Recursos de colección

Caltech Authors (143.226 recursos)

Repository of works by Caltech published authors.

Status = In Press

Mostrando recursos 1 - 20 de 1.509

  1. Accelerated Oxygen Atom Transfer and C−H Bond Oxygenation by Remote Redox Changes in Fe_3Mn-Iodosobenzene Adducts

    De Ruiter, Graham; Carsch, Kurtis M.; Gul, Sheraz; Chatterjee, Ruchira; Thompson, Niklas B.; Takase, Michael K.; Yano, Junko; Agapie, Theodor
    We report the synthesis, characterization, and reactivity of [Lfe_3(PhPz)_3OMn(^sPhIO)][OTf]_x (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene–metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, ^(57)Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (Fe^(III)_2Fe^(II)Mn^(II) vs. Fe^(III)_3Mn^(II)) influence oxygen atom transfer in tetranuclear Fe_3Mn clusters. In particular, a one-electron redox change at a distal metal site leads to a change in oxygen atom transfer reactivity by ca. two...

  2. MicroRNA regulation and analytical methods in cancer cell metabolism

    Zhang, Ling-Fei; Jiang, Shuai; Liu, Mo-Fang
    The reprogramming of glucose metabolism from oxidative to glycolytic metabolism, known as the Warburg effect, is an anomalous characteristic of cancer cell metabolism. Recent studies have revealed a subset of microRNAs (miRNAs) that play critical roles in regulating the reprogramming of glucose metabolism in cancer cells. These miRNAs regulate cellular glucose metabolism by directly targeting multiple metabolic genes, including those encoding key glycolytic enzymes. In the first part of this review, we summarized the recent knowledge of miRNA regulation in the reprogramming of glucose metabolism in cancer cells and discussed the potential utilization of the key miRNA regulators as metabolic...

  3. Atomimetic Mechanical Structures with Nonlinear Topological Domain Evolution Kinetics

    Frazier, Michael J.; Kochmann, Dennis M.
    A mechanical metamaterial, a simple, periodic mechanical structure, is reported, which reproduces the nonlinear dynamic behavior of materials undergoing phase transitions and domain switching at the structural level. Tunable multistability is exploited to produce switching and transition phenomena whose kinetics are governed by the same Allen–Cahn law commonly used to describe material-level, structural-transition processes. The reported purely elastic mechanical system displays several key features commonly found in atomic- or mesoscale physics of solids. The rotating-mass network shows qualitatively analogous features as, e.g., ferroic ceramics or phase-transforming solids, and the discrete governing equation is shown to approach the phase field equation...

  4. Rate-and-state friction properties of the Longitudinal Valley Fault from kinematic and dynamic modeling of seismic and aseismic slip

    Thomas, Marion Y.; Avouac, Jean-Philippe; Lapusta, Nadia
    The Longitudinal Valley Fault (LVF, Taiwan) is a fast slipping fault (∼5 cm/yr), which exhibits both seismic, and aseismic slip. Geodetic and seismological observations (1992-2010) were used to infer the temporal evolution of fault slip [Thomas et al., 2014a]. This kinematic model is used here to estimate spatial variations of steady-state velocity dependence of fault friction and to develop a simplified fully-dynamic rate-and-state model of the LVF. Based on the postseismic slip, we estimate that the rate-and-state parameter (a – b) σ[over-bar] decreases from ∼1.2 MPa near the surface to near velocity-neutral at 19 km depth. The inferred (a −...

  5. Low-Coherence Frames from Group Fourier Matrices

    Thill, Matthew; Hassibi, Babak
    Many problems in areas such as compressive sensing and coding theory seek to design a set of equal-norm vectors with large angular separation. This idea is essentially equivalent to constructing a frame with low coherence. The elements of such frames can in turn be used to build high-performance spherical codes, quantum measurement operators, and compressive sensing measurement matrices, to name a few applications. In this work, we allude to the group-frame construction first described by Slepian and further explored in the works of Vale and Waldron. We present a method for selecting representations of a nite group to construct a...

