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MIT Open Access Articles

Mostrando recursos 1 - 20 de 20.547

  1. Turning a Shove into a Nudge? A “Labeled Cash Transfer” for Education

    Benhassine, Najy; Devoto, Florencia; Duflo, Esther; Dupas, Pascaline; Pouliquen, Victor
    Conditional Cash Transfers (CCTs) have been shown to increase human capital investments, but their standard features make them expensive. We use a large randomized experiment in Morocco to estimate an alternative government-run program, a "labeled cash transfer" (LCT): a small cash transfer made to fathers of school-aged children in poor rural communities, not conditional on school attendance but explicitly labeled as an education support program. We document large gains in school participation. Adding conditionality and targeting mothers made almost no difference in our context. The program increased parents' belief that education was a worthwhile investment, a likely pathway for the...

  2. Toxoplasma gondii effectors are master regulators of the inflammatory response

    Melo, Mariane Bandeira; Jensen, Kirk D. C.; Saeij, Jeroen
    Toxoplasma is a highly successful parasite that establishes a life-long chronic infection. To do this, it must carefully regulate immune activation and host cell effector mechanisms. Here we review the latest developments in our understanding of how Toxoplasma counteracts the immune response of the host, and in some cases provokes it, through the use of specific parasite effector proteins. An emerging theme from these discoveries is that Toxoplasma effectors are master regulators of the pro-inflammatory response, which elicits many of the toxoplasmacidal mechanisms of the host. We speculate that combinations of these effectors present in certain Toxoplasma strains work to...

  3. Chemistry with an Artificial Primer of Polyhydroxybutyrate Synthase Suggests a Mechanism for Chain Termination

    Buckley, Rachael Marie; Stubbe, JoAnne
    Polyhydroxybutyrate (PHB) synthases (PhaCs) catalyze the conversion of 3-(R)-hydroxybutyryl CoA (HBCoA) to PHB, which is deposited as granules in the cytoplasm of microorganisms. The class I PhaC from Caulobacter crescentus (PhaC[subscript Cc]) is a highly soluble protein with a turnover number of 75 s[superscript –1] and no lag phase in coenzyme A (CoA) release. Studies with [1-[superscript 14]C]HBCoA and PhaC[subscript Cc] monitored by sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS–PAGE) and autoradiography reveal that the rate of elongation is much faster than the rate of initiation. Priming with the artificial primer [[superscript 3]H]sTCoA and monitoring for CoA release reveal a single...

  4. Heat conduction mechanisms in nanofluids and suspensions

    Wang, Jianjian; Gao, Jinwei; Chen, Gang; Zheng, Ruiting
    Nanofluids, liquids containing suspensions of nanoparticles, have been reported by some groups to exhibit substantially higher thermal conductivity than that of their corresponding base fluids that cannot be explained by existing theories. However, the reported high thermal conductivity sometimes cannot be reproduced by others. Potential mechanisms leading to this enhancement are still under scrutiny. In this paper, we first take a critical review of heat conduction mechanisms proposed in literature, and then summarize our work. Our experimental studies demonstrate that nanoparticle clustering is the key contributor to the thermal conductivity enhancement. Guided by this insight, we use graphite flakes as...

  5. Breakdown of the integer and fractional quantum Hall states in a quantum point contact

    Dillard, Colin R.; Lin, Xi; Kastner, Marc A.; Pfeiffer, L. N.; West, K. W.
    The integer and fractional quantum Hall states are known to breakdown at high dc bias, exhibiting deviation from the ideal incompressible behavior. We measure breakdown of the ν=2, 3, 4, 5 integer and the ν=4/3 and 5/3 fractional states in a quantum point contact (QPC) of lithographic width ∼600 nm. Dependence of the critical current on magnetic field, QPC gate voltage, and QPC width are presented. Of particular interest, the critical current of the 4/3 and 5/3 fractional states shows the opposite dependence on QPC width compared to the integer states. This previously unobserved result is not explained by current...

