Recursos de colección

Caltech Authors (147.369 recursos)

Repository of works by Caltech published authors.

Status = In Press

Mostrando recursos 1 - 20 de 1.619

  1. Olefin Hydroarylation Catalyzed by (Pyridyl-Indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects

    Suslick, Benjamin A.; Liberman-Martin, Allegra L.; Wambach, Truman C.; Tilley, T. Don
    A series of Pt(II) complexes of the type (N–N)PtPh(SR2) (N-N = 2,2′-pyridyl-indolate) were prepared, and their performance as catalysts for the hydroarylation of olefins was assessed. Evidence that the catalysis is homogeneous and Pt mediated is provided by control experiments with added hindered base (2,6-di-tert-butyl-4-methylpyridine) and Hg(0). Two potential catalytic intermediates, (tBuPyInd)PtPh(C2H4) and (tBuPyInd)Pt(CH2CH2Ph)(C2H4), were synthesized and their catalytic efficacy was explored. Additionally, decomposition and deactivation pathways, including styrene formation via β-hydride elimination and ligand reductive demetallation, were identified.

  2. Polycyclic Furanobutenolide-Derived Cembranoid and Norcembranoid Natural Products: Biosynthetic Connections and Synthetic Efforts

    Craig, Robert A., II; Stoltz, Brian M.
    The polycyclic furanobutenolide-derived cembranoid and norcembranoid natural products are a family of congested, stereochemically complex, and extensively oxygenated polycyclic diterpenes and norditerpenes. Although the elegant architectures and biological activity profiles of these natural products have captured the attention of chemists since the isolation of the first members of the family in the 1990s, the de novo synthesis of only a single polycyclic furanobutenolide-derived cembranoid and norcembranoid has been accomplished. This article begins with a brief discussion of the proposed biosyntheses and biosynthetic connections among the polycyclic furanobutenolide-derived cembranoids and norcembranoids and then provides a comprehensive review of the synthetic efforts...

  3. Visual-Inertial Curve SLAM: Creating a Sparse Structured World without Feature Points

    Meier, Kevin; Chung, Soon-Jo; Hutchinson, Seth
    We present a simultaneous localization and mapping (SLAM) algorithm that uses Bézier curves as static landmark primitives rather than feature points. Our approach allows us to estimate the full 6-DOF pose of a robot while providing a structured map which can be used to assist a robot in motion planning and control. We demonstrate how to reconstruct the 3-D location of curve landmarks from a stereo pair and how to compare the 3-D shape of curve landmarks between chronologically sequential stereo frames to solve the data association problem. We also present a method to combine curve landmarks for mapping purposes,...

  4. Improved Synthesis for Modular Ascarosides Uncovers Biological Activity

    Zhang, Ying K.; Sanchez-Ayala, Marco A.; Sternberg, Paul W.; Srinivasan, Jagan; Schroeder, Frank C.
    A versatile synthesis of modular ascarosides, a family of signaling molecules from Caenorhabditis elegans and other nematodes, via hydrogenolysis of a cyclic sulfate derived from methyl-α-l-rhamnopyranoside is reported. The route enables selective introduction of different side chains at the 1, 2, and 4 positions of the sugar, as demonstrated for ascarosides from C. elegans and Pristionchus pacificus. Bioassays with synthetic samples of 4′-tigloyl ascaroside mbas#3 revealed its role as an avoidance or dispersal signal.

  5. Control of Hsp90 chaperone and its clients by N-terminal acetylation and the N-end rule pathway

    Oh, Jang-Hyun; Hyun, Ju-Yeon; Varshavsky, Alexander
    We found that the heat shock protein 90 (Hsp90) chaperone system of the yeast Saccharomyces cerevisiae is greatly impaired in naa10Δ cells, which lack the NatA N^α-terminal acetylase (Nt-acetylase) and therefore cannot N-terminally acetylate a majority of normally N-terminally acetylated proteins, including Hsp90 and most of its cochaperones. Chk1, a mitotic checkpoint kinase and a client of Hsp90, was degraded relatively slowly in wild-type cells but was rapidly destroyed in naa10Δ cells by the Arg/N-end rule pathway, which recognized a C terminus-proximal degron of Chk1. Diverse proteins (in addition to Chk1) that are shown here to be targeted for degradation...