  6. p97/VCP promotes degradation of CRBN substrate glutamine synthetase and neosubstrates

    Nguyen, Van Thang; Li, Jing; Lu, Chin-Chun (Jean); Mamrosh, Jennifer L.; Lu, Gang; Cathers, Brian E.; Deshaies, Raymond J.
    Glutamine synthetase (GS) plays an essential role in metabolism by catalyzing the synthesis of glutamine from glutamate and ammonia. Our recent study showed that CRBN, a direct protein target for the teratogenic and antitumor activities of immunomodulatory drugs such as thalidomide, lenalidomide, and pomalidomide, recognizes an acetyl degron of GS, resulting in ubiquitylation and degradation of GS in response to glutamine. Here, we report that valosin-containing protein (VCP)/p97 promotes the degradation of ubiquitylated GS, resulting in its accumulation in cells with compromised p97 function. Notably, p97 is also required for the degradation of all four known CRBN neo-substrates [Ikaros family...

  7. Sensing relative signal in the Tgf-β/Smad pathway

    Frick, Christopher L.; Yarka, Clare; Nunns, Harry; Goentoro, Lea
    How signaling pathways function reliably despite cellular variation remains a question in many systems. In the transforming growth factor-β (Tgf-β) pathway, exposure to ligand stimulates nuclear localization of Smad proteins, which then regulate target gene expression. Examining Smad3 dynamics in live reporter cells, we found evidence for fold-change detection. Although the level of nuclear Smad3 varied across cells, the fold change in the level of nuclear Smad3 was a more precise outcome of ligand stimulation. The precision of the fold-change response was observed throughout the signaling duration and across Tgf-β doses, and significantly increased the information transduction capacity of the...

  8. Effect of environmental conditions on the relationship between solar induced fluorescence and gross primary productivity at an OzFlux grassland site

    Verma, Manish; Schimel, David; Evans, Bradley; Frankenberg, Christian; Beringer, Jason; Drewry, Darren T.; Magney, Troy; Marang, Ian; Hutley, Lindsay; Moore, Caitlin; Eldering, Annmarie
    Recent studies have utilized coarse spatial and temporal resolution remotely sensed solar induced fluorescence (SIF) for modeling terrestrial gross primary productivity (GPP) at regional scales. Although these studies have demonstrated the potential of SIF, there have been concerns about the ecophysiological basis of the relationship between SIF and GPP in different environmental conditions. Launched in 2014, the Orbiting Carbon Observatory-2 (OCO-2) has enabled fine scale (1.3-by-2.5 km) retrievals of SIF that are comparable with measurements recorded at eddy covariance towers. In this study, we examine the effect of environmental conditions on the relationship of OCO-2 SIF with tower GPP over the...

  9. High Temperature Electronic and Thermal Transport Properties of EuGa_(2−x)In_xSb_2

    Chanakian, Sevan; Weber, Rochelle; Aydemir, Umut; Ormeci, Alim; Fleurial, Jean-Pierre; Bux, Sabah; Snyder, G. Jeffrey
    The Zintl phase EuGa_2Sb_2 was synthesized via ball milling followed by hot pressing. The crystal structure of EuGa_2Sb_2 is comprised of a 3-D network of polyanionic [Ga_2Sb_2]^(2−) tunnels filled with Eu cations that provide charge balance (Eu^(2+)[Ga_2Sb_2]^(2−)). Here we report the temperature-dependent resistivity, Hall Effect, Seebeck coefficient and thermal conductivity for EuGa_(2−x)In_xSb_2 (x = 0, 0.05, 0.1) from 300 K to 775 K. Experimental results demonstrate that the material is a p-type semiconductor. However, a small band gap (∼0.1 eV) prevents EuGa_2Sb_2 from having high zT at higher temperatures. Isoelectronic substitution of In on the Ga site leads to point...

