327 research outputs found
Two dimensional Berezin-Li-Yau inequalities with a correction term
We improve the Berezin-Li-Yau inequality in dimension two by adding a
positive correction term to its right-hand side. It is also shown that the
asymptotical behaviour of the correction term is almost optimal. This improves
a previous result by Melas.Comment: 6 figure
Rigorous proof of Luttinger liquid behavior in the 1d Hubbard model
We give the first rigorous (non perturbative) proof of Luttinger liquid
behavior in the one dimensional Hubbard model, for small repulsive interaction
and values of the density different from half filling. The analysis is based on
the combination of multiscale analysis with Ward identities bases on a hidden
and approximate local chiral gauge invariance. No use is done of exact
solutions or special integrability properties of the Hubbard model, and the
results can be in fact easily generalized to include non local interactions,
magnetic fields or interaction with external potential
Crystallographic studies on decameric Brucella spp. lumazine synthase: A novel quaternary arrangement evolved for a new function?
The enzyme lumazine synthase (LS) catalyzes the penultimate step of riboflavin biosynthesis in plants, fungi and bacteria. The quaternary structure of the polypeptide differs between species, existing as pentamers or as icosahedrally arranged dodecamers of pentamers with 60 subunits. The pathogen Brucella spp. expresses two proteins that exhibit LS activity, RibH1 and RibH2. The latter enzyme belongs to a novel third category of quaternary arrangement for LS, that of a decameric structure assembled as a head-to-head oriented dimer of pentamers. In contrast, the RibH1 enzyme is assembled as a pentamer, as noted for several other LS enzymes. RibH1 appears to be the functional LS in Brucella spp., whereas RibH2, an enzyme of lower catalytic activity, is a virulence factor presumably acting in response to oxidative stress. The latter observation prompted us to further investigate the structural and catalytic properties of RibH2 in order to clarify the biological function of this enzyme. Here, we present a detailed analysis of two new crystallographic forms of RibH2 that explain the low catalytic activity of this enzyme in comparison with RibH1 and other LSs. Additionally, we analyze the effect of pH on the structure of this enzyme, and the binding of riboflavin and 6,7-dimethyl-8-ribityllumazine to its active site.Fil: Klinke, Sebastian. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Parque Centenario. Instituto de Investigaciones BioquĂmicas de Buenos Aires. FundaciĂłn Instituto Leloir. Instituto de Investigaciones BioquĂmicas de Buenos Aires; ArgentinaFil: Zylberman, Vanesa. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Parque Centenario. Instituto de Investigaciones BioquĂmicas de Buenos Aires. FundaciĂłn Instituto Leloir. Instituto de Investigaciones BioquĂmicas de Buenos Aires; ArgentinaFil: Vega, Daniel Roberto. ComisiĂłn Nacional de EnergĂa AtĂłmica; ArgentinaFil: GuimarĂŁes, Beatriz G.. Laboratorio Nacional de Luz Sincrotron; BrasilFil: Braden, Bradford C.. Bowie State University; Estados UnidosFil: Goldbaum, Fernando Alberto. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Parque Centenario. Instituto de Investigaciones BioquĂmicas de Buenos Aires. FundaciĂłn Instituto Leloir. Instituto de Investigaciones BioquĂmicas de Buenos Aires; Argentin
Interstellar MHD Turbulence and Star Formation
This chapter reviews the nature of turbulence in the Galactic interstellar
medium (ISM) and its connections to the star formation (SF) process. The ISM is
turbulent, magnetized, self-gravitating, and is subject to heating and cooling
processes that control its thermodynamic behavior. The turbulence in the warm
and hot ionized components of the ISM appears to be trans- or subsonic, and
thus to behave nearly incompressibly. However, the neutral warm and cold
components are highly compressible, as a consequence of both thermal
instability in the atomic gas and of moderately-to-strongly supersonic motions
in the roughly isothermal cold atomic and molecular components. Within this
context, we discuss: i) the production and statistical distribution of
turbulent density fluctuations in both isothermal and polytropic media; ii) the
nature of the clumps produced by thermal instability, noting that, contrary to
classical ideas, they in general accrete mass from their environment; iii) the
density-magnetic field correlation (or lack thereof) in turbulent density
fluctuations, as a consequence of the superposition of the different wave modes
in the turbulent flow; iv) the evolution of the mass-to-magnetic flux ratio
(MFR) in density fluctuations as they are built up by dynamic compressions; v)
the formation of cold, dense clouds aided by thermal instability; vi) the
expectation that star-forming molecular clouds are likely to be undergoing
global gravitational contraction, rather than being near equilibrium, and vii)
the regulation of the star formation rate (SFR) in such gravitationally
contracting clouds by stellar feedback which, rather than keeping the clouds
from collapsing, evaporates and diperses them while they collapse.Comment: 43 pages. Invited chapter for the book "Magnetic Fields in Diffuse
Media", edited by Elisabete de Gouveia dal Pino and Alex Lazarian. Revised as
per referee's recommendation
Validation of Agent-Based Models in Economics and Finance
Since the survey by Windrum et al. (Journal of Artificial Societies and Social Simulation 10:8, 2007), research on empirical validation of agent-based models in economics has made substantial advances, thanks to a constant flow of high-quality contributions. This Chapter attempts to take stock of such recent literature to offer an updated critical review of the existing validation techniques. We sketch a simple theoretical framework that conceptualizes existing validation approaches, which we examine along three different dimensions: (i) comparison between artificial and real-world data; (ii) calibration and estimation of model parameters; and (iii) parameter space exploration. Finally, we discuss open issues in the field of ABM validation and estimation. In particular, we argue that more research efforts should be devoted toward advancing hypothesis testing in ABM, with specific emphasis on model stationarity and ergodicity
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Structural basis for the Pr-Pfr long-range signaling mechanism of a full-length bacterial phytochrome at the atomic level.
