8,543 research outputs found
Depletion forces between two spheres in a rod solution
We study the depletion interaction between spherical particles of radius R
immersed in a dilute solution of rigid rods of length L. The computed
interaction potential is, within numerical accuracy, exact for any value of
L/R. In particular we find that for L of order R, the depth of the depletion
well is smaller than the prediction of the Derjaguin approximation. Our results
bring new light into the discussion on the lack of phase separation in
colloidal mixtures of spheres and rods.Comment: 12 pages including figures. 5 eps figures. LaTeX with REVTe
Polymer collapse in miscible good solvents is a generic phenomenon driven by preferential adsorption
Water and alcohol, such as methanol or ethanol, are miscible and, individually, good solvents for poly(N-isopropylacrylamide) (PNIPAm), but this polymer precipitates in water–alcohol mixtures. The intriguing behaviour of solvent mixtures that cannot dissolve a given polymer or a given protein, while the same macromolecule dissolves well in each of the cosolvents, is called cononsolvency. It is a widespread phenomenon, relevant for many formulation steps in the physicochemical and pharmaceutical industry, that is usually explained by invoking specific chemical details of the mixtures: as such, it has so far eluded any generic explanation. Here, by using a combination of simulations and theory, we present a simple and universal treatment that requires only the preferential interaction of one of the cosolvents with the polymer. The results show striking quantitative agreement with experiments and chemically specific simulations, opening a new perspective towards an operational understanding of macromolecular solubility
Role of microRNAs in the age-associated decline of pancreatic beta cell function in rat islets
This is a freely-available open access publication. Please cite the published version which is available via the DOI link in this record.AIMS/HYPOTHESIS: Ageing can lead to reduced insulin sensitivity and loss of pancreatic beta cell function, predisposing individuals to the development of diabetes. The aim of this study was to assess the contribution of microRNAs (miRNAs) to age-associated beta cell dysfunction. METHODS: The global mRNA and miRNA profiles of 3- and 12-month-old rat islets were collected by microarray. The functional impact of age-associated differences in miRNA expression was investigated by mimicking the observed changes in primary beta cells from young animals. RESULTS: Beta cells from 12-month-old rats retained normal insulin content and secretion, but failed to proliferate in response to mitotic stimuli. The islets of these animals displayed modifications at the level of several miRNAs, including upregulation of miR-34a, miR-124a and miR-383, and downregulation of miR-130b and miR-181a. Computational analysis of the transcriptomic modifications observed in the islets of 12-month-old rats revealed that the differentially expressed genes were enriched for miR-34a and miR-181a targets. Indeed, the induction of miR-34a and reduction of miR-181a in the islets of young animals mimicked the impaired beta cell proliferation observed in old animals. mRNA coding for alpha-type platelet-derived growth factor receptor, which is critical for compensatory beta cell mass expansion, is directly inhibited by miR34a and is likely to be at least partly responsible for the effects of this miRNA. CONCLUSIONS/INTERPRETATION: Changes in the level of specific miRNAs that occur during ageing affect the proliferative capacity of beta cells. This might reduce their ability to expand under conditions of increased insulin demand, favouring the development of type 2 diabetes.Swiss National Science FoundationFondation Francophone pour la Recherche sur le DiabèteWellcome Trust Senior Investigator AwardMRC Programme GrantRoyal Society Wolfson Research Merit AwardWellcome Trust project gran
Processing The Interspecies Quorum-Sensing Signal Autoinducer-2 (AI-2) Characterization Of Phospho-(S)-4,5-Dihydroxy-2,3-Pentanedione Isomerization By LsrG Protein