  6. Real-time Optimal Power Flow

    Tang, Yujie; Dvijotham, Krishnamurthy; Low, Steven
    Future power networks are expected to incorporate a large number of distributed energy resources, which introduce randomness and fluctuations as well as fast control capabilities. But traditional optimal power flow methods are only appropriate for applications that operate on a slow timescale. In this paper, we build on recent work to develop a real-time algorithm for AC optimal power flow, based on quasi-Newton methods. The algorithm uses second order information to provide suboptimal solutions on a fast timescale, and can be shown to track the optimal power flow solution when the estimated second order information is sufficiently accurate. We also...

  7. Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

    Liao, Bolin; Zhao, Huan; Najafi, Ebrahim; Yan, Xiaodong; Tian, He; Tice, Jesse; Minnich, Austin J.; Wang, Han; Zewail, Ahmed H.
    As an emerging single elemental layered material with a low symmetry in-plane crystal lattice, black phosphorus (BP) has attracted significant research interest owing to its unique electronic and optoelectronic properties, including its widely tunable bandgap, polarization-dependent photoresponse and highly anisotropic in-plane charge transport. Despite extensive study of the steady-state charge transport in BP, there has not been direct characterization and visualization of the hot carriers dynamics in BP immediately after photoexcitation, which is crucial to understanding the performance of BP-based optoelectronic devices. Here we use the newly developed scanning ultrafast electron microscopy (SUEM) to directly visualize the motion of photoexcited...

  8. Multiple stages of aqueous alteration along fractures in mudstone and sandstone strata in Gale Crater, Mars

    Yen, A. S.; Ming, D. W.; Vaniman, D. T.; Gellert, R.; Blake, D. F.; Morris, R. V.; Morrison, S. M.; Bristow, T. F.; Chipera, S. J.; Edgett, K. S.; Treiman, A. H.; Clark, B. C.; Downs, R. T.; Farmer, J. D.; Grotzinger, J. P.; Rampe, E. B.; Schmidt, M. E.; Sutter, B.; Thompson, L. M.
    The Mars rover Curiosity in Gale crater conducted the first-ever direct chemical and mineralogical comparisons of samples that have clear parent (unaltered) and daughter (altered) relationships. The mineralogy and chemistry of samples within and adjacent to alteration halos in a sandstone formation were established by the Chemistry and Mineralogy (CheMin) X-ray diffraction (XRD) instrument and the Alpha Particle X-ray Spectrometer (APXS), respectively. The Stimson formation sandstones unconformably overlie the Murray mudstone formation and represent the youngest stratigraphic unit explored by Curiosity to date. Aqueous alteration of the parent sandstone resulted in a loss of half of the original crystalline mineral...

  9. Ubiquitin- and ATP-dependent unfoldase activity of P97/VCP•NPLOC4•UFD1L is enhanced by a mutation that causes multisystem proteinopathy

    Blythe, Emily E.; Olson, Kristine C.; Chau, Vincent; Deshaies, Raymond J.
    p97 is a “segregase” that plays a key role in numerous ubiquitin (Ub)-dependent pathways such as ER-associated degradation. It has been hypothesized that p97 extracts proteins from membranes or macromolecular complexes to enable their proteasomal degradation; however, the complex nature of p97 substrates has made it difficult to directly observe the fundamental basis for this activity. To address this issue, we developed a soluble p97 substrate—Ub-GFP modified with K48-linked ubiquitin chains—for in vitro p97 activity assays. We demonstrate that WT p97 can unfold proteins and that this activity is dependent on the p97 adaptor NPLOC4-UFD1L, ATP hydrolysis, and substrate ubiquitination,...

  10. Energetic cost of building a virus

    Mahmoudabadi, Gita; Milo, Ron; Phillips, Rob
    Viruses are incapable of autonomous energy production. Although many experimental studies make it clear that viruses are parasitic entities that hijack the molecular resources of the host, a detailed estimate for the energetic cost of viral synthesis is largely lacking. To quantify the energetic cost of viruses to their hosts, we enumerated the costs associated with two very distinct but representative DNA and RNA viruses, namely, T4 and influenza. We found that, for these viruses, translation of viral proteins is the most energetically expensive process. Interestingly, the costs of building a T4 phage and a single influenza virus are nearly...