  6. Niosome Encapsulation of Curcumin: Characterization and Cytotoxic Effect on Ovarian Cancer Cells

    Xu, Ying-Qi; Chen, Wen-Rong; Tsosie, Jonathan; Xie, Xi; Li, Peng; Wan, Jian-Bo; He, Cheng-Wei; Chen, Mei-He
    Curcumin, a natural chemical compound found in Curcuma longa, has been applied in multiple medicinal areas from antibiotic to antitumor treatment. However, the chemical structure of curcumin results in poor stability, low solubility, and rapid degradation in vivo, hindering its clinical utilization. To address these issues, we have developed a novel niosome system composed of nonionic surfactants: Span 80, Tween 80, and Poloxamer 188. Curcumin was encapsulated in the niosomes with a high entrapment efficiency of 92.3 [plus or minus] 0.4%. This system provided controlled release of curcumin, thereby improving its therapeutic capability. Dynamic dialysis was conducted to evaluate the...

  7. Direct interfacial Y[subscript 731] oxidation in α[subscript 2] by a photoβ[subscript 2] subunit of E. coli class Ia ribonucleotide reductase

    Song, David Y.; Pizano, Arturo A.; Holder, Patrick G.; Stubbe, JoAnne; Nocera, Daniel G.
    Proton-coupled electron transfer (PCET) is a fundamental mechanism important in a wide range of biological processes including the universal reaction catalysed by ribonucleotide reductases (RNRs) in making de novo, the building blocks required for DNA replication and repair. These enzymes catalyse the conversion of nucleoside diphosphates (NDPs) to deoxynucleoside diphosphates (dNDPs). In the class Ia RNRs, NDP reduction involves a tyrosyl radical mediated oxidation occurring over 35 Å across the interface of the two required subunits (β[subscript 2] and α[subscript 2]) involving multiple PCET steps and the conserved tyrosine triad [Y[subscript 356](β[subscript 2])–Y[subscript 731](α[subscript 2])–Y[subscript 730](α2)]. We report the synthesis...

  8. Trigger Selection Strategies to Stabilize Program Verifiers

    Leino, K. R. M.; Pit-Claudel, Clement Francois
    SMT-based program verifiers often suffer from the so-called butterfly effect, in which minor modifications to the program source cause significant instabilities in verification times, which in turn may lead to spurious verification failures and a degraded user experience. This paper identifies matching loops (ill-behaved quantifiers causing an SMT solver to repeatedly instantiate a small set of quantified formulas) as a significant contributor to these instabilities, and describes some techniques to detect and prevent them. At their core, the contributed techniques move the trigger selection logic away from the SMT solver and into the high-level verifier: this move allows authors of...

  9. Intravascular fibrin molecular imaging improves the detection of unhealed stents assessed by optical coherence tomography in vivo

    Hara, Tetsuya; Ughi, Giovanni J.; McCarthy, Jason R.; Erdem, S. Sibel; Mauskapf, Adam; Lyon, Samantha C.; Fard, Ali M.; Edelman, Elazer R.; Tearney, Guillermo J.; Jaffer, Farouc A.
    Aims: Fibrin deposition and absent endothelium characterize unhealed stents that are at heightened risk of stent thrombosis. Optical coherence tomography (OCT) is increasingly used for assessing stent tissue coverage as a measure of healed stents, but cannot precisely identify whether overlying tissue represents physiological neointima. Here we assessed and compared fibrin deposition and persistence on bare metal stent (BMS) and drug-eluting stent (DES) using near-infrared fluorescence (NIRF) molecular imaging in vivo, in combination with simultaneous OCT stent coverage. Methods and results: Rabbits underwent implantation of one BMS and one DES without overlap in the infrarenal aorta (N = 20 3.5 ×...

