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DSpace at MIT (80,380 recursos)
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MIT Open Access Articles

Mostrando recursos 41 - 60 de 15,246

41. SPATIALLY RESOLVED STAR FORMATION IMAGE AND THE ULTRALUMINOUS X-RAY SOURCE POPULATION IN NGC 2207/IC 2163 - Mineo, S.; Rappaport, Saul A.; Steinhorn, Benjamin S.; Levine, Alan M.; Gilfanov, M.; Pooley, D.
The colliding galaxy pair NGC 2207/IC 2163, at a distance of ~39 Mpc, was observed with Chandra, and an analysis reveals 28 well resolved X-ray sources, including 21 ultraluminous X-ray sources (ULXs) with L X ≿1039 erg s[superscript –1], as well as the nucleus of NGC 2207. The number of ULXs is comparable with the largest numbers of ULXs per unit mass in any galaxy yet reported. In this paper we report on these sources, and quantify how their locations correlate with the local star formation rates seen in spatially resolved star formation rate density images that we have constructed...

42. Technique for Efficient Evaluation of SRAM Timing Failure - Qazi, Masood; Tikekar, Mehul; Dolecek, Lara; Shah, Devavrat; Chandrakasan, Anantha P.
This brief presents a technique to evaluate the timing variation of static random access memory (SRAM). Specifically, a method called loop flattening, which reduces the evaluation of the timing statistics in the complex highly structured circuit to that of a single chain of component circuits, is justified. Then, to very quickly evaluate the timing delay of a single chain, a statistical method based on importance sampling augmented with targeted high-dimensional spherical sampling can be employed. The overall methodology has shown 650× or greater speedup over the nominal Monte Carlo approach with 10.5% accuracy in probability. Examples based on both the...

43. Mechanisms for mechanical trapping of geologically sequestered carbon dioxide - Cohen, Yossi; Rothman, Daniel H.
Carbon dioxide (CO[subscript 2]) sequestration in subsurface reservoirs is important for limiting atmospheric CO[subscript 2] concentrations. However, a complete physical picture able to predict the structure developing within the porous medium is lacking. We investigate theoretically reactive transport in the long-time evolution of carbon in the brine–rock environment. As CO[subscript 2] is injected into a brine–rock environment, a carbonate-rich region is created amid brine. Within the carbonate-rich region minerals dissolve and migrate from regions of high-to-low concentration, along with other dissolved carbonate species. This causes mineral precipitation at the interface between the two regions. We argue that precipitation in a...

44. A 249-Mpixel/s HEVC Video-Decoder Chip for 4K Ultra-HD Applications - Tikekar, Mehul; Huang, Chao-Tsung; Sze, Vivienne; Chandrakasan, Anantha P.; Juvekar, Chiraag Shashikant
High Efficiency Video Coding, the latest video standard, uses larger and variable-sized coding units and longer interpolation filters than [H.264 over AVC] to better exploit redundancy in video signals. These algorithmic techniques enable a 50% decrease in bitrate at the cost of computational complexity, external memory bandwidth, and, for ASIC implementations, on-chip SRAM of the video codec. This paper describes architectural optimizations for an HEVC video decoder chip. The chip uses a two-stage subpipelining scheme to reduce on-chip SRAM by 56 kbytes-a 32% reduction. A high-throughput read-only cache combined with DRAM-latency-aware memory mapping reduces DRAM bandwidth by 67%. The chip...

45. Sparsity-Driven Synthetic Aperture Radar Imaging: Reconstruction, autofocusing, moving targets, and compressed sensing - Cetin, Mujdat; Stojanovic, Ivana; Onhon, Ozben; Varshney, Kush; Samadi, Sadegh; Karl, William Clem; Willsky, Alan S.
This article presents a survey of recent research on sparsity-driven synthetic aperture radar (SAR) imaging. In particular, it reviews 1) the analysis and synthesis-based sparse signal representation formulations for SAR image formation together with the associated imaging results, 2) sparsity-based methods for wide-angle SAR imaging and anisotropy characterization, 3) sparsity-based methods for joint imaging and autofocusing from data with phase errors, 4) techniques for exploiting sparsity for SAR imaging of scenes containing moving objects, and 5) recent work on compressed sensing (CS)-based analysis and design of SAR sensing missions.

