2,075 research outputs found
W asymmetries at CDF and D0
We present recent W and charged lepton asymmetry measurements from the CDF
and D0 experiments. Theoretical predictions agree with the CDF W asymmetry,
measured using a new matrix element technique. These theoretical predictions
are less consistent with the latest lepton asymmetry measurements from D0 and
CDF, especially for high charged lepton transverse momentum.Comment: 6 pages, 6 figure
A Model for the Thermodynamics of Globular Proteins
Comments: 6 pages RevTeX, 6 Postscript figures. We review a statistical
mechanics treatment of the stability of globular proteins based on a simple
model Hamiltonian taking into account protein self interactions and
protein-water interactions. The model contains both hot and cold folding
transitions. In addition it predicts a critical point at a given temperature
and chemical potential of the surrounding water. The universality class of this
critical point is new
Pathways in Two-State Protein Folding
The thermodynamics of proteins indicate that folding/unfolding takes place
either through stable intermediates or through a two-state process without
intermediates. The rather short folding times of the two-state process indicate
that folding is guided. We reconcile these two seemingly contradictory
observations quantitatively in a schematic model of protein folding. We propose
a new dynamical transition temperature which is lower than the thermodynamic
one, in qualitative agreement with in vivo measurement of protein stability
using E.coli. Finally we demonstrate that our framework is easily generalized
to encompass cold unfolding, and make predictions that relate the sharpness of
the cold and hot unfolding transitions.Comment: 4 pages RevTeX, 5 Postscript figur
The D0 Run IIb Luminosity Measurement
An assessment of the recorded integrated luminosity is presented for data
collected with the D0 detector at the Fermilab Tevatron Collider from June 2006
to September 2011 (Run IIb). In addition, a measurement of the effective cross
section for inelastic interactions, also referred to as the luminosity
constant, is reported. This measurement incorporates new features that lead to
a substantial improvement in the precision of the result. A luminosity constant
of \sigma_{LM} = 48.3\pm1.9\pm0.6 mb is obtained, where the first uncertainty
is due to the accuracy of the inelastic cross section used by both CDF and D0,
and the second uncertainty is due to D0 sources. The recorded luminosity for
the highest E_T jet trigger is L_rec = 9.2 \pm 0.4 fb^{-1}, with a relative
uncertainty of 4.3%.Comment: 20 pages, 23 figure
Metabolic Futile Cycles and Their Functions: A Systems Analysis of Energy and Control
It has long been hypothesized that futile cycles in cellular metabolism are
involved in the regulation of biochemical pathways. Following the work of
Newsholme and Crabtree, we develop a quantitative theory for this idea based on
open-system thermodynamics and metabolic control analysis. It is shown that the
{\it stoichiometric sensitivity} of an intermediary metabolite concentration
with respect to changes in steady-state flux is governed by the effective
equilibrium constant of the intermediate formation, and the equilibrium can be
regulated by a futile cycle. The direction of the shift in the effective
equilibrium constant depends on the direction of operation of the futile cycle.
High stoichiometric sensitivity corresponds to ultrasensitivity of an
intermediate concentration to net flow through a pathway; low stoichiometric
sensitivity corresponds to super-robustness of concentration with respect to
changes in flux. Both cases potentially play important roles in metabolic
regulation. Futile cycles actively shift the effective equilibrium by expending
energy; the magnitude of changes in effective equilibria and sensitivities is a
function of the amount of energy used by a futile cycle. This proposed
mechanism for control by futile cycles works remarkably similarly to kinetic
proofreading in biosynthesis. The sensitivity of the system is also intimately
related to the rate of concentration fluctuations of intermediate metabolites.
The possibly different roles of the two major mechanisms for cellular
biochemical regulation, namely reversible chemical modifications via futile
cycles and shifting equilibrium by macromolecular binding, are discussed.Comment: 11 pages, 5 figure
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
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