Phytochromes constitute a widespread photoreceptor family that typically interconverts between two photostates called Pr (red light–absorbing) and Pfr (far-red light–absorbing). The lack of full-length structures solved at the (near-)atomic level in both pure Pr and Pfr states leaves gaps in the structural mechanisms involved in the signal transmission pathways during the photoconversion. Here, we present the crystallographic structures of three versions from the plant pathogen Xanthomonas campestris virulence regulator XccBphP bacteriophytochrome, including two full-length proteins, in the Pr and Pfr states. The structures show a reorganization of the interaction networks within and around the chromophore-binding pocket, an α-helix/β-sheet tongue transition, and specific domain reorientations, along with interchanging kinks and breaks at the helical spine as a result of the photoswitching, which subsequently affect the quaternary assembly. These structural findings, combined with multidisciplinary studies, allow us to describe the signaling mechanism of a full-length bacterial phytochrome at the atomic level
Hepatitis Vaccination of Men Who Have Sex with Men at Gay Pride Events
Prevention researchers have advocated primary prevention such as vaccination in alternative venues. However, there have been major questions about both the attendance of, and the ability to, vaccinate high-risk individuals in such settings. The current study seeks to assess the feasibility of vaccinating high-risk men who have sex with men (MSM) at Gay Pride events. The research questions are: Do gay men who are sampled at Gay Pride events engage in more or less risky behavior than gay men sampled at other venues? Do the gay men who receive hepatitis vaccinations at Gay Pride engage in more or less risky behavior than gay men at Gay Pride who do not receive hepatitis vaccination? Of the 3689 MSM that completed the Field Risk Assessment (FRA), 1095/3689 = 29.68% were recruited at either the 2006 or 2007 Long Beach, California Gay Pride events. The remaining, 2594/3689 = 70.32% were recruited at Long Beach gay bars, gay community organizations and institutions, and through street recruitment in various gay enclaves in the Long Beach area. Logistic regression analysis yielded eight factors that were associated with non-attendance of Gay Pride: Age, had sex while high in the last 12 months, had unprotected anal intercourse (UAI) in the last 12 months, had sex for drugs/money in the last 12 months, been diagnosed with a sexually transmitted infection (STI) in the last 12 months, used nitrites (poppers) in the last 12 months, and used methamphetamine in the last 12 months. Identifying as White, Asian, or African American compared to Hispanic was also associated with non-attendance. Bivariate analysis indicated that, of the MSM sampled at Gay Pride, 280/1095 = 25.57% received a hepatitis vaccination there. The MSM sampled at Gay Pride who reported engaging in UAI or having used any stimulant (cocaine, crack-cocaine, or methamphetamine) in the last 12 months were more likely to receive hepatitis vaccination on-site. The results provide evidence for the viability of successfully vaccinating high-risk MSM at Gay Pride events. However, it is vital that no-cost vaccinations are also funded in other community settings such as STI clinics, drug treatment programs, prisons, universities, and other community resource centers in order to reach those additional high-risk MSM who do not attend Gay Pride
Relevance of the Diversity among Members of the Trypanosoma Cruzi Trans-Sialidase Family Analyzed with Camelids Single-Domain Antibodies
The sialic acid present in the protective surface mucin coat of
Trypanosoma cruzi is added by a membrane anchored
trans-sialidase (TcTS), a modified sialidase that is expressed from a large gene
family. In this work, we analyzed single domain camelid antibodies produced
against trans-sialidase. Llamas were immunized with a recombinant
trans-sialidase and inhibitory single-domain antibody fragments were obtained by
phage display selection, taking advantage of a screening strategy using an
inhibition test instead of the classic binding assay. Four single domain
antibodies displaying strong trans-sialidase inhibition activity against the
recombinant enzyme were identified. They share the same
complementarity-determining region 3 length (17 residues) and have very similar
sequences. This result indicates that they likely derived from a unique clone.
Probably there is only one structural solution for tight binding inhibitory
antibodies against the TcTS used for immunization. To our surprise, this single
domain antibody that inhibits the recombinant TcTS, failed to inhibit the
enzymatic activity present in parasite extracts. Analysis of individual
recombinant trans-sialidases showed that enzymes expressed from different genes
were inhibited to different extents (from 8 to 98%) by the llama
antibodies. Amino acid changes at key positions are likely to be responsible for
the differences in inhibition found among the recombinant enzymes. These results
suggest that the presence of a large and diverse trans-sialidase family might be
required to prevent the inhibitory response against this essential enzyme and
might thus constitute a novel strategy of T. cruzi to evade the
host immune system
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