Recursos de colección

Caltech Authors (166.525 recursos)

Repository of works by Caltech published authors.

Status = Published

Mostrando recursos 1 - 20 de 22.980

  1. Genomic DNA functionalized 3D printed materials for drug capture

    Yee, Daryl; Schulz, Michael; Blumenfeld, Carl; Grubbs, Robert; Greer, Julia
    Since the discovery of nitrogen mustard as an effective anti-cancer agent in the 1940s, and consequently, the concept of chemotherapy, researchers around the world have been actively developing new and more effective chemotherapeutic agents to better treat cancer. Traditionally, chemotherapeutic agents work by interfering with cell division. However, by virtue of their mechanism of action, healthy normal cells can also be targeted and destroyed. As a result, while chemotherapy is an effective way of managing cancer, the resulting side effects limits its use. One approach currently taken to reduce these side effects is to deliver the chemotherapy drugs directly to the tumor via transarterial chemoembolization,...

  2. Even-handed active space selection in projection-based wavefunction-in-DFT embedding

    Welborn, Matthew; Miller, Thomas F.
    Projection-based embedding offers a simple framework for embedding wavefunction theories in d. functional theory. Underlying this embedding is a heuristic for translating the chem.-intuitive idea of a set of embedded atoms into a set of embedded orbitals on which the embedded wavefunction calcn. will be performed. For single-point calcns., a successful heuristic has been to first localize the occupied Kohn-Sham orbitals, and then assign these localized orbitals to the embedded region based on at. population anal. However, during a large geometry change - such as a chem. reaction - the nature of the localized orbitals, as well as their at. populations can change dramatically. This can...

  3. Operando Raman interrogation of the synthesis and activation of a CoSe HER catalyst

    Rinaldi, Katherine Z.; Carim, Azhar; Lewis, Nathan S.
    A highly active cobalt selenide electrocatalyst for the hydrogen-evolution reaction (HER) in acidic media was electrochem. synthesized using a two-step process consisting of potentiostatic electrodeposition from aq. soln. followed by galvanostatic conditioning in the HER potential regime. Ex-situ Raman spectroscopy and energy-dispersive X-Ray spectroscopy indicated that the initial deposition step generated a material contg. CoSe as well as Se in multiple phases. Operando Raman anal. during the initial galvanostatic operation indicated a structural evolution of the Se phases including the partial elimination of one phase. Despite operating in the HER regime, the faradaic efficiency towards hydrogen evolution was low until this structural change was complete, demonstrating...

  4. Forces on nascent polypeptides during membrane insertion and translocation via the Sec translocon

    Niesen, Michiel J. M.; Miller, Thomas
    Co-translational insertion into and translocation across the cell membrane via the Sec translocon are key steps in the biogenesis of membrane proteins and secreted proteins. However, both exptl. and computational studies of these processes are difficult due to the role of long-timescale non-equil. dynamics during ribosomal translation. We have developed a coarsegrained (CG) simulation approach that is capable of reaching exptl. relevant (i.e. minute) timescales, while retaining the level of detail required to capture the effect of individual amino-acid substitutions. The CG model is applied to uncover the mechanism underlying exptl. obsd. forces acting on hydrophilic, hydrophobic, and charged nascent polypeptides as they are synthesized by...

  5. Photoelectrochemical performance of BiVO_4 photoanodes integrated with [NiFe]-layered double hydroxide water oxidation nanocatalysts

    Mueller, Astrid M.; Sinclair, Timothy S.; Gray, Harry B.
    We integrated laser-made highly active nickel iron layered double hydroxide ([NiFe]-LDH) water oxidn. nanocatalysts, which we developed in our group, with BiVO_4 photoanodes and tested their photoelectrochem. performance under simulated sunlight illumination. We demonstrated decreased aggregation and increased photocurrent generation with nanocatalysts that contain dipos. metals ([NiFe]-LDH and cobalt oxide) as citrate surfactant selectively ligated the catalyst nanoparticles. We also optimized catalyst mass loading, which is a tradeoff between most efficient depletion of photogenerated holes that drive catalytic turnover and parasitic light absorption by the catalyst particles. Integrated [NiFe]-LDH-BiVO_4 photoanodes enhanced photocurrent generation by a factor of 3.3 compared to bare BiVO_4. Comparison of photoelectrochem. performance...