  10. Biomass Torrefaction Models: 1. Volatile and Solid Product Evolution Kinetics

    Bates, Richard B.; Ghoniem, Ahmed F.
    The aim of this work is the development of a kinetics model for the evolution of the volatile and solid product composition during torrefaction conditions between 200 and 300 °C. Coupled to an existing two step solid mass loss kinetics mechanism, this model describes the volatile release kinetics in terms of a set of identifiable chemical components, permitting the solid product composition to be estimated by mass conservation. Results show that most of the volatiles released during the first stage include highly oxygenated species such as water, acetic acid, and carbon dioxide, while volatiles released during the second step are...

  11. Mechanical and thermal performance of aerogel-filled sandwich panels for building insulation

    Chen, Kevin; Neugebauer, Adam Halbert; Goutierre, Thomas; Tang, Astera S.; Glicksman, Leon R.; Gibson, Lorna J.
    Monolithic silica aerogels provide exceptional thermal insulation but have low mechanical properties. For instance, their flexural strength is typically 0.03–0.08 MPa. Here, we report on the development of a truss-core sandwich panel filled with compacted aerogel granules, designed to provide both mechanical support and thermal insulation. Mechanical and thermal properties of the sandwich panel prototype were measured and compared with theoretical models available in the literature. The models give a good description of the properties of aerogel-filled truss-core sandwich panels.

  12. Thermal conductivity and characterization of compacted, granular silica aerogel

    Neugebauer, Adam Halbert; Chen, Kevin; Tang, Astera S.; Allgeier, A.; Glicksman, Leon R.; Gibson, Lorna J.
    Monolithic silica aerogels are well known for their low thermal conductivity (approximately 15 mW/(m K)) (Aegerter et al. (Eds.), 2011. Aerogels Handbook, first ed., Springer-Verlag New York, LLC, New York, NY). Their low relative density (typically less than 5%) reduces conduction through the solid and their small pore size, typically less than one hundred nanometers, on the order of the mean free path of air, reduces conduction through air, as well as convection and radiation. As they are fragile and brittle, they are often used in a granular form in thermal insulation, with some increase in their thermal conductivity from...

  13. Box modeling of the Eastern Mediterranean sea

    Ashkenazy, Yosef; Stone, Peter H.; Rizzoli, Paola M.
    In ∼1990 a new source of deep water formation in the Eastern Mediterranean was found in the southern part of the Aegean sea. Till then, the only source of deep water formation in the Eastern Mediterranean was in the Adriatic sea; the rate of the deep water formation of the new Aegean source is 1 Sv, three times larger than the Adriatic source. We develop a simple three-box model to study the stability of the thermohaline circulation of the Eastern Mediterranean sea. The three boxes represent the Adriatic sea, Aegean sea, and the Ionian seas. The boxes exchange heat and...

  14. Suppression of grain growth by additive in nanostructured p-type bismuth antimony tellurides

    Zhang, Qian; Zhang, Qinyong; Chen, Shuo; Liu, Weishu; Lukas, Kevin; Yan, Xiao; Wang, Hengzhi; Wang, Dezhi; Opeil, Cyril; Chen, Gang; Ren, Zhifeng
    Grain growth is a major issue in the preparation of nanostructured bismuth-antimony-tellurides during hot pressing the nanopowders into dense bulk samples. To prevent grain agglomeration during ball milling and growth during hot pressing, organic agent (Oleic Acid, OA) as additive was added into the materials at the beginning of the ball milling process. With different concentrations of OA (0.5, 1.0, 1.5, 2.0, and 2.5 wt%), grains with different sizes are obtained. Structural analysis clearly shows that it is the particle size of the nanopowders that determines the final grain size in the densely compacted bulk samples. A combination of small...

