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Caltech Authors (133.905 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 50.220

  1. A Redox-Switchable α-Cyclodextrin-Based [2]Rotaxane

    Zhao, Yan-Li; Dichtel, William R.; Trabolsi, Ali; Saha, Sourav; Aprahamian, Ivan; Stoddart, J. Fraser
    A bistable [2]rotaxane comprising an α-cyclodextrin (α-CD) ring and a dumbbell component containing a redox-active tetrathiafulvalene (TTF) ring system within its rod section has been synthesized using the Cu(I)-catalyzed azide−alkyne cycloaddition, and the redox-driven movements of the α-CD ring between the TTF and newly formed triazole ring systems have been elucidated. Microcalorimetric titrations on model complexes suggested that the α-CD ring prefers to reside on the TTF rather than on the triazole ring system by at least an order of magnitude. The fact that this situation does pertain in the bistable [2]rotaxane has not only been established quantitatively by electrochemical...

  2. The space groups of point group C_3: some corrections, some comments

    Marsh, Richard E.
    A survey of the October 2001 release of the Cambridge Structural Database [Cambridge Structural Database (1992). Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, England] has uncovered approximately 675 separate apparently reliable entries under space groups P3, P3_1, P3_2 and R3; in approximately 100 of these entries, the space-group assignment appears to be incorrect. Other features of these space groups are also discussed.

  3. Structure of MrsD, an FAD-binding protein of the HFCD family

    Blaesse, Michael; Kupke, Thomas; Huber, Robert; Steinbacher, Stefan
    MrsD from Bacillus sp. HIL-Y85/54728 is a member of the HFCD (homo-oligomeric flavin-containing Cys decarboxylases) family of flavoproteins and is involved in the biosynthesis of the lantibiotic mersacidin. It catalyses the oxidative decarboxylation of the C-terminal cysteine residue of the MrsA precursor peptide of mersacidin, yielding a (Z)-enethiol intermediate as the first step in the formation of the unusual amino acid S-[(Z)-2-aminovinyl]-methyl-D-cysteine. Surprisingly, MrsD was found to bind FAD, in contrast to the three other characterized members of the HFCD family, which bind FMN. To determine the molecular discriminators of FAD binding within the HFCD family, the crystal structure of...

  4. NIHAO project II: halo shape, phase-space density and velocity distribution of dark matter in galaxy formation simulations

    Butsky, Iryna; Macciò, Andrea V.; Dutton, Aaron A.; Wang, Liang; Obreja, Aura; Stinson, Greg S.; Penzo, Camilla; Kang, Xi; Keller, Ben W.; Wadsley, James
    We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects) cosmological simulations to study the effects of galaxy formation on key properties of dark matter (DM) haloes. NIHAO consists of ≈90 high-resolution smoothed particle hydrodynamics simulations that include (metal-line) cooling, star formation, and feedback from massive stars and supernovae, and cover a wide stellar and halo mass range: 10^6 ≲ M^*/M_⊙ ≲ 10^(11)(10^(9.5) ≲ M_(halo)/M_⊙ ≲ 10^(12.5)). When compared to DM-only simulations, the NIHAO haloes have similar shapes at the virial radius, R_(vir), but are substantially rounder inside ≈0.1Rvir. In NIHAO simulations, c/a increases with halo mass and integrated star...

  5. Shock–turbulence interaction in core-collapse supernovae

    Abdikamalov, Ernazar; Zhaksylykov, Azamat; Radice, David; Berdibek, Shapagat
    Nuclear shell burning in the final stages of the lives of massive stars is accompanied by strong turbulent convection. The resulting fluctuations aid supernova explosion by amplifying the non-radial flow in the post-shock region. In this work, we investigate the physical mechanism behind this amplification using a linear perturbation theory. We model the shock wave as a one-dimensional planar discontinuity and consider its interaction with vorticity and entropy perturbations in the upstream flow. We find that, as the perturbations cross the shock, their total turbulent kinetic energy is amplified by a factor of ∼2, while the average linear size of...