  10. Re-evaluating the foundations of lncRNA–Polycomb function

    Blanco, Mario R.; Guttman, Mitchell
    Many lncRNAs are thought to interact with the polycomb repressive complex 2 (PRC2) in order to regulate gene expression. A central example of this lncRNA–PRC2 paradigm in gene regulation is HOTAIR. In this issue of The EMBO Journal, a study (Portoso et al, 2017) reports that while HOTAIR binds PRC2 with high affinity, the complex itself is dispensable for HOTAIR‐mediated transcriptional silencing. This study raises important questions about the role of PRC2 interactions for lncRNA‐mediated functions and argues for a re‐evaluation of this lncRNA–PRC2 functional paradigm.

  11. Embedded Mean-Field Theory with Block-Orthogonalized Partitioning

    Ding, Feizhi; Manby, Frederick R.; Miller, Thomas F., III
    Embedded mean-field theory (EMFT) provides a simple, flexible framework for describing subsystems at different levels of mean-field theory. Subsystems are defined by partitioning a one-particle basis set, with a natural choice being the atomic orbital (AO) basis. Although generally well behaved, EMFT with AO partitioning can exhibit unphysical collapse of the self-consistent solution. To avoid this issue, we introduce subsystem partitioning of a block-orthogonalized (BO) basis set; this eliminates the unphysical collapse without significantly increasing computational cost. We also investigate a non-self-consistent implementation of EMFT, in which the density matrix is obtained using BO partitioning and the final energy evaluated...

  12. Approaches to homogeneously catalyzed CO hydrogenation: A personal retrospective

    Labinger, Jay A.
    Homogeneous catalysis of CO hydrogenation has long been a topic of interest in the organometallic community. In this review article I discuss a variety of approaches to the problem that have been a significant component of my research program for over 40 years.

  13. Negative regulation of Smad1 pathway and collagen IV expression by store-operated Ca2+ entry in glomerular mesangial cells

    Wu, Peiwen; Ren, Yuezhong; Ma, Yuhong; Wang, Yanxia; Jiang, Hui; Chaudhari, Sarika; Davis, Mark E.; Zuckerman, Jonathan E.; Ma, Rong
    Collagen IV (Col IV) is a major component of expanded glomerular extracellular matrix in diabetic nephropathy and Smad1 is a key molecule regulating Col IV expression in mesangial cells (MCs). The present study was conducted to determine if Smad1 pathway and Col IV protein abundance were regulated by store-operated Ca2+ entry (SOCE). In cultured human MCs, pharmacological inhibition of SOCE significantly increased the total amount of Smad1 protein. Activation of SOCE blunted high glucose-increased Smad1 protein content. Treating human MCs with angiotensin II at 1 µM for 15 min, or high glucose for 3 days, or TGF-β1 at 5 ng/ml...

  14. Ancient evolutionary origin of vertebrate enteric neurons from trunk-derived neural crest

    Green, Stephen A.; Uy, Benjamin R.; Bronner, Marianne E.
    The enteric nervous system of jawed vertebrates arises primarily from vagal neural crest cells that migrate to the foregut and subsequently colonize and innervate the entire gastrointestinal tract. Here we examine development of the enteric nervous system in the basal jawless vertebrate the sea lamprey (Petromyzon marinus) to gain insight into its evolutionary origin. Surprisingly, we find no evidence for the existence of a vagally derived enteric neural crest population in the lamprey. Rather, labelling with the lipophilic dye DiI shows that late-migrating cells, originating from the trunk neural tube and associated with nerve fibres, differentiate into neurons within the...

  15. Evaluation of flow schemes for near-neutral pH electrolytes in solar-fuel generators

    Singh, Meenesh R.; Xiang, Chengxiang; Lewis, Nathan S.
    The electrochemical performance of three different types of membrane-containing electrolyte-flow schemes for solar-driven water splitting has been studied quantitatively using 1-dimensional and 2-dimensional multi-physics models. The three schemes include a recirculation scheme with a well-mixed bulk electrolyte, a recirculation scheme with laminar flow fields, and a fresh-feed scheme with laminar flow fields. The Nernstian potential loss associated with pH gradients at the electrode surfaces, the resistive loss between the cathode and anode, the product-gas crossovers, and the required pumping energy in all three schemes have been evaluated as a function of the operational current density, the flow rates for the...