46. Topological pumping over a photonic Fibonacci quasicrystal - Verbin, Mor; Zilberberg, Oded; Lahini, Yoav; Kraus, Yaacov E.; Silberberg, Yaron
Quasiperiodic lattices have recently been shown to be a nontrivial topological phase of matter. Charge pumping—one of the hallmarks of topological states of matter—was recently realized for photons in a one-dimensional off-diagonal Harper model implemented in a photonic waveguide array. However, if the relationship between topological pumps and quasiperiodic systems is generic, one might wonder how to observe it in the canonical and most studied quasicrystalline system in one dimension—the Fibonacci chain. This chain is expected to facilitate a similar phenomenon, yet its discrete nature hinders the experimental study of such topological effects. Here, we overcome this obstacle by utilizing...

47. Multimaterial coatings with reduced thermal noise - Yam, William; Gras, Slawek; Evans, Matthew J.
The most sensitive measurements of time and space are made with resonant optical cavities, and these measurements are limited by coating thermal noise. The mechanical and optical performance requirements placed on coating materials, especially for interferometric gravitational wave detectors, have proven extremely difficult to meet despite a lengthy search. In this paper we propose a new approach to high performance coatings, the use of multiple materials at different depths in the coating. To support this we generalize previous work on thermal noise in two-material coatings to an arbitrary multimaterial stack, and develop a means of estimating absorption in these multimaterial...

48. Elite male faculty in the life sciences employ fewer women - Sheltzer, Jason Meyer; Smith, Joan C.
Women make up over one-half of all doctoral recipients in biology-related fields but are vastly underrepresented at the faculty level in the life sciences. To explore the current causes of women’s underrepresentation in biology, we collected publicly accessible data from university directories and faculty websites about the composition of biology laboratories at leading academic institutions in the United States. We found that male faculty members tended to employ fewer female graduate students and postdoctoral researchers (postdocs) than female faculty members did. Furthermore, elite male faculty—those whose research was funded by the Howard Hughes Medical Institute, who had been elected to...

49. Engineering adenylate cyclases regulated by near-infrared window light - Ryu, Min-Hyung; Kang, In-Hye; Nelson, Mathew D.; Jensen, Tricia M.; Lyuksyutova, Anna I.; Siltberg-Liberles, Jessica; Raizen, David M.; Gomelsky, Mark
Bacteriophytochromes sense light in the near-infrared window, the spectral region where absorption by mammalian tissues is minimal, and their chromophore, biliverdin IXα, is naturally present in animal cells. These properties make bacteriophytochromes particularly attractive for optogenetic applications. However, the lack of understanding of how light-induced conformational changes control output activities has hindered engineering of bacteriophytochrome-based optogenetic tools. Many bacteriophytochromes function as homodimeric enzymes, in which light-induced conformational changes are transferred via α-helical linkers to the rigid output domains. We hypothesized that heterologous output domains requiring homodimerization can be fused to the photosensory modules of bacteriophytochromes to generate light-activated fusions. Here,...

50.  β2-Adrenergic receptor agonist ameliorates phenotypes and corrects microRNA-mediated IGF1 deficits in a mouse model of Rett syndrome - Mellios, Nikolaos; Woodson, Jonathan; Garcia, Rodrigo; Crawford, Benjamin; Sharma, Jitendra; Sheridan, Steven; Sur, Mriganka; Haggarty, Stephen J.
Rett syndrome is a severe childhood onset neurodevelopmental disorder caused by mutations in methyl-CpG–binding protein 2 (MECP2), with known disturbances in catecholamine synthesis. Here, we show that treatment with the β2-adrenergic receptor agonist clenbuterol increases survival, rescues abnormalities in respiratory function and social recognition, and improves motor coordination in young male Mecp2-null (Mecp2−/y) mice. Importantly, we demonstrate that short-term treatment with clenbuterol in older symptomatic female heterozygous (Mecp2−/+) mice rescues respiratory, cognitive, and motor coordination deficits, and induces an anxiolytic effect. In addition, we reveal abnormalities in a microRNA-mediated pathway, downstream of brain-derived neurotrophic factor that affects insulin-like growth factor...