  6. High-energy gamma-ray observations of the accreting black hole V404 Cygni during its 2015 June outburst

    Loh, A.; Corbel, S.; Dubus, G.; Rodriguez, J.; Grenier, I.; Hovatta, T.; Pearson, T.; Readhead, A.; Fender, R.; Mooley, K.
    We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in 2015 June–July. Detailed analyses reveal a possible excess of γ-ray emission on 2015 26 June, with a very soft spectrum above 100 MeV, at a position consistent with the direction of V404 Cyg (within the 95 per cent confidence region and a chance probability of 4 × 10−4). This emission cannot be associated with any previously known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the light curve of V404 Cyg, at the end of...

  7. Symmetry and Orbit Detection via Lie-Algebra Voting

    Shi, Zeyun; Alliez, Pierre; Desbrun, Mathieu; Bao, Hujun; Huang, Jin
    In this paper, we formulate an automatic approach to the detection of partial, local, and global symmetries and orbits in arbitrary 3D datasets. We improve upon existing voting-based symmetry detection techniques by leveraging the Lie group structure of geometric transformations. In particular, we introduce a logarithmic mapping that ensures that orbits are mapped to linear subspaces, hence unifying and extending many existing mappings in a single Lie-algebra voting formulation. Compared to previous work, our resulting method offers significantly improved robustness as it guarantees that our symmetry detection of an input model is frame, scale, and reflection invariant. As a consequence,...

  8. Superalgebraically convergent smoothly windowed lattice sums for doubly periodic Green functions in three-dimensional space

    Bruno, Oscar P.; Shipman, Stephen P.; Turc, Catalin; Venakides, Stephanos
    This work, part I in a two-part series, presents: (i) a simple and highly efficient algorithm for evaluation of quasi-periodic Green functions, as well as (ii) an associated boundary-integral equation method for the numerical solution of problems of scattering of waves by doubly periodic arrays of scatterers in three-dimensional space. Except for certain ‘Wood frequencies’ at which the quasi-periodic Green function ceases to exist, the proposed approach, which is based on smooth windowing functions, gives rise to tapered lattice sums which converge superalgebraically fast to the Green function—that is, faster than any power of the number of terms used. This...

  9. Atmospheric mixing ratios of methyl ethyl ketone (2-butanone) in tropical, boreal, temperate and marine environments

    Yáñez-Serrano, A. M.; Nölscher, A. C.; Bourtsoukidis, E.; Derstroff, B.; Zannoni, N.; Gros, V.; Lanza, M.; Brito, J.; Noe, S. M.; House, E.; Hewitt, C. N.; Langford, B.; Nemitz, E.; Behrendt, T.; Williams, J.; Artaxo, P.; Andreae, M. O.; Kesselmeier, J.
    Methyl ethyl ketone (MEK) enters the atmosphere following direct emission from vegetation and anthropogenic activities, as well as being produced by the gas-phase oxidation of volatile organic compounds (VOCs) such as n-butane. This study presents the first overview of ambient MEK measurements at six different locations, characteristic of forested, urban and marine environments. In order to understand better the occurrence and behaviour of MEK in the atmosphere, we analyse diel cycles of MEK mixing ratios, vertical profiles, ecosystem flux data, and HYSPLIT back trajectories, and compare with co-measured VOCs. MEK measurements were primarily conducted with proton-transfer-reaction mass spectrometer (PTR-MS) instruments....

  10. Synthesis, Characterization, and Properties of Metal Phosphide Catalysts for the Hydrogen-Evolution Reaction

    Callejas, Juan F.; Read, Carlos G.; Roske, Christopher W.; Lewis, Nathan S.; Schaak, Raymond E.
    Hydrogen gas obtained by the electrolysis of water has long been proposed as a clean and sustainable alternative to fossil fuels. Noble metals such as Pt are capable of splitting water at low overpotentials, but the implementation of inexpensive solar-driven water-splitting systems and electrolyzers could benefit from the development of robust, efficient, and abundant alternatives to noble metal catalysts. Transition metal phosphides (M_xP_y) have recently been identified as a promising family of Earth abundant electrocatalysts for the hydrogen-evolution reaction (HER) and are capable of operating with low overpotentials at operationally relevant current densities while exhibiting stability under strongly acidic conditions....