  11. Use of Supramolecular Assemblies as Lithographic Resists

    Lewis, Scott M.; Fernandez, Antonio; DeRose, Guy A.; Hunt, Matthew S.; Whitehead, George F. S.; Lagzda, Agnese; Alty, Hayden R.; Ferrando-Soria, Jesus; Varey, Sarah; Kostopoulos, Andreas K.; Schedin, Fredrik; Muryn, Christopher A.; Timco, Grigore A.; Scherer, Axel; Yeates, Stephen G.; Winpenny, Richard E. P.
    A new resist material for electron beam lithography has been created that is based on a supramolecular assembly. Initial studies revealed that with this supramolecular approach, high-resolution structures can be written that show unprecedented selectivity when exposed to etching conditions involving plasmas.

  12. Probing the C-O bond-formation step in metalloporphyrin catalyzed C-H oxygenation reactions

    Liu, Wei; Cheng, Mu-Jeng; Nielsen, Robert J.; Goddard, William A., III; Groves, John T.
    The oxygen rebound mechanism, proposed four decades ago, is invoked in a wide range of oxygen and hetero-atom transfer reactions. In this process, a high-valent metal-oxo species abstracts a hydrogen atom from the substrate to generate a carbon-centered radical, which immediately recombines with the hydroxometal intermediate with very fast rate constants that can be in the ns to ps regime. In addition to catalyzing C-O bond formation, we found that manganese porphyrins can also directly catalyze C-H halogenations and pseudohalogenations, including chlorination, bromination and fluorination as well as C-H azidation. For these cases, we showed that long-lived substrate radicals are...

  13. Psi4 1.1: An Open-Source Electronic Structure Program Emphasizing Automation, Advanced Libraries, and Interoperability

    Parrish, Robert M.; Sokolov, Alexander Yu.
    Psi4 is an ab initio electronic structure program providing methods such as Hartree–Fock, density functional theory, configuration interaction, and coupled-cluster theory. The 1.1 release represents a major update meant to automate complex tasks, such as geometry optimization using complete-basis-set extrapolation or focal-point methods. Conversion of the top-level code to a Python module means that Psi4 can now be used in complex workflows alongside other Python tools. Several new features have been added with the aid of libraries providing easy access to techniques such as density fitting, Cholesky decomposition, and Laplace denominators. The build system has been completely rewritten to simplify...

  14. N_2-to-NH_3 Conversion by a triphos-Iron Catalyst and Enhanced Turnover under Photolysis

    Buscagan, Trixia M.; Oyala, Paul H.; Peters, Jonas C.
    Bridging iron hydrides are proposed to form at the active site of MoFe-nitrogenase during catalytic dinitrogen reduction to ammonia and may be key in the binding and activation of N_2 via reductive elimination of H_2. This possibility inspires the investigation of well-defined molecular iron hydrides as precursors for catalytic N_2-to-NH_3 conversion. Herein, we describe the synthesis and characterization of new P_2^(P′Ph)Fe(N_2)(H)_x systems that are active for catalytic N_2-to-NH_3 conversion. Most interestingly, we show that the yields of ammonia can be significantly increased if the catalysis is performed in the presence of mercury lamp irradiation. Evidence is provided to suggest that...

  15. Design, simulation, and characterization of a radial opposed migration ion and aerosol classifier (ROMIAC)

    Mui, Wilton; Mai, Huajun; Downard, Andrew J.; Seinfeld, John H.; Flagan, Richard C.
    We present the design, simulation, and characterization of the radial opposed migration ion and aerosol classifier (ROMIAC), a compact differential electrical mobility classifier. We evaluate the performance of the ROMIAC using a combination of finite element modeling and experimental validation of two nearly identical instruments using tetra-alkyl ammonium halide mass standards and sodium chloride particles. Mobility and efficiency calibrations were performed over a wide range of particle diameters and flow rates to characterize ROMIAC performance under the range of anticipated operating conditions. The ROMIAC performs as designed, though performance deviates from that predicted using simplistic models of the instrument. The...

