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Caltech Authors (157.532 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 11.346

  1. Fiber taper characterization by optical backscattering reflectometry

    Lai, Yu-Hung; Yang, Ki Youl; Suh, Myoung-Gyun; Vahala, Kerry J.
    Fiber tapers provide a way to rapidly measure the spectra of many types of optical microcavities. Proper fabrication of the taper ensures that its width varies sufficiently slowly (adiabatically) along the length of the taper so as to maintain single spatial mode propagation. This is usually accomplished by monitoring the spectral transmission through the taper. In addition to this characterization method it is also helpful to know the taper width versus length. By developing a model of optical backscattering within the fiber taper, it is possible to use backscatter measurements to characterize the taper width versus length. The model uses...

  2. Sensing Floquet-Majorana fermions via heat transfer

    Molignini, Paolo; van Nieuwenburg, Evert; Chitra, R.
    Time periodic modulations of the transverse field in the closed XY spin-1/2 chain generate a very rich dynamical phase diagram, with a hierarchy of Z_n topological phases characterized by differing numbers of Floquet-Majorana modes. This rich phase diagram survives when the system is coupled to dissipative end reservoirs. Circumventing the obstacle of preparing and measuring quasienergy configurations endemic to Floquet-Majorana detection schemes, we show that stroboscopic heat transport and spin density are robust observables to detect both the dynamical phase transitions and Majorana modes in dissipative settings. We find that the heat current provides very clear signatures of these Floquet...

  3. Mechanisms of Tail-Anchored Membrane Protein Targeting and Insertion

    Chio, Un Seng; Cho, Hyunjun; Shan, Shu-ou
    Proper localization of membrane proteins is essential for the function of biological membranes and for the establishment of organelle identity within a cell. Molecular machineries that mediate membrane protein biogenesis need to not only achieve a high degree of efficiency and accuracy, but also prevent off-pathway aggregation events that can be detrimental to cells. The posttranslational targeting of tail-anchored proteins (TAs) provides tractable model systems to probe these fundamental issues. Recent advances in understanding TA-targeting pathways reveal sophisticated molecular machineries that drive and regulate these processes. These findings also suggest how an interconnected network of targeting factors, cochaperones, and quality...

  4. Anti-Markovnikov alkene oxidation by metal-oxo–mediated enzyme catalysis

    Hammer, Stephan C.; Kubik, Grzegorz; Watkins, Ella; Huang, Shan; Minges, Hannah; Arnold, Frances H.
    Catalytic anti-Markovnikov oxidation of alkene feedstocks could simplify synthetic routes to many important molecules and solve a long-standing challenge in chemistry. Here we report the engineering of a cytochrome P450 enzyme by directed evolution to catalyze metal-oxo–mediated anti-Markovnikov oxidation of styrenes with high efficiency. The enzyme uses dioxygen as the terminal oxidant and achieves selectivity for anti-Markovnikov oxidation over the kinetically favored alkene epoxidation by trapping high-energy intermediates and catalyzing an oxo transfer, including an enantioselective 1,2-hydride migration. The anti-Markovnikov oxygenase can be combined with other catalysts in synthetic metabolic pathways to access a variety of challenging anti-Markovnikov functionalization reactions.

  5. Search for a heavy resonance decaying to a top quark and a vector-like top quark at √s = 13 TeV

    Sirunyan, A. M.; Anderson, D.; Bendavid, J.; Bornheim, A.; Bunn, J.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A search is presented for massive spin-1 Z′ resonances decaying to a top quark and a heavy vector-like top quark partner T. The search is based on a 2.6 fb^(−1) sample of proton-proton collisions at 13 TeV collected with the CMS detector at the LHC. The analysis is optimized for final states in which the T quark decays to a W boson and a bottom quark. The focus is on all-jet final states in which both the W boson and the top quark decay into quarks that evolve into jets. The decay products of the top quark and of the...