51. Planktonic Euryarchaeota are a significant source of archaeal tetraether lipids in the ocean - Summons, Roger Everett; DeLong, Edward; Lincoln, Sara A.; Wai, Brenner; Eppley, John M.; Church, Matthew J.
Archaea are ubiquitous in marine plankton, and fossil forms of archaeal tetraether membrane lipids in sedimentary rocks document their participation in marine biogeochemical cycles for >100 million years. Ribosomal RNA surveys have identified four major clades of planktonic archaea but, to date, tetraether lipids have been characterized in only one, the Marine Group I Thaumarchaeota. The membrane lipid composition of the other planktonic archaeal groups—all uncultured Euryarchaeota—is currently unknown. Using integrated nucleic acid and lipid analyses, we found that Marine Group II Euryarchaeota (MG-II) contributed significantly to the tetraether lipid pool in the North Pacific Subtropical Gyre at shallow to...

52. Tautomerism provides a molecular explanation for the mutagenic properties of the anti-HIV nucleoside 5-aza-5,6-dihydro-2'-deoxycytidine - Li, Deyu; Fedeles, Bogdan I.; Singh, Vipender; Peng, Chunte Sam; Silvestre, Katherine J.; Simi, Allison K.; Simpson, Jeffrey H.; Essigmann, John M.; Tokmakoff, Andrei
Viral lethal mutagenesis is a strategy whereby the innate immune system or mutagenic pool nucleotides increase the error rate of viral replication above the error catastrophe limit. Lethal mutagenesis has been proposed as a mechanism for several antiviral compounds, including the drug candidate 5-aza-5,6-dihydro-2′-deoxycytidine (KP1212), which causes A-to-G and G-to-A mutations in the HIV genome, both in tissue culture and in HIV positive patients undergoing KP1212 monotherapy. This work explored the molecular mechanism(s) underlying the mutagenicity of KP1212, and specifically whether tautomerism, a previously proposed hypothesis, could explain the biological consequences of this nucleoside analog. Establishing tautomerism of nucleic acid...

53. Platelets guide the formation of early metastatic niches - Labelle, Myriam; Begum, Shahinoor; Hynes, Richard O.
During metastasis, host cells are recruited to disseminated tumor cells to form specialized microenvironments (“niches”) that promote metastatic progression, but the mechanisms guiding the assembly of these niches are largely unknown. Tumor cells may autonomously recruit host cells or, alternatively, host cell-to-host cell interactions may guide the formation of these prometastatic microenvironments. Here, we show that platelet-derived rather than tumor cell-derived signals are required for the rapid recruitment of granulocytes to tumor cells to form “early metastatic niches.” Granulocyte recruitment relies on the secretion of CXCL5 and CXCL7 chemokines by platelets upon contact with tumor cells. Blockade of the CXCL5/7...

54. Rapid innovation diffusion in social networks - Kreindler, Gabriel Emanuel; Young, H. Peyton
Social and technological innovations often spread through social networks as people respond to what their neighbors are doing. Previous research has identified specific network structures, such as local clustering, that promote rapid diffusion. Here we derive bounds that are independent of network structure and size, such that diffusion is fast whenever the payoff gain from the innovation is sufficiently high and the agents’ responses are sufficiently noisy. We also provide a simple method for computing an upper bound on the expected time it takes for the innovation to become established in any finite network. For example, if agents choose log-linear...

55. Structures of benzylsuccinate synthase elucidate roles of accessory subunits in glycyl radical enzyme activation and activity - Funk, Michael Andrew; Drennan, Catherine L.; Judd, Evan T.; Marsh, E. Neil G.; Elliott, Sean J.
Anaerobic degradation of the environmental pollutant toluene is initiated by the glycyl radical enzyme benzylsuccinate synthase (BSS), which catalyzes the radical addition of toluene to fumarate, forming benzylsuccinate. We have determined crystal structures of the catalytic α-subunit of BSS with its accessory subunits β and γ, which both bind a [4Fe-4S] cluster and are essential for BSS activity in vivo. We find that BSSα has the common glycyl radical enzyme fold, a 10-stranded β/α-barrel that surrounds the glycyl radical cofactor and active site. Both accessory subunits β and γ display folds related to high potential iron–sulfur proteins but differ substantially...

