnetometry or the search
for violations of parity and time reversal symmetry are feasible
Mechanical response of plectonemic DNA: an analytical solution
We consider an elastic rod model for twisted DNA in the plectonemic regime.
The molecule is treated as an impenetrable tube with an effective, adjustable
radius. The model is solved analytically and we derive formulas for the contact
pressure, twisting moment and geometrical parameters of the supercoiled region.
We apply our model to magnetic tweezer experiments of a DNA molecule subjected
to a tensile force and a torque, and extract mechanical and geometrical
quantities from the linear part of the experimental response curve. These
reconstructed values are derived in a self-contained manner, and are found to
be consistent with those available in the literature.Comment: 14 pages, 4 figure
Traffic-Related Air Pollution and QT Interval: Modification by Diabetes, Obesity, and Oxidative Stress Gene Polymorphisms in the Normative Aging Study
BACKGROUND. Acute exposure to ambient air pollution has been associated with acute changes in cardiac outcomes, often within hours of exposure. OBJECTIVES. We examined the effects of air pollutants on heart-rate-corrected QT interval (QTc), an electrocardiographic marker of ventricular repolarization, and whether these associations were modified by participant characteristics and genetic polymorphisms related to oxidative stress. METHODS. We studied repeated measurements of QTc on 580 men from the Veterans Affairs Normative Aging Study (NAS) using mixed-effects models with random intercepts. We fitted a quadratic constrained distributed lag model to estimate the cumulative effect on QTc of ambient air pollutants including fine particulate matter †2.5 Όm in aerodynamic diameter (PM2.5), ozone (O3), black carbon (BC), nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2) concentrations during the 10 hr before the visit. We genotyped polymorphisms related to oxidative stress and analyzed pollution-susceptibility score interactions using the genetic susceptibility score (GSS) method. RESULTS. Ambient traffic pollutant concentrations were related to longer QTc. An interquartile range (IQR) change in BC cumulative during the 10 hr before the visit was associated with increased QTc [1.89 msec change; 95% confidence interval (CI), -0.16 to 3.93]. We found a similar association with QTc for an IQR change in 1-hr BC that occurred 4 hr before the visit (2.54 msec change; 95% CI, 0.28-4.80). We found increased QTc for IQR changes in NO2 and CO, but the change was statistically insignificant. In contrast, we found no association between QTc and PM2.5, SO2, and O3. The association between QTc and BC was stronger among participants who were obese, who had diabetes, who were nonsmokers, or who had higher GSSs. CONCLUSIONS. Traffic-related pollutants may increase QTc among persons with diabetes, persons who are obese, and nonsmoking elderly individuals; the number of genetic variants related to oxidative stress increases this effect.National Institute of Environmental Health Sciences (ES014663-01A2, P01 ES09825); United States Environmental Protection Agency (R827353, R83241601
3D reconstruction and comparison of shapes of DNA minicircles observed by cryo-electron microscopy
We use cryo-electron microscopy to compare 3D shapes of 158 bp long DNA minicircles that differ only in the sequence within an 18 bp block containing either a TATA box or a catabolite activator protein binding site. We present a sorting algorithm that correlates the reconstructed shapes and groups them into distinct categories. We conclude that the presence of the TATA box sequence, which is believed to be easily bent, does not significantly affect the observed shapes
Loitering with intent: dealing with human-intensive systems
This paper discusses the professional roles of information systems analysts and users, focusing on a perspective of human intensive, rather than software intensive information systems. The concept of âmeaningful useâ is discussed in re-lation to measures of success/failure in IS development. The authors consider how a number of different aspects of reductionism may distort analyses, so that processes of inquiry cannot support organizational actors to explore and shape their requirements in relation to meaningful use. Approaches which attempt to simplify complex problem spaces, to render them more susceptible to âsolutionâ are problematized. Alternative perspectives which attempt a systematic, holistic complexification, by supporting contextual dependencies to emerge, are advocated as a way forward
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