Recursos de colección

Caltech Authors (158.172 recursos)

Repository of works by Caltech published authors.

Type = Conference or Workshop Item

Mostrando recursos 1 - 20 de 228

  1. Total synthesis and study of myrmicarin alkaloids

    Ondrus, Alison Evelynn; Movassaghi, Mohammad
    A convergent total synthesis of myrmicarin alkaloids and related derivatives based on a potentially biomimetic dimerization of optically active pyrroloindolizines is described. A variety of dimerization strategies are discussed that take advantage of the unique chemistry of pyrroloindolizines. Mechanistic insights regarding the high level of substrate directed stereoselection in these dimerization reactions will be discussed.

  2. Twists and turns of lactam research

    Stoltz, Brian M.
    The importance of the amide bond cannot be overstated. Typical amides are planar structures, however, amide bonds can be highly twisted such as in bicyclic bridgehead lactams. The distortion of the orbitals from planarity and the pyramidalization of the nitrogen from sp toward sp dramatically affect the stability and reactivity of anti-Bredt amides. In 2006, our group published the first unambiguous synthesis and characterization of 2- quinuclidonium tetrafluoroborate. Since then we have been fascinated with the synthesis and characterization of such strained reactive lactams. We have also pursued the synthesis of a series of novel stereogenic lactams that can be employed as intermediates in multi-step synthesis....

  3. Fully quantum simulation of surface enhanced Raman scattering from real-time ab-initio methods

    Kretchmer, Joshua; Chan, Garnet
    Surface enhanced Raman scattering (SERS) is the huge enhancement of the Raman signal of a mol. near a metal or nanoparticle surface due to coupling with the plasmon of the metal nanostructure. Although SERS was discovered several decades ago, a complete picture of the underlying mechanism still remains elusive due to the difficulty of treating the coupled metal-mol. system fully quantum mech.; the majority of previous studies have treated the plasmon classically accounting for only an electromagnetic enhancement of the Raman signal. However, it is well known that the SERS enhancement can be orders of magnitude larger than that predicted by electromagnetic enchancement....

  4. Improving thermodynamics teaching for chemistry students

    Yu, Ted
    Thermodn. is an important subject matter covered in first year physics and chem. It is also one of the required upper-level courses in subjects like Chem., Mech. Engineering, Materials Engineering, and Chem. Engineering. Due to the traditionally math. nature of this course, the use of "atomistic" visual aids can greatly enhance a chem. student's learning of this subject. By introducing the simple concept of the quantum vol. (QV) of a gas atom first, the idea is to start thermodn. from an atomistic point of view. The QV is the amt. of vol. that an atom's wavelength occupies derived from the cube of the DeBroglie Wavelength....

  5. Volatile nematode sex pheromones

    Shinya, Ryoji; Gronquist, Mathew; Leighton, Daniel; Hsueh, Yen-Ping; Schroeder, Frank; Sternberg, Paul
    Nematodes produce a staggering array of sol. social cues, glycosides of dideoxy-sugars such as the ascarylose (ascarosides), that are secondary metabolites and communicate information such as population d. and location of sex partners. We found that predatory fungi respond to certain ascarosides by making sticky traps that trap and kill nematodes. This finding suggested that one driver of the evolution of ascaroside prodn. is avoiding predation. We have recently identified using GC-MS of the head-space of cultures of two distant genera of nematodes - the fungal-feeding and tree-parasitic Bursaphelenchus and the fruit-assocd., free-living Caenorhabditis - a set of volatile compds. that are produced by nematodes and...

  6. Synthesis and evaluation of dithiolate-modified ruthenium olefin metathesis catalysts

    Montgomery, Timothy; Grubbs, Robert
    A premier method for the formation of carbon-carbon double bonds, olefin metathesis has found widespread use in synthetic org., biol., and materials chem. Recent advancements made using ruthenium olefin metathesis catalysts have employed dithiolate X-type ligands, giving rise to a kinetically (E)-selective olefin metathesis process through stereoretention of the starting olefin geometry. Modification of the dithiolate ligands provides a path to metathesis catalysts contg. interesting electronic and stereochem. properties. These catalysts deliver a better understanding of the structure-activity relationships obsd. in olefin metathesis catalysts.