  6. Measurement of the tt production cross section using events with one lepton and at least one jet in pp collisions at √s = 13 TeV

    Sirunyan, A. M.; Anderson, D.; Bendavid, J.; Bornheim, A.; Bunn, J.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A measurement of the tt production cross section at √s = 13 TeV is presented using proton-proton collisions, corresponding to an integrated luminosity of 2.2 fb^(−1), collected with the CMS detector at the LHC. Final states with one isolated charged lepton (electron or muon) and at least one jet are selected and categorized according to the accompanying jet multiplicity. From a likelihood fit to the invariant mass distribution of the isolated lepton and a jet identified as coming from the hadronization of a bottom quark, the cross section is measured to be σ_(tt) = 888 ± 2,(stat)^(+26)_(−28)(syst)±20(lumi)pb, in agreement with...

  7. Canyon effect and seasonal variability of deep-sea organisms in the NW Mediterranean: synchronous, year-long captures of "swimmers" from near-bottom sediment traps in a submarine canyon and its adjacent open slope

    Romano, C.; Flexas, M. M.; Segura, M.; Román, S.; Bahamon, N.; Gili, J. M.; Sanchez-Vidal, A.; Martin, D.
    Numerous organisms, including both passive sinkers and active migrators, are captured in sediment traps together with sediments. By capturing these “swimmers”, the traps become an extraordinarily tool to obtain relevant information on the biodiversity and dynamics of deep-sea organisms. Here we analyze near-bottom swimmers larger than 500 μm and their fluxes collected from eight near-bottom sediment traps installed on instrumented moorings deployed nearby Blanes Canyon (BC). Our data, obtained from November 2008 to October 2009 with a sampling rate of 15 days, constitutes the first year-long, continuous time series of the whole swimmers’ community collected at different traps and bottom...

  8. Clarifying the Intracellular and Extracellular Lifestyles of CF Microbes in Three Dimensions

    Jorth, P.; Newman, D. K.
    Imaging of sputum smears and thin sections have suggested that P. aeruginosa persists as extracellular bacterial aggregates, or biofilms, during chronic CF infections. However, emerging evidence indicates that host cells may also provide a protective reservoir for P. aeruginosa during infection. While phagocytes that flood the CF airways normally kill infecting bacteria, CFTR-deficient murine macrophages fail to kill P. aeruginosa, raising the possibility that CF macrophages could house viable bacteria. Epithelial cells may also provide an intracellular home for P. aeruginosa, since P. aeruginosa can invade epithelial cells in tissue culture systems. Together, these results have led to our hypothesis that the...

  9. Aggregation of Nontuberculous Mycobacteria in Vitro and in Situ

    DePas, W.; Bergkessel, M.; Newman, D. K.
    The incidence of nontuberculous mycobacterial (NTM) infections in cystic fibrosis (CF) patients is increasing, with some CF clinics in the US reporting NTM prevalence upwards of 25%. In addition, the current treatment regimen for NTM involves long courses of antibiotic cocktails that demonstrate limited efficacy and cause frequent and serious side effects. Mycobacterium abscessus, in particular, is difficult to treat and correlates with a more rapid decline in lung function compared to Mycobacterium avium complex. Studies with zebrafish and human cell cultures have demonstrated that M. abscessus is more virulent when aggregated into cord-like biofilms, in part because of the decreased ability...

  10. Real-Time In Vivo Intraocular Pressure Monitoring using an Optomechanical Implant and an Artificial Neural Network

    Kim, Kun ho; Lee, Jeong Oen; Du, Juan; Sretavan, David; Choo, Hyuck
    Optimized glaucoma therapy requires frequent monitoring and timely lowering of elevated intraocular pressure (IOP). A recently developed microscale IOP-monitoring implant, when illuminated with broadband light, reflects a pressure-dependent optical spectrum that is captured and converted to measure IOP. However, its accuracy is limited by background noise and the difficulty of modeling non-linear shifts of the spectra with respect to pressure changes. Using an end-to-end calibration system to train an artificial neural network (ANN) for signal demodulation we improved the speed and accuracy of pressure measurements obtained with an optically probed IOP-monitoring implant and make it suitable for real-time in vivo...