  6. Catalysis of the oxygen evolution reaction in strongly acidic electrolytes with earth-abundant crystalline nickel-manganese antimonate

    Moreno-Hernandez, Ivan A.; MacFarland, Clara A.; Read, Carlos G.; Papadantonakis, Kimberly; Brunschwig, Bruce S.; Lewis, Nathan S.
    Many technologies that store renewable energy in chem. bonds rely on the oxygen evolution reaction (OER) to produce a renewable oxidant. In systems with an alk. electrolyte the OER is catalyzed by earth-abundant oxyhydroxides of the first-row transition metals, whereas systems with acidic electrolytes rely on noble metal oxides such as RuO_2 or IrO_2. The discrepancy in earth-abundance of OER catalysts in alk. and acidic electrolytes corresponds to the thermodn. instability of many first-row transition metal binary oxides in acidic conditions. Understanding how to stabilize first-row transition metals in acidic electrolytes while retaining catalytic activity towards the OER is a key milestone towards the development...

  7. Sunlight-driven hydrogen formation by membrane-supported photoelectrochemical water splitting

    Lewis, Nathan S.
    We are developing an artificial photosynthetic system that will utilize sunlight and water as inputs and will produce hydrogen and oxygen as outputs using a modular, parallel development approach in which the three distinct primary components-the photoanode, the photocathode, and the product-sepg. but ion-conducting membrane-are fabricated and optimized sep. before assembly into a water-splitting system. The design principles incorporate two sep., photosensitive semiconductor/liq. junctions that will collectively generate the 1.7-1.9 Vat open circuit to support both the oxidn. of H2_O (or OH-) and the redn. of H+ (or H_2O). The photoanode and photocathode will consist of rod-like semiconductor components, with attached heterogeneous multielectron transfer catalysts, needed...

  8. Strategies for controlled bacterial assembly resulting in activation of a quorum-sensing circuit

    Kozlowski, Mark; Silverman, Bradley; Johnstone, Christopher; Tirrell, David
    Bacterial communities show promise in a wide range of applications, such as environmental remediation and multi-step biochem. synthesis. Here we present methods by which bacteria can be assembled in two distinct architectures through controlled surface display of assocg. proteins (SpyTag and SpyCatcher or proteins contg. coiled-coil peptides including SynZip 17 and SynZip 18). We can tune the size of the aggregates by addn. of a "stoichiometric excess" of one type of cell in an interacting pair, as well as by modulating the amt. of protein displayed on the surface. We further demonstrate reversibility of the SynZip-mediated interactions, permanence of the SpyTag-SpyCatcher mediated clustering, and construction...

  9. Nature of the active sites for carbon dioxide reduction on metal nanoparticles: suggestions for optimizing performance

    Cheng, Tao; Wang, Zhijiang; Goddard, William A., III
    Metal nanoparticles exhibit superior carbon dioxide (CO_2) redn. performance due to the existence of unique surface active sites. For example, gold (Au) nanoparticles (NPs) show high efficiency in reducing CO_2 to carbon monoxide (CO), and copper (Cu) NPs efficiently convert CO to C2 productions. However, due to the difficulties in operando measurements, the at. details of these actives site have never been revealed. In this work, we employed multi-scale simulation technologies to "computationally synthesized" the Au NPs and Cu NPs on carbon nanotube (CNT) support by closely simulating the chem. vapor deposition (CVD) expts. The large-scale reactive force field simulations with up to million atoms...

  10. Structured Si/Co-P photocathodes: Designs for efficient light absorption in earth abundant solar fuels devices

    Kempler, Paul; Gonzalez, Miguel; Papadantonakis, Kimberly; Lewis, Nathan S.
    Designs for solar fuels devices frequently employ electrocatalysts integrated directly on the light-absorbing surfaces of semiconductors. Improvements that are gained in catalytic efficiency can be offset by the losses in light-conversion efficiency that result from parasitic absorption by the catalyst. We report two methods of circumventing parasitic absorption in these systems, using silicon light-absorbers decorated with electrodeposited Co-P catalyst as model integrated photocathodes. (1) Co-P catalyst films on planar silicon were restructured during operation to form nanoscale catalyst islands, which led to a doubling of the max. AM1.5 power conversion from 0.56 mW/cm^2 to 1.1 mW/cm^2• (2) Higher loadings of Co-Pon structured n+p-Si photocathodes achieved greater...