  11. A Model Reduction Method for Elliptic PDEs with Random Input Using the Heterogeneous Stochastic FEM Framework

    Hou, Thomas Y.; Liu, Pengfei; Zhang, Zhiwen
    We introduce a model reduction method for elliptic PDEs with random input, which follows the heterogeneous stochastic finite element method framework and exploits the compactness of the solution operator in the stochastic direction on local regions of the spatial domain. This method consists of two stages and suits the multi-query setting. In the offline stage, we adaptively construct local stochastic basis functions that can capture the stochastic structure of the solution space in local regions of the domain. This is achieved through local Hilbert-Karhunen-Loève expansions of sampled stochastic solutions with randomly chosen forcing functions. In the online stage, for given...

  12. The vertical structure of upper ocean variability at the Porcupine Abyssal Plain during 2012-2013

    Damerell, Gillian M.; Heywood, Karen J.; Thompson, Andrew F.; Binetti, Umberto; Kaiser, Jan
    This study presents the characterization of variability in temperature, salinity and oxygen concentration, including the vertical structure of the variability, in the upper 1000 m of the ocean over a full year in the northeast Atlantic. Continuously profiling ocean gliders with vertical resolution between 0.5 and 1 m provide more information on temporal variability throughout the water column than time series from moorings with sensors at a limited number of fixed depths. The heat, salt and dissolved oxygen content are quantified at each depth. While the near surface heat content is consistent with the net surface heat flux, heat content...

  13. Heat advection processes leading to El Niño events as depicted by an ensemble of ocean assimilation products

    Ballester, Joan; Bordoni, Simona; Petrova, Desislava; Rodó, Xavier
    The oscillatory nature of El Niño-Southern Oscillation results from an intricate superposition of near-equilibrium balances and out-of-phase disequilibrium processes between the ocean and the atmosphere. The main objective of the present work is to perform an exhaustive spatiotemporal analysis of the upper ocean heat budget in an ensemble of state-of-the-art ocean assimilation products. We put specific emphasis on the ocean heat advection mechanisms, and their representation in individual ensemble members and in the different stages of the ENSO oscillation leading to EN events. Our analyses consistently show that the initial subsurface warming in the western equatorial Pacific is advected to...

  14. Disk Detective: Discovery of New Circumstellar Disk Candidates through Citizen Science

    Kuchner, Marc J.; Silverberg, Steven M.; Bans, Alissa S.; Bhattacharjee, Shambo; Kenyon, Scott J.; Debes, John H.; Currie, Thayne; García, Luciano; Jung, Dawoon; Lintott, Chris; McElwain, Michael; Padgett, Deborah L.; Rebull, Luisa M.; Wisniewski, John P.; Nesvold, Erika; Schawinski, Kevin; Thaller, Michelle L.; Grady, Carol A.; Biggs, Joseph; Bosch, Milton; C̆ernohous, Tadeás̆; Durantini Luca, Hugo A.; Hyogo, Michiharu; Wah, Lily Lau Wan; Piipuu, Art; Piñeiro, Fernanda
    The Disk Detective citizen science project aims to find new stars with 22 μm excess emission from circumstellar dust using data from NASA's Wide-field Infrared Survey Explorer (WISE) mission. Initial cuts on the AllWISE catalog provide an input catalog of 277,686 sources. Volunteers then view images of each source online in 10 different bands to identify false positives (galaxies, interstellar matter, image artifacts, etc.). Sources that survive this online vetting are followed up with spectroscopy on the FLWO Tillinghast telescope. This approach should allow us to unleash the full potential of WISE for finding new debris disks and protoplanetary disks....