  11. SutA is a Global Transcriptional Regulator Operating during Slow Growth in Pseudomonas Aeruginosa

    Bergkessel, M.; Newman, D. K.; Babin, B.; Tirrell, D. A.
    Chronic Pseudomonas aeruginosa infections, a major cause of morbidity and mortality for CF patients, are highly resistant to antibiotic treatment, and many lines of evidence suggest that this is due in part to slow-growing states imposed by the lung environment. In previously published work (Babin BM, et al. Proc Natl Acad Sci 2016;113(5):E597-605), we identified a novel RNA polymerase-binding protein that is post-transcriptionally upregulated under such slow-growth conditions. We named this small, acidic protein SutA, and showed using RNA-Seq and ChIP-Seq that it associates with RNA polymerase (RNAP) at many genomic regions, and tends to positively affect transcript levels for genes with...

  12. Genetic and Molecular Analysis of Phosphorus Homeostasis Regulatory Genes. A Key Nutrient for Aquaculture Sustainability

    Suarez-Bregua, P.; Torres-Nunez, E.; Prober, D.; Cerda-Reverter, J. M.; Guerreiro, P. M.; Du, S. J.; Canestro, C.; Rotllant, J.
    [no abstract]

  13. Highly Efficient Excitation of Surface Plasmons Using a Si Gable Tip

    Dewanjee, Arnab; Alam, M. Z.; Aitchison, J. Stewart; Mojahedi, Mo
    A compact and highly efficient technique to excite SPP mode at an Au/SiO2 interface by using an engineered high index (silicon) gabled tip at the 1550 nm wavelength has been proposed. The optimized geometry of the Si tip enables a highly efficient excitation of the single interface SPP mode through near field interaction in an ultra-compact setup. An experimental demonstration of the proposed scheme is also presented in the paper which converts 25.5% of the total input power to an SPP mode. With an improved fabrication, this efficiency can reach as high as 52%. The device is compact, facilitates on-chip...

  14. A New Zealand perspective on centennial-scale Southern Hemisphere westerly wind shifts during the last two millennia

    Hinojosa, Jessica L.; Moy, Christopher M.; Stirling, Claudine H.; Wilson, Gary S.; Eglinton, Timothy I.
    The strength and latitudinal position of the Southern Hemisphere westerly winds control regional climate and influence the global carbon cycle by physically regulating Southern Ocean CO_2 exchange with the atmosphere. However, the mechanisms driving interannual to millennial variability of the westerlies remain poorly understood. Here, we present an 1800-yr record of westerly wind variability recorded in New Zealand fjord sediments. Located west of the Southern Alps, fjord basins receive large amounts of westerly-driven orographic precipitation (>6 m yr^(−1)) and strong winds lead to vigorous fjord mixing. Because of these links, reconstructing precipitation and fjord circulation provides information on westerly wind...

  15. Measurement of the top quark mass using single top quark events in proton-proton collisions at √s = 8 TeV

    Sirunyan, A. M.; Anderson, D.; Bendavid, J.; Bornheim, A.; Bunn, J.; Duarte, J.; Lawhorn, J. M.; Mott, A.; Newman, H. B.; Pena, C.; Spiropulu, M.; Vlimant, J. R.; Xie, S.; Zhu, R. Y.; Dubinin, M.
    A measurement of the top quark mass is reported in events containing a single top quark produced via the electroweak t channel. The analysis is performed using data from proton-proton collisions collected with the CMS detector at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb^(−1). Top quark candidates are reconstructed from their decay to a W boson and a b quark, with the W boson decaying leptonically to a muon and a neutrino. The final state signature and kinematic properties of single top quark events in the t channel are used...