  11. Ligand-Specific Charge Localization in the MLCT Excited State of Ru(bpy)_2(dpphen)^(2+) Monitored by Time-Resolved Resonance Raman Spectroscopy

    Turro, Claudia; Bossmann, Stefan H.; Leroi, George E.; Barton, Jacqueline K.; Turro, Nicholas J.
    Time-resolved resonance Raman spectroscopy has been employed to examine the location of the promoted electron in the metal-to-ligand charge-transfer (MLCT) excited state of Ru(bpy)_2(dpphen)^(2+) (bpy) = 2,2'-bipyridine; dpphen = 4,7-diphenyl-1,10-phenanthroline). Variations in the environment about Ru(bpy)_2(dpphen)^(2+) shift the localization of charge in the MLCT excited state from bpy in neutral micelles (Brij 35) to dpphen in the presence of DNA and anionic surfactants (C_(12)H_(25)OSO_3Na, C_(10)H_(23)OSO_3Na, and C_8H_(21)OSO_3Na), whereas in water the electron is localized on both ligands. The shifts in the electronic absorption spectrum and the dependence of the ground-state resonance Raman (rR) signal with excitation wavelengths coincident with the high- and low-energy sides...

  12. New Syntheses of Benzobarrelenes

    Pu, Lin; Grubbs, Robert H.
    New syntheses of both substituted and unsubstituted benzobarrelenes are described. Treatment of 3,5-cyclohexadiene-cis-1,2-diol with benzaldehyde dimethylacetal in the presence of a catalytic amount of p-toluenesulfonic acid gave 1,2-(benzylidenedioxy)-3,5-cyclohexadiene(2). Addition of benzynes to 2 provided 3 and 4. Treatment of 3 and 4 with excess LDA and potassium tert-butoxide afforded benzobarrelenes 1 and 5 in good yields.

  13. Ethanol production in baker's yeast: Application of experimental perturbation techniques for model development and resultant changes in flux control analysis

    Schlosser, Paul M.; Riedy, Gerard; Bailey, James E.
    Recent theoretical results (Schlosser and Bailey, 1990; Schlosser et al., 1993) suggest that it is possible to model elements of metabolism to the extent necessary for flux control analysis using only the results of perturbation experiments. In particular, the development of detailed, mechanistic descriptions of certain relevant processes (which would otherwise require prohibitive effort) can be avoided by direct use of experimental results. Anaerobic glycolysis in Saccharomyces cerevisiae AMW-13C was studied both under unperturbed conditions and under several experimental perturbations. Changes in pathway fluxes and metabolite levels relative to the unperturbed values were observed and analyzed to determine the relationships...

  14. Oxidation of Zeise's Salt by [PtCl_6]^(2-): A Mechanistic Model for Hydrocarbon Oxidation

    Luinstra, Gerrit A.; Wang, Lin; Stahl, Shannon S.; Labinger, Jay A.; Bercaw, John E.
    Among a variety of organometallic approaches to C-H bond activation, reactions with electrophilic metal centers such as Pt(II), Pd(II), and Hg(Il) currently appear to offer the best prospects for productive alkane functionalization. Our working mechanistic scheme for conversion of alkanes to alcohols and alkyl chlorides by aqueous solutions of [PtCl_4]^(2-) and [PtCl_6]^(2-) (eq 1) consists of electrophilic C-H activation to generate a platinum(II) alkyl, transformation to a platinum(IV) alkyl, and nucleophilic attack by water or chloride, displacing platinum(II) and generating the product alcohol or alkyl chloride.

  15. Award Address (Herbert C. Brown Award for Creative Research in Synthetic Methods sponsored by the Purdue Borane Research Fund and the Herbert C. Brown Award Endowment). Nucleophilic substitution reactions: A radical alternative to S_N1 and S_N2 reactions

    Fu, Gregory C.
    Classical methods for achieving nucleophilic substitutions of alkyl electrophiles (S_N1 and S_N2) have limited scope and are not generally amenable to enantioselective variants that employ readily available racemic electrophiles. In this presentation, we will describe how the combination of radical chem. and transition-metal catalysis has opened the door to addressing the challenges of reactivity and of enantioselectivity in nucleophilic substitution reactions of secondary and tertiary alkyl electrophiles.

