189 research outputs found
Entropic Tension in Crowded Membranes
Unlike their model membrane counterparts, biological membranes are richly
decorated with a heterogeneous assembly of membrane proteins. These proteins
are so tightly packed that their excluded area interactions can alter the free
energy landscape controlling the conformational transitions suffered by such
proteins. For membrane channels, this effect can alter the critical membrane
tension at which they undergo a transition from a closed to an open state, and
therefore influence protein function \emph{in vivo}. Despite their obvious
importance, crowding phenomena in membranes are much less well studied than in
the cytoplasm.
Using statistical mechanics results for hard disk liquids, we show that
crowding induces an entropic tension in the membrane, which influences
transitions that alter the projected area and circumference of a membrane
protein. As a specific case study in this effect, we consider the impact of
crowding on the gating properties of bacterial mechanosensitive membrane
channels, which are thought to confer osmoprotection when these cells are
subjected to osmotic shock. We find that crowding can alter the gating energies
by more than in physiological conditions, a substantial fraction of
the total gating energies in some cases.
Given the ubiquity of membrane crowding, the nonspecific nature of excluded
volume interactions, and the fact that the function of many membrane proteins
involve significant conformational changes, this specific case study highlights
a general aspect in the function of membrane proteins.Comment: 20 pages (inclduing supporting information), 4 figures, to appear in
PLoS Comp. Bio
Improved Measurement of the K+ to pi+ nu nubar Branching Ratio
An additional event near the upper kinematic limit for K+ to pi+ nu nubar has
been observed by Experiment E949 at Brookhaven National Laboratory. Combining
previously reported and new data, the branching ratio is B(K+ to pi+ nu nubar)=
1.47 (+1.30, - 0.89) x 10-10 based on three events observed in the pion
momentum region 211<P<229 MeV/c. At the measured central value of the branching
ratio, the additional event had a signal-to-background ratio of 0.9
A Pedagogical Review of Electroweak Symmetry Breaking Scenarios
We review different avenues of electroweak symmetry breaking explored over
the years. This constitutes a timely exercise as the world's largest and the
highest energy particle accelerator, namely, the Large Hadron Collider (LHC) at
CERN near Geneva, has started running whose primary mission is to find the
Higgs or some phenomena that mimic the effects of the Higgs, i.e. to unravel
the mysteries of electroweak phase transition. In the beginning, we discuss the
Standard Model Higgs mechanism. After that we review the Higgs sector of the
Minimal Supersymmetric Standard Model. Then we take up three relatively recent
ideas: Little Higgs, Gauge-Higgs Unification, and Higgsless scenarios. For the
latter three cases, we first present the basic ideas and restrict our
illustration to some instructive toy models to provide an intuitive feel of the
underlying dynamics, and then discuss, for each of the three cases, how more
realistic scenarios are constructed and how to decipher their experimental
signatures. Wherever possible, we provide enough pedagogical details, which the
beginners might find useful.Comment: 45 pages, Review based on a series of lectures; v2: 63 pages,
substantially expanded, references added, to appear in `Reports on Progress
in Physics
Upper Limit on the Branching Ratio for the Decay
A sample of kinematically identified decays obtained
with the E949 detector was used to search for the helicity-suppressed decay
resulting in an upper limit of at
90% confidence level.The upper limit is also applicable to decays into
unknown weakly interacting particles.Comment: 9 pages, 3 figures; no change in the results, accepted for
publication in Physical Review
Study of the decay K+ => pi+,nu,nubar in the momentum region 140<Ppi<199 MeV/c
Experiment E949 at Brookhaven National Laboratory has observed three new
events consistent with the decay K+ => pi+,nu,nubar in the pion momentum region
140 < P_pi < 199 MeV/c in an exposure of 1.71e12 stopped kaons with an
estimated total background of 0.93+-0.17(stat.)+0.32-0.24(syst.) events. This
brings the total number of observed K+ => pi+,nu,nubar events to seven.
Combining this observation with previous results, assuming the pion spectrum
predicted by the standard model, results in a branching ratio of
(1.73+1.15-1.05)e-10. An interpretation of the results for alternative models
of the decay K^ => pi+,nothing is also presented.Comment: Submitted to Phys.Rev.
Solar and Atmospheric Neutrino Oscillations and Lepton Flavor Violation in Supersymmetric Models with Right-handed Neutrinos
Taking the solar and the atmospheric neutrino experiments into account we
discuss the lepton flavor violating processes, such as or
, in the minimal supersymmetric standard model with
right-handed neutrinos (MSSMRN) and the supersymmetric SU(5) GUT with
right-handed neutrinos (SU(5)RN). The predicted branching ratio of in the MSSMRN with the MSW large angle solution is so large that it
goes beyond the current experimental bound if the second-generation
right-handed Majorana mass is greater than GeV for . When we take the MSW small angle solution,
the rate is at most about 1/100 of that of the MSW large angle
solution. The 'just so' solution implies of that of the MSW large
angle solution. Also, in the SU(5)RN the large rate naturally
follows from the MSW large angle solution, and the predicted rate is beyond the
current experimental bound if the typical right-handed Majorana mass is
larger than GeV for , similarly to
the MSSMRN. We show the multimass insertion formulas and their applications to
and .Comment: 55 pages, 18 figures, Latex, the final version to be published in
Phys. Rev.
Magnetic resonance imaging of anterior cruciate ligament rupture
BACKGROUND: Magnetic resonance (MR) imaging is a useful diagnostic tool for the assessment of knee joint injury. Anterior cruciate ligament repair is a commonly performed orthopaedic procedure. This paper examines the concordance between MR imaging and arthroscopic findings. METHODS: Between February, 1996 and February, 1998, 48 patients who underwent magnetic resonance (MR) imaging of the knee were reported to have complete tears of the anterior cruciate ligament (ACL). Of the 48 patients, 36 were male, and 12 female. The average age was 27 years (range: 15 to 45). Operative reconstruction using a patellar bone-tendon-bone autograft was arranged for each patient, and an arthroscopic examination was performed to confirm the diagnosis immediately prior to reconstructive surgery. RESULTS: In 16 of the 48 patients, reconstructive surgery was cancelled when incomplete lesions were noted during arthroscopy, making reconstructive surgery unnecessary. The remaining 32 patients were found to have complete tears of the ACL, and therefore underwent reconstructive surgery. Using arthroscopy as an independent, reliable reference standard for ACL tear diagnosis, the reliability of MR imaging was evaluated. The true positive rate for complete ACL tear diagnosis with MR imaging was 67%, making the possibility of a false-positive report of "complete ACL tear" inevitable with MR imaging. CONCLUSIONS: Since conservative treatment is sufficient for incomplete ACL tears, the decision to undertake ACL reconstruction should not be based on MR findings alone
Measurement of the Branching Ratio
Experiment E949 at Brookhaven National Laboratory studied the rare decay
\ and other processes with an exposure of 's. The data were analyzed using a blind analysis technique
yielding one candidate event with an estimated background of
events. Combining this result with the observation of two candidate events by
the predecessor experiment E787 gave the branching ratio
{\calB}(K^+\to\pi^+\nu\bar{\nu})=(1.47^{+1.30}_{-0.89})\times 10^{-10},
consistent with the Standard Model prediction of . This is a more detailed report of results previously published in
Physical Review Letters.Comment: 99 pages, 32 figures, 12 tables. Added authors, corrected typos and
modify the text suggested by the referees. Accepted for publication in PR
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