  15. HST Grism Confirmation of Two z ~ 2 Structures from the Clusters around Radio-loud AGN (CARLA) Survey

    Noirot, Gaël; Vernet, Joël; De Breuck, Carlos; Wylezalek, Dominika; Galametz, Audrey; Stern, Daniel; Mei, Simona; Brodwin, Mark; Cooke, Elizabeth A.; Gonzalez, Anthony H.; Hatch, Nina A.; Rettura, Alessandro; Stanford, Spencer Adam
    Using Hubble Space Telescope slitless grism data, we report the spectroscopic confirmation of two distant structures at z ~ 2 associated with powerful high-redshift radio-loud active galactic nuclei (AGNs). These rich structures, likely (forming) clusters, are among the most distant structures currently known, and were identified on the basis of Spitzer/IRAC [3.6]–[4.5] color. We spectroscopically confirm nine members in the field of MRC 2036−254, comprising eight star-forming galaxies and the targeted radio galaxy. The median redshift is z = 2.000. We spectroscopically confirm 10 members in the field of B3 0756+406, comprising 8 star-forming galaxies and 2 AGNs, including the...

  16. Substrate Pre-Folding and Water Molecule Organization Matters for Terpene Cyclase Catalyzed Conversion of Unnatural Substrates

    Hammer, Stephan C.; Syrén, Per-Olof; Hauer, Bernhard
    Terpene cyclase enzymes have recently been challenged with terpene substrate derivatives to generate additional chemical complexity beyond to what is currently found in nature. Herein, molecular dynamics and biocatalysis are used to shed light on the flexibility and inherent limitation of a triterpene cyclase in converting unnatural substrates. Our studies suggest that populating binding modes which allows for concerted reaction pathways is a key element towards an expanded substrate scope and new chemistries displayed by terpene cyclases. Additionally, we show that the spatial organization of water, which is influenced by both the substrate architecture as well as the active site...

  17. Late Ediacaran post-collisional A-type syenites with shoshonitic affinities, northern Arabian-Nubian Shield: a possible mantle-derived A-type magma

    Gahlan, Hisham; Asimow, Paul; Al-Kahtany, Khaled
    The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments...

  18. Upper and lower bounds on a system’s bandwidth based on its zero-value time constants

    Hong, B.; Hajimiri, A.
    It is shown that for systems with no zeros and no complex poles, the classical estimate of the 3 dB cutoff frequency based on the sum of the zero-value time constants (ZVTs) is always conservative. The opposite problem is also solved, whereby a non-trivial upper bound on the cutoff frequency which depends only on the sum of the ZVTs and the system’s order is derived. It is demonstrated that both bounds are tight – specifically, the lower bound is approached by making one of the system’s poles increasingly dominant, whereas the best possible bandwidth is achieved when all of the system’s...

  19. Absorption cross-sections of ozone in the ultraviolet and visible spectral regions: Status report 2015

    Orphal, Johannes; Viatte, Camille
    The activity “Absorption Cross-Sections of Ozone” (ACSO) started in 2008 as a joint initiative of the International Ozone Commission (IO3C), the World Meteorological Organization (WMO) and the IGACO (“Integrated Global Atmospheric Chemistry Observations”) O3/UV subgroup to study, evaluate, and recommend the most suitable ozone absorption cross-section laboratory data to be used in atmospheric ozone measurements. The evaluation was basically restricted to ozone absorption cross-sections in the UV range with particular focus on the Huggins band. Up until now, the data of Bass and Paur published in 1985 (BP, 1985) are still officially recommended for such measurements. During the last decade...

  20. Kirchhoff’s theory for optical diffraction, its predecessor and subsequent development: the resilience of an inconsistent theory

    Buchwald, Jed Z.; Yeang, Chen-Pang
    Kirchhoff’s 1882 theory of optical diffraction forms the centerpiece in the long-term development of wave optics, one that commenced in the 1820s when Fresnel produced an empirically successful theory based on a reinterpretation of Huygens’ principle, but without working from a wave equation. Then, in 1856, Stokes demonstrated that the principle was derivable from such an equation albeit without consideration of boundary conditions. Kirchhoff’s work a quarter century later marked a crucial, and widely influential, point for he produced Fresnel’s results by means of Green’s theorem and function under specific boundary conditions. In the late 1880s, Poincaré uncovered an inconsistency...

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