  16. Olefin trimerization and ethylene tandem conversion to linear low density polyethylene

    Bercaw, John E.
    Supporting a titanium catalyst (FI)TiCl_3 (FI = (N-(5-methyl-3-(1-adamantyl)salicylidene)-2'-(2"-methoxyphenyl) anilinato) onto MAO/silica affords a more active catalytic system for ethylene and 1-olefin trimerization. Cosupporting the (FI)TiCl_3 catalyst with the ethylene polymn. catalyst (CpSiNR)TiCl_2 (CpSiNR = (C5Me4) SiMe_2NCMe_3), either on sep. or the same MAO/silica particles, affords a catalyst system for producing butyl-branched LLDPE from and ethylene-only feed. Recent expts. aimed at defining the mechanism and LLDPE structure will be described.

  17. Oxidation of Methanol on 2nd and 3rd Row Group VIII Transition Metals (Pt, Ir, Os, Pd, Rh, and Ru): Application to Direct Methanol Fuel Cells

    Kua, Jeremy; Goddard, William A., III
    Using first principles quantum mechanics [nonlocal density functional theory (B3LYP)], we calculated the 13 most likely intermediate species for methanol oxidation on clusters of all 2nd and 3rd row Group VIII transition metals for all three likely binding sites (top, bridge, and cap). This comprehensive set of binding energies and structures allows a detailed analysis of possible reaction mechanisms and how they change for different metals. This illustrates the role in which modern quantum chemical methods can be used to provide data for combinatorial strategies for discovering and designing new catalysts. We find that methanol dehydrogenation is most facile on...

  18. Slow Evaporation of Water from Hydrated Salen Transition Metal Complexes in the Gas Phase Reveals Details of Metal Ligand Interactions

    Lee, Sang-Won; Chang, Sukbok; Kossakovski, Dmitri; Cox, Heather; Beauchamp, J. L.
    Water clusters of salen [N,N‘-ethylenebis(salicylideneaminato)] transition metal complexes [(salen)M, M = Cr^(3+), Mn^(3+), Co^(3+)] formed by electrospray source have been investigated using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Kinetics of water evaporation from the cluster ions is observed to be highly dependent on the central metal ion. For example, the evaporation rate of water from solvated salen chromium ion ([SCr + nH_2O]^+) is significantly slower than that from solvated salen manganese ([SMn + nH_2O]+) and solvated salen cobalt ([SCo + nH_2O]^+) ions. Furthermore, the clusters of salen chromium ions with two waters attached exhibit special stability, indicated...

  19. Alkyl Rearrangement Processes in Organozirconium Complexes. Observation of Internal Alkyl Complexes during Hydrozirconation

    Chirik, Paul J.; Day, Michael W.; Labinger, Jay A.; Bercaw, John E.
    Isotopically labeled alkyl zirconocene complexes of the form (CpR_n)_2Zr(CH_2CDR‘_2)(X) (CpR_n = alkyl-substituted cyclopentadienyl; R‘ = H, alkyl group; X = H, D, Me) undergo isomerization of the alkyl ligand as well as exchange with free olefin in solution under ambient conditions. Increasing the substitution on the Cp ring results in slower isomerization reactions, but these steric effects are small. In contrast, changing X has a very large effect on the rate of isomerization. Pure σ-bonding ligands such as methyl and hydride promote rapid isomerization, whereas π-donor ligands inhibit β-H elimination and hence alkyl isomerization. For (η^5-C_5H_5)2Zr(R)(Cl), internal alkyl complexes have...

  20. A search for clustering around Herbig Ae/Be stars. II. Atlas of the observed sources

    Testi, L.; Palla, F.; Natta, A.
    We present large field infrared images of a sample of 45 Herbig Ae/Be stars. Stellar parameters, such as age and luminosity, have been derived for all of them in a consistent way. The images have been used to identify stellar groups or clusters associated with the Herbig Ae/Be star. The results presented in this paper form the database for a study of clustering around intermediate mass stars (Testi et al. 1998).

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.