14,967 research outputs found
Finite subgroups of simple algebraic groups with irreducible centralizers
We determine all finite subgroups of simple algebraic groups that have
irreducible centralizers - that is, centralizers whose connected component does
not lie in a parabolic subgroup.Comment: 24 page
Quantum Feynman-Kac perturbations
We develop fully noncommutative Feynman-Kac formulae by employing quantum
stochastic processes. To this end we establish some theory for perturbing
quantum stochastic flows on von Neumann algebras by multiplier cocycles.
Multiplier cocycles are constructed via quantum stochastic differential
equations whose coefficients are driven by the flow. The resulting class of
cocycles is characterised under alternative assumptions of separability or
Markov regularity. Our results generalise those obtained using classical
Brownian motion on the one hand, and results for unitarily implemented flows on
the other.Comment: 27 pages. Minor corrections to version 2. To appear in the Journal of
the London Mathematical Societ
LHC Limits on the Top-Higgs in Models with Strong Top-Quark Dynamics
LHC searches for the standard model Higgs Boson in WW or ZZ decay modes place
strong constraints on the top-Higgs state predicted in many models with new
dynamics preferentially affecting top quarks. Such a state couples strongly to
top-quarks, and is therefore produced through gluon fusion at a rate enhanced
relative to the rate for the standard model Higgs boson. A top-Higgs state with
mass less than 300 GeV is excluded at 95% CL if the associated top-pion has a
mass of 150 GeV, and the constraint is even stronger if the mass of the
top-pion state exceeds the top-quark mass or if the top-pion decay constant is
a substantial fraction of the weak scale. These results have significant
implications for theories with strong top dynamics, such as topcolor-assisted
technicolor, top-seesaw models, and certain Higgsless models.Comment: 9 pages, 6 pdf figures included. Updated with 2011 Lepton-Photon
conference Higgs limits. Order of figures reversed, and conclusion slightly
expanded and clarifie
Structure of Mandelate Racemase with Bound Intermediate Analogues Benzohydroxamate and Cupferron
Mandelate racemase (MR, EC 5.1.2.2) from Pseudomonas putida catalyzes the Mg2+-dependent interconversion of the enantiomers of mandelate, stabilizing the altered substrate in the transition state by 26 kcal/mol relative to the substrate in the ground state. To understand the origins of this binding discrimination, we determined the X-ray crystal structures of wild-type MR complexed with two analogues of the putative aci-carboxylate intermediate, benzohydroxamate and Cupferron, to 2.2-Å resolution. Benzohydroxamate is shown to be a reasonable mimic of the transition state and/or intermediate because its binding affinity for 21 MR variants correlates well with changes in the free energy of transition state stabilization afforded by these variants. Both benzohydroxamate and Cupferron chelate the active site divalent metal ion and are bound in a conformation with the phenyl ring coplanar with the hydroxamate and diazeniumdiolate moieties, respectively. Structural overlays of MR complexed with benzohydroxamate, Cupferron, and the ground state analogue (S)-atrolactate reveal that the para carbon of the substrate phenyl ring moves by 0.8−1.2 Å between the ground state and intermediate state, consistent with the proposal that the phenyl ring moves during MR catalysis while the polar groups remain relatively fixed. Although the overall protein structure of MR with bound intermediate analogues is very similar to that of MR with bound (S)-atrolactate, the intermediate−Mg2+ distance becomes shorter, suggesting a tighter complex with the catalytic Mg2+. In addition, Tyr 54 moves closer to the phenyl ring of the bound intermediate analogues, contributing to an overall constriction of the active site cavity. However, site-directed mutagenesis experiments revealed that the role of Tyr 54 in MR catalysis is relatively minor, suggesting that alterations in enzyme structure that contribute to discrimination between the altered substrate in the transition state and the ground state by this proficient enzyme are extremely subtle
Adhesion signalling complexes
SummaryIntercellular communication in metazoa not only requires autocrine, paracrine and exocrine signalling systems, but it also relies on the structural and positional information encoded in extracellular matrices (ECMs). Most cells in tissues are structurally and functionally integrated with their surrounding ECM in a highly organised manner involving thousands of dynamic connections. On the intracellular face of these linkages, adhesion receptors — principally integrins and syndecans — link the cytoskeleton to the plasma membrane and compartmentalise cytoplasmic signalling events, whereas at the extracellular face the same receptors direct and organise the deposition of the ECM itself. Adhesion receptors transduce mechanical force bidirectionally across the plasma membrane by tethering variably deformable ECMs to the contractile cytoskeleton (Figure 1), and they translate the topography and composition of the ECM into chemical signals that determine behaviour. The membrane-proximal functions of adhesion receptors in turn trigger distal processes within cells, such as alterations in the direction of cell movement and the regulation of gene transcription, and long-range effects outside cells, such as the construction of ECM networks and consequent shaping of higher-order tissue structure. Given the diverse and fundamental roles attributed to adhesion, it is understandable that adhesion receptor engagement has been reported to alter the flux through virtually all major signalling pathways
Intraspecific trait variation and coordination: Root and leaf economics spectra in coffee across environmental gradients
Hypotheses on the existence of a universal “Root Economics Spectrum” (RES) have received arguably the least attention of all trait spectra, despite the key role root trait variation plays in resource acquisition potential. There is growing interest in quantifying intraspecific trait variation (ITV) in plants, but there are few studies evaluating (i) the existence of an intraspecific RES within a plant species, or (ii) how a RES may be coordinated with other trait spectra within species, such as a leaf economics spectrum (LES). Using Coffea arabica (Rubiaceae) as a model species, we measured seven morphological and chemical traits of intact lateral roots, which were paired with information on four key LES traits. Field collections were completed across four nested levels of biological organization. The intraspecific trait coefficient of variation (cv) ranged from 25 to 87% with root diameter and specific root tip density showing the lowest and highest cv, respectively. Between 27 and 68% of root ITV was explained by site identity alone for five of the seven traits measured. A single principal component explained 56.2% of root trait covariation, with plants falling along a RES from resource acquiring to conserving traits. Multiple factor analysis revealed significant orthogonal relationships between root and leaf spectra. RES traits were strongly orthogonal with respect to LES traits, suggesting these traits vary independently from one another in response to environmental cues. This study provides among the first evidence that plants from the same species differentiate from one another along an intraspecific RES. We find that in one of the world's most widely cultivated crops, an intraspecific RES is orthogonal to an intraspecific LES, indicating that above and belowground responses of plants to managed (or natural) environmental gradients are likely to occur independently from one another. (Résumé d'auteur
Discovering Strong Top Dynamics at the LHC
We analyze the phenomenology of the top-pion and top-Higgs states in models
with strong top dynamics, and translate the present LHC searches for the
Standard Model Higgs into bounds on these scalar states. We explore the
possibility that the new state at a mass of approximately 125 GeV observed at
the LHC is consistent with a neutral pseudoscalar top-pion state. We
demonstrate that a neutral pseudoscalar top-pion can generate the diphoton
signal at the observed rate. However, the region of model parameter space where
this is the case does not correspond to classic topcolor-assisted technicolor
scenarios with degenerate charged and neutral top-pions and a top-Higgs mass of
order twice the top mass; rather, additional isospin violation would need to be
present and the top dynamics would be more akin to that in top seesaw models.
Moreover, the interpretation of the new state as a top-pion can be sustained
only if the ZZ (four-lepton) and WW (two-lepton plus missing energy) signatures
initially observed at the 3? level decline in significance as additional data
is accrued.Comment: 25 pages, pdf embedded figures. Submission extensively revised to
reflect discovery of a 125 GeV boso
- …