  15. Asteroid 21 Lutetia: Low Mass, High Density

    Patzold, M.; Andert, T. P.; Asmar, S. W.; Anderson, J. D.; Barriot, J.- P.; Bird, M. K.; Hausler, B.; Hahn, M.; Tellmann, S.; Sierks, H.; Lamy, P.; Weiss, Benjamin P.
    Asteroid 21 Lutetia was approached by the Rosetta spacecraft on 10 July 2010. The additional Doppler shift of the spacecraft radio signals imposed by 21 Lutetia’s gravitational perturbation on the flyby trajectory were used to determine the mass of the asteroid. Calibrating and correcting for all Doppler contributions not associated with Lutetia, a least-squares fit to the residual frequency observations from 4 hours before to 6 hours after closest approach yields a mass of (1.700 ± 0.017) × 1018 kilograms. Using the volume model of Lutetia determined by the Rosetta Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera, the...

  16. Timescales and regions of the sensitivity of Atlantic meridional volume and heat transport: Toward observing system design

    Heimbach, Patrick; Wunsch, Carl Isaac; Ponte, Rui M.; Forget, Gael; Hill, Christopher N.; Utke, Jean
    A dual (adjoint) model is used to explore elements of the oceanic state influencing the meridional volume and heat transports (MVT and MHT) in the sub-tropical North Atlantic so as to understand their variability and to provide the elements of useful observational program design. Focus is on the effect of temperature (and salinity) perturbations. On short timescales (months), as expected, the greatest sensitivities are to local disturbances, but as the timescales extend back to a decade and longer, the region of influence expands to occupy much of the Atlantic basin and significant areas of the global ocean, although the influence...

  17. What processes drive the ocean heat transport?

    Ferrari, Raffaele; Ferreira, David
    The ocean contributes to regulating the Earth’s climate through its ability to transport heat from the equator to the poles. In this study we use long simulations of an ocean model to investigate whether the heat transport is carried primarily by wind-driven gyres or whether it is dominated by deep circulations associated with abyssal mixing and high latitude convection. The heat transport is computed as a function of temperature classes. In the Pacific and Indian ocean, the bulk of the heat transport is associated with wind-driven gyres confined to the thermocline. In the Atlantic, the thermocline gyres account for only...

  18. Ironing without control

    Toikka, Juuso T.
    I extend Myersonʼs [R. Myerson, Optimal auction design, Math. Oper. Res. 6 (1981) 58–73] ironing technique to more general objective functions. The approach is based on a generalized notion of virtual surplus which can be maximized pointwise even when the monotonicity constraint implied by incentive compatibility binds. It is applicable to quasilinear principal-agent models where the standard virtual surplus is weakly concave in the allocation or appropriately separable in the allocation and type. No assumptions on allocation rules are required beyond monotonicity.

  19. Nanoscale interfaces to biology

    Park, Sunho; Hamad, Kimberly S.
    Nanotechnology has held great promise for revolutionizing biology. The biological behavior of nanomaterials depends primarily on how they interface to biomolecules and their surroundings. Unfortunately, interface issues like non-specific adsorption are still the biggest obstacles to the success of nanobiotechnology and nanomedicine, and have held back widespread practical use of nanotechnology in biology. Not only does the biological interface of nanoparticles (NPs) need to be understood and controlled, but also NPs must be treated as biological entities rather than inorganic ones. Furthermore, one can adopt an engineering perspective of the NP–biological interface, realizing that it has unique, exploitable properties.

  20. Continuous-flow synthesis and purification of atropine with sequential in-line separations of structurally similar impurities

    Dai, Chunhui; Snead, David; Zhang, Ping; Jamison, Timothy F.
    Flow chemistry has attracted significant interest in pharmaceutical development, where substantial efforts have been directed toward the design of continuous processes. Here, we report a total synthesis of atropine in flow that features an unusual hydroxymethylation and separation of several byproducts with high structural similarity to atropine. Using a combination of careful pH control in three sequential liquid-liquid extractions and a functionalized resin, atropine is delivered by the flow system with >98% purity.

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