56. Robust excitons inhabit soft supramolecular nanotubes - Rebentrost, Frank Patrick; Lloyd, Seth; Nelson, Keith Adam; Bawendi, Moungi G.; Eisele, Dorthe M.; Arias, Dylan H.; Fu, Xiaofeng; Bloemsma, Erik A.; Steiner, Colby P.; Jensen, Russell A.; Eisele, Holger; Tokmakoff, Andrei; Nicastro, Daniela; Knoester, Jasper
Nature's highly efficient light-harvesting antennae, such as those found in green sulfur bacteria, consist of supramolecular building blocks that self-assemble into a hierarchy of close-packed structures. In an effort to mimic the fundamental processes that govern nature’s efficient systems, it is important to elucidate the role of each level of hierarchy: from molecule, to supramolecular building block, to close-packed building blocks. Here, we study the impact of hierarchical structure. We present a model system that mirrors nature’s complexity: cylinders self-assembled from cyanine-dye molecules. Our work reveals that even though close-packing may alter the cylinders’ soft mesoscopic structure, robust delocalized excitons...

57. Three-dimensionally printed biological machines powered by skeletal muscle - Chan, Vincent; Sakar, Mahmut Selman; Asada, Harry; Cvetkovic, Caroline; Raman, Ritu; Williams, Brian J.; Tolish, Madeline; Bajaj, Piyush; Saif, M. Taher A.; Bashir, Rashid
Combining biological components, such as cells and tissues, with soft robotics can enable the fabrication of biological machines with the ability to sense, process signals, and produce force. An intuitive demonstration of a biological machine is one that can produce motion in response to controllable external signaling. Whereas cardiac cell-driven biological actuators have been demonstrated, the requirements of these machines to respond to stimuli and exhibit controlled movement merit the use of skeletal muscle, the primary generator of actuation in animals, as a contractile power source. Here, we report the development of 3D printed hydrogel “bio-bots” with an asymmetric physical...

58. Genetic analysis of the Complexin trans-clamping model for cross-linking SNARE complexes in vivo - Littleton, J. Troy; Cho, Richard William; Kummel, Daniel; Li, Feng; Baguley, Stephanie Wood; Coleman, Jeff; Rothman, James E.
Complexin (Cpx) is a SNARE-binding protein that regulates neurotransmission by clamping spontaneous synaptic vesicle fusion in the absence of Ca[superscript 2+] influx while promoting evoked release in response to an action potential. Previous studies indicated Cpx may cross-link multiple SNARE complexes via a trans interaction to function as a fusion clamp. During Ca[superscript 2+] influx, Cpx is predicted to undergo a conformational switch and collapse onto a single SNARE complex in a cis-binding mode to activate vesicle release. To test this model in vivo, we performed structure–function studies of the Cpx protein in Drosophila. Using genetic rescue approaches with cpx...

59. Functional recovery with recombinant human IGF1 treatment in a mouse model of Rett Syndrome - Garcia, Rodrigo; Castro, Jorge; Kwok, Showming; Banerjee, Abhishek; Petravicz, Jeremy C.; Woodson, Jonathan; Mellios, Nikolaos; Tropea, Daniela; Sur, Mriganka
Rett Syndrome is a neurodevelopmental disorder that arises from mutations in the X-linked gene methyl-CpG binding protein 2 (MeCP2). MeCP2 has a large number of targets and a wide range of functions, suggesting the hypothesis that functional signaling mechanisms upstream of synaptic and circuit maturation may contribute to our understanding of the disorder and provide insight into potential treatment. Here, we show that insulin-like growth factor-1 (IGF1) levels are reduced in young male Mecp2-null (Mecp2 [superscript − over y]) mice, and systemic treatment with recombinant human IGF1 (rhIGF1) improves lifespan, locomotor activity, heart rate, respiration patterns, and social and anxiety...

60. On the Choice of Average Solar Zenith Angle - Cronin, Timothy W.
Idealized climate modeling studies often choose to neglect spatiotemporal variations in solar radiation, but doing so comes with an important decision about how to average solar radiation in space and time. Since both clear-sky and cloud albedo are increasing functions of the solar zenith angle, one can choose an absorption-weighted zenith angle that reproduces the spatial- or time-mean absorbed solar radiation. Calculations are performed for a pure scattering atmosphere and with a more detailed radiative transfer model and show that the absorption-weighted zenith angle is usually between the daytime-weighted and insolation-weighted zenith angles but much closer to the insolation-weighted zenith...

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