The molecule (S)-4,5-dihydroxy-2,3-pentanedione (DPD) is produced by many different species of bacteria and is the precursor of the signal molecule autoinducer-2 (AI-2). AI-2 mediates interspecies communication and facilitates regulation of bacterial behaviors such as biofilm formation and virulence. A variety of bacterial species have the ability to sequester and process the AI-2 present in their environment, thereby interfering with the cell-cell communication of other bacteria. This process involves the AI-2-regulated lsr operon, comprised of the Lsr transport system that facilitates uptake of the signal, a kinase that phosphorylates the signal to phospho-DPD (P-DPD), and enzymes (like LsrG) that are responsible for processing the phosphorylated signal. Because P-DPD is the intracellular inducer of the lsr operon, enzymes involved in P-DPD processing impact the levels of Lsr expression. Here we show that LsrG catalyzes isomerization of P-DPD into 3,4,4-trihydroxy-2-pentanone-5-phosphate. We present the crystal structure of LsrG, identify potential catalytic residues, and determine which of these residues affects P-DPD processing in vivo and in vitro. We also show that an lsrG deletion mutant accumulates at least 10 times more P-DPD than wild type cells. Consistent with this result, we find that the lsrG mutant has increased expression of the lsr operon and an altered profile of AI-2 accumulation and removal. Understanding of the biochemical mechanisms employed by bacteria to quench signaling of other species can be of great utility in the development of therapies to control bacterial behavior
R-symmetric Gauge Mediation and the MRSSM
This is an invited summary of a seminar talk given at various institutions in
the United States and Canada. After a brief introduction, a review of the
minimal R-symmetric supersymmetric standard model is given, and the benefits to
the flavor sector are discussed. R-symmetric gauge mediation is an attempt to
realize this model using metastable supersymmetry breaking techniques. Sample
low energy spectra are presented and tuning is discussed. Various other
phenomenological results are summarized.Comment: 14 pages, invited Brief Review, submitted to Modern Physics Letters
A; v2: replaced Figure 1, updated acknowledgments, fixed typo
A Study of Two-Temperature Non-Equilibrium Ising Models: Critical Behavior and Universality
We study a class of 2D non-equilibrium Ising models based on competing
dynamics induced by contact with heat-baths at two different temperatures. We
make a comparative study of the non-equilibrium versions of Metropolis, heat
bath/Glauber and Swendsen-Wang dynamics and focus on their critical behavior in
order to understand their universality classes. We present strong evidence that
some of these dynamics have the same critical exponents and belong to the same
universality class as the equilibrium 2D Ising model. We show that the bond
version of the Swendsen-Wang update algorithm can be mapped into an equilibrium
model at an effective temperature.Comment: 12 pages of LaTeX plus 18 pages of postscript figures in a uuencoded
file (608k
Quaternion-Octonion SU(3) Flavor Symmetry
Starting with the quaternionic formulation of isospin SU(2) group, we have
derived the relations for different components of isospin with quark states.
Extending this formalism to the case of SU(3) group we have considered the
theory of octonion variables. Accordingly, the octonion splitting of SU(3)
group have been reconsidered and various commutation relations for SU(3) group
and its shift operators are also derived and verified for different iso-spin
multiplets i.e. I, U and V- spins.
Keywords: SU(3), Quaternions, Octonions and Gell Mann matrices
PACS NO: 11.30.Hv: Flavor symmetries; 12.10-Dm: Unified field theories and
models of strong and electroweak interaction
Phase transition of a two dimensional binary spreading model
We investigated the phase transition behavior of a binary spreading process
in two dimensions for different particle diffusion strengths (). We found
that cluster mean-field approximations must be considered to get
consistent singular behavior. The approximations result in a continuous
phase transition belonging to a single universality class along the phase transition line. Large scale simulations of the particle density
confirmed mean-field scaling behavior with logarithmic corrections. This is
interpreted as numerical evidence supporting that the upper critical dimension
in this model is .The pair density scales in a similar way but with an
additional logarithmic factor to the order parameter. At the D=0 endpoint of
the transition line we found DP criticality.Comment: 8 pages, 10 figure
Notes on SUSY and R-Symmetry Breaking in Wess-Zumino Models
We study aspects of Wess-Zumino models related to SUSY and R-symmetry
breaking at tree-level. We present a recipe for constructing a wide class of
tree-level SUSY and R-breaking models. We also deduce a general property shared
by all tree-level SUSY breaking models that has broad application to model
building. In particular, it explains why many models of direct gauge mediation
have anomalously light gauginos (even if the R-symmetry is broken spontaneously
by an order one amount). This suggests new approaches to dynamical SUSY
breaking which can generate large enough gaugino masses.Comment: 23 pages. v2: references added, minor changes. v3: comment on
non-renormalizable case adde
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