  16. Physical principles for DNA tile self-assembly

    Evans, Constantine G.; Winfree, Erik
    DNA tiles provide a promising technique for assembling structures with nanoscale resolution through self-assembly by basic interactions rather than top-down assembly of individual structures. Tile systems can be programmed to grow based on logical rules, allowing for a small number of tile types to assemble large, complex assemblies that can retain nanoscale resolution. Such algorithmic systems can even assemble different structures using the same tiles, based on inputs that seed the growth. While programming and theoretical analysis of tile self-assembly often makes use of abstract logical models of growth, experimentally implemented systems are governed by nanoscale physical processes that can...

  17. Models for shock-induced ignition evaluated by detailed chemical kinetics for hydrogen/air in the context of deflagration-to-detonation transition

    Boeck, L. R.; Mével, R.; Sattelmayer, T.
    Deflagration-to-detonation transition can occur in industrial explosions involving highly reactive gas mixtures such as H_2/air. Large degrees of confinement and congestion support strong flame acceleration and the transition to detonation. Various mechanisms can accomplish the onset of detonation, such as mixing processes and instabilities or shock-induced ignition. This paper focuses on ignition by normal shock reflection. Two ignition modes behind reflected shock waves are distinguished: strong ignition, which enables direct detonation initiation; and weak ignition, leading to distributed ignition kernels (deflagrations). Models for the prediction of ignition modes by Voevodsky and Soloukhin, Meyer and Oppenheim, Radulescu et al., Grogan and...

  18. Mass inventory of the giant-planet formation zone in a solar nebula analogue

    Zhang, Ke; Bergin, Edwin A.; Blake, Geoffrey A.; Cleeves, L. Ilsedore; Schwarz, Kamber R.
    The initial mass distribution in the solar nebula is a critical input to planet formation models that seek to reproduce today’s Solar System. Traditionally, constraints on the gas mass distribution are derived from observations of the dust emission from disks, but this approach suffers from large uncertainties in dust opacity and gas-to-dust ratio. On the other hand, previous observations of gas tracers only probe surface layers above the bulk mass reservoir. Here we present the first partially spatially resolved observations of the ^(13)C^(18)O J = 3–2 line emission in the closest protoplanetary disk, TW Hydrae, a gas tracer that probes the bulk...

  19. Proposal for gravitational-wave detection beyond the standard quantum limit through EPR entanglement

    Ma, Yiqiu; Miao, Haixing; Pang, Belinda Heyun; Evans, Matthew; Zhao, Chunnong; Harms, Jan; Schnabel, Roman; Chen, Yanbei
    In continuously monitored systems the standard quantum limit is given by the trade-off between shot noise and back-action noise. In gravitational-wave detectors, such as Advanced LIGO, both contributions can be simultaneously squeezed in a broad frequency band by injecting a spectrum of squeezed vacuum states with a frequency-dependent squeeze angle. This approach requires setting up an additional long baseline, low-loss filter cavity in a vacuum system at the detector’s site. Here, we show that the need for such a filter cavity can be eliminated, by exploiting Einstein–Podolsky–Rosen (EPR)-entangled signals and idler beams. By harnessing their mutual quantum correlations and the...

  20. Mineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater, Mars

    Rampe, E. B.; Grotzinger, J. P.; Siebach, K.
    The Mars Science Laboratory Curiosity rover has been traversing strata at the base of Aeolis Mons (informally known as Mount Sharp) since September 2014. The Murray formation makes up the lowest exposed strata of the Mount Sharp group and is composed primarily of finely laminated lacustrine mudstone intercalated with rare crossbedded sandstone that is prodeltaic or fluvial in origin. We report on the first three drilled samples from the Murray formation, measured in the Pahrump Hills section. Rietveld refinements and FULLPAT full pattern fitting analyses of X-ray diffraction patterns measured by the MSL CheMin instrument provide mineral abundances, refined unit-cell...

Aviso de cookies: Usamos cookies propias y de terceros para mejorar nuestros servicios, para análisis estadístico y para mostrarle publicidad. Si continua navegando consideramos que acepta su uso en los términos establecidos en la Política de cookies.