  16. Progress and Potential of Electron Cryotomography as Illustrated by Its Application to Bacterial Chemoreceptor Arrays

    Briegel, Ariane; Jensen, Grant
    Electron cryotomography (ECT) can produce three-dimensional images of biological samples such as intact cells in a near-native, frozen-hydrated state to macromolecular resolution (∼4 nm). Because one of its first and most common applications has been to bacterial chemoreceptor arrays, ECT’s contributions to this field illustrate well its past, present, and future. Although X-ray crystallography and nuclear magnetic resonance spectroscopy have revealed the structures of nearly all the individual components of chemoreceptor arrays, ECT has revealed the mesoscale information about how the components are arranged within cells. Receptors assemble into a universally conserved 12-nm hexagonal lattice linked by CheA/CheW rings. Membrane-bound...

  17. Reliable Performance Characterization of Mediated Photocatalytic Water-Splitting Half Reactions

    Han, Lihao; Lin, Meng; Haussener, Sophia
    Photocatalytic approaches using two sets of semiconductor particles and a pair of redox-shuttle mediators are considered as a safe and economic solution for solar water splitting. Here, accurate experimental characterization techniques for photocatalytic half reactions are reported, investigating the gas as well as the liquid products. The methods are exemplified utilizing photocatalytic titania particles in an iron-based aqueous electrolyte for effective oxygen evolution and mediator reduction reactions under illumination. Several product characterization methods, including an optical oxygen sensor, pressure sensor, gas chromatography, and UV/Vis spectroscopy are used and compared for accurate, high-resolution gas-products and mediator conversion measurements. Advantages of each...

  18. Enantioselective Iridium-Catalyzed Allylic Alkylation Reactions of Masked Acyl Cyanide Equivalents

    Hethcox, J. Caleb; Shockley, Samantha E.; Stoltz, Brian M.
    The first enantioselective iridium-catalyzed allylic alkylation reaction of a masked acyl cyanide (MAC) reagent has been developed. The transformation allows for the use of an umpoled synthon, which serves as a carbon monoxide equivalent. The reaction proceeds with good yield and excellent selectivity up to gram scale for a wide range of substituted allylic electrophiles, delivering products amenable to the synthesis of highly desirable, enantioenriched vinylated α-aryl carbonyl derivatives.

  19. Deep-sea coral evidence for lower Southern Ocean surface nitrate concentrations during the last ice age

    Wang, Xingchen Tony; Sigman, Daniel M.; Prokopenko, Maria G.; Adkins, Jess F.; Robinson, Laura F.; Hines, Sophia K.; Chai, Junyi; Studer, Anja S.; Martínez-García, Alfredo; Chen, Tianyu; Haug, Gerald H.
    The Southern Ocean regulates the ocean’s biological sequestration of CO_2 and is widely suspected to underpin much of the ice age decline in atmospheric CO_2 concentration, but the specific changes in the region are debated. Although more complete drawdown of surface nutrients by phytoplankton during the ice ages is supported by some sediment core-based measurements, the use of different proxies in different regions has precluded a unified view of Southern Ocean biogeochemical change. Here, we report measurements of the ^(15)N/^(14)N of fossil-bound organic matter in the stony deep-sea coral Desmophyllum dianthus, a tool for reconstructing surface ocean nutrient conditions. The...

  20. Distributed optimization decomposition for joint economic dispatch and frequency regulation

    Cai, Desmond; Mallada, Enrique; Wierman, Adam
    Economic dispatch and frequency regulation are typically viewed as fundamentally different problems in power systems and, hence, are typically studied separately. In this paper, we frame and study a joint problem that co-optimizes both slow timescale economic dispatch resources and fast timescale frequency regulation resources. We show how the joint problem can be decomposed without loss of optimality into slow and fast timescale sub-problems that have appealing interpretations as the economic dispatch and frequency regulation problems respectively. We solve the fast timescale sub-problem using a distributed frequency control algorithm that preserves network stability during transients. We solve the slow timescale...

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