  16. On asymmetric coverings and covering numbers

    Applegate, David; Rains, E. M.; Sloane, N. J. A.
    An asymmetric covering D(n,R) is a collection of special subsets S of an n-set such that every subset T of the n-set is contained in at least one special S with |S| - |T| ≤ R. In this paper we compute the smallest size of any D(n,1) for n ≤ 8. We also investigate “continuous” and “banded” versions of the problem. The latter involves the classical covering numbers C(n, k, k-1), and we determine the following new values: C(10, 5, 4) = 51, C(11, 7, 6) = 84, C(12, 8, 7) = 126, C(13, 9, 8) = 185 and C(14,...

  17. The EKG Sequence

    Lagarias, J. C.; Rains, E. M.; Sloane, N. J. A.
    The EKC or electrocardiogram sequence is defined by a(1) = 1, a(2) = 2 and, for n ≥ 3, a(n) is the smallest natural number not already in the sequence with the property that gcd{a(n − 1),a(n)} > 1. In spite of its erratic local behavior, which when plotted resembles an electrocardiogram, its global behavior appears quite regular. We conjecture that almost all a(n) satisfy the asymptotic formula a(n) = n(1+1/(3logn)) + o(n/log n) as n → ∞ and that the exceptional values a(n) = p and a(n) = 3p, for p a prime, produce the spikes in the EKG...

  18. Artificial Micromotors in the Mouse’s Stomach: A Step toward in Vivo Use of Synthetic Motors

    Gao, Wei; Dong, Renfeng; Thamphiwatana, Soracha; Li, Jinxing; Gao, Weiwei; Zhang, Liangfang; Wang, Joseph
    Artificial micromotors, operating on locally supplied fuels and performing complex tasks, offer great potential for diverse biomedical applications, including autonomous delivery and release of therapeutic payloads and cell manipulation. Various types of synthetic motors, utilizing different propulsion mechanisms, have been fabricated to operate in biological matrices. However, the performance of these man-made motors has been tested exclusively under in vitro conditions (outside the body); their behavior and functionalities in an in vivo environment (inside the body) remain unknown. Herein, we report an in vivo study of artificial micromotors in a living organism using a mouse model. Such in vivo evaluation...

  19. Motion-based threat detection using microrods: experiments and numerical simulations

    Ezhilan, Barath; Gao, Wei; Pei, Allen; Rozen, Isaac; Dong, Renfeng; Jurado-Sanchez, Beatriz; Wang, Joseph; Saintillan, David
    Motion-based chemical sensing using microscale particles has attracted considerable recent attention. In this paper, we report on new experiments and Brownian dynamics simulations that cast light on the dynamics of both passive and active microrods (gold wires and gold–platinum micromotors) in a silver ion gradient. We demonstrate that such microrods can be used for threat detection in the form of a silver ion source, allowing for the determination of both the location of the source and concentration of silver. This threat detection strategy relies on the diffusiophoretic motion of both passive and active microrods in the ionic gradient and on...

  20. The Orbiting Carbon Observatory-2 early science investigations of regional carbon dioxide fluxes

    Eldering, A.; Wennberg, P. O.; Crisp, D.; Schimel, D. S.; Gunson, M. R.; Chatterjee, A.; Liu, J.; Schwander, F. M.; Sun, Y.; O'Dell, C. W.; Frankenberg, C.; Taylor, T.; Fisher, B.; Osterman, G. B.; Wunch, D.; Hakkarainen, J.; Tamminen, J.; Weir, B.
    NASA’s Orbiting Carbon Observatory-2 (OCO-2) mission was motivated by the need to diagnose how the increasing concentration of atmospheric carbon dioxide (CO_2) is altering the productivity of the biosphere and the uptake of CO_2 by the oceans. Launched on 2 July 2014, OCO-2 provides retrievals of the column-averaged CO_2 dry-air mole fraction (XCO_2) as well as the fluorescence from chlorophyll in terrestrial plants. The seasonal pattern of uptake by the terrestrial biosphere is recorded in fluorescence and the drawdown of XCO_2 during summer. Launched just before one of the most intense El Niños of the past century, OCO-2 measurements of...

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