  7. Synthesis and evaluation of dithiolate-modified ruthenium olefin metathesis catalysts

    Montgomery, Timothy; Grubbs, Robert
    A premier method for the formation of carbon-carbon double bonds, olefin metathesis has found widespread use in synthetic org., biol., and materials chem. Recent advancements made using ruthenium olefin metathesis catalysts have employed dithiolate X-type ligands, giving rise to a kinetically (E)-selective olefin metathesis process through stereoretention of the starting olefin geometry. Modification of the dithiolate ligands provides a path to metathesis catalysts contg. interesting electronic and stereochem. properties. These catalysts deliver a better understanding of the structure-activity relationships obsd. in olefin metathesis catalysts.

  8. Degradation and mineralization of common pharmaceuticals using nitrogen-doped carbon monolith 3D electrode with 3D printed electrochemical reactor

    Liu, Kai; Yu, Menglin; Jasper, Justin; Hoffmann, Michael
    The oxidative degrdn. of common pharmaceuticals including propranolol, carbamazepine, ranitidine, metoprolol, trimethoprim, cimetidine, ciprofloxacin, and acetaminophen have been studied by using nitrogendoped carbon monolith 3D cathode in a continuous electro-peroxone process with novel 3D printed electrochem. reactor. The carbon monolith electrode was prepd. from polymer precursor with the ease of control over nitrogen dopant concn. and structure of the 3D porosity, which tunes oxygen redn. selectivity to facilitate hydrogen peroxide prodn. and improves mass transfer resp. The effect of parameters including c.d., soln. pH, and flow rate on the pharmaceutical degrdn. in aq. soln. was detd. The mineralization of pharmaceuticals and...

  9. Direct simulation of non-adiabatic dynamics in large-scale enzymatic systems

    Kretchmer, Joshua; Miller, Thomas
    The simulation of general non-adiabatic processes, such as electron transfer, proton-transfer, and protoncoupled electron transfer, in enzymic systems presents a challenge for theor. methods due to the large system-sizes, the coupling between classical and quantum mech. motions, and the multiple time-scales present in these systems. To address these challenges we have developed the kinetically-constrained ring-polymer mol. dynamics (KC-RPMD) method for the direct simulation of non-adiabatic processes in large-scale biol. and chem. systems. KC-RPMD, like the conventional ring-polymer mol. dynamics (RPMD), is based on Feynman's path-integral formulation of statistical mechanics and is able to simulate quantum dynamical processes using purely classical dynamics; KC-RPMD goes...

  10. Cellular structural biology probing prokaryotic and eukaryotic membrane protein complexes in-situ at atomic resolution

    Kaplan, Mohammed
    Cellular structural biol. aims at studying the structure and dynamics of biomols. at at. resoln. in their physiol. settings. Over the past few decades, solid-state NMR (ssNMR) has proven to be a powerful technique that can provide at.-resoln. information on membrane proteins in different lipid environments. Recently, such studies have been extended to include small membrane proteins in native milieu. However, larger complexes pose further challenges in terms of sample prepn., data anal. and sensitivity. Here, we show that sensitivity-enhanced ssNMR through Dynamic Nuclear Polarization (DNP) can be used to investigate large prokaryotic and eukaryotic membrane protein complexes in-situ at at....

  11. Cellular structural biology probing prokaryotic and eukaryotic membrane protein complexes in-situ at atomic resolution

    Kaplan, Mohammed
    Cellular structural biol. aims at studying the structure and dynamics of biomols. at at. resoln. in their physiol. settings. Over the past few decades, solid-state NMR (ssNMR) has proven to be a powerful technique that can provide at.-resoln. information on membrane proteins in different lipid environments. Recently, such studies have been extended to include small membrane proteins in native milieu. However, larger complexes pose further challenges in terms of sample prepn., data anal. and sensitivity. Here, we show that sensitivity-enhanced ssNMR through Dynamic Nuclear Polarization (DNP) can be used to investigate large prokaryotic and eukaryotic membrane protein complexes in-situ at at....

  12. Noncovalent modulation of protien energy landscapes with targeted molecular binders

    Bunck, David; Atsavapranee, Beatriz; Museth, Katrine; Vander Velde, David; Heath, James
    Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by the deterioration of motor neurons that abates essential biol. functions and exhibits survival times of 3 - 5 years after diagnosis. One driver of this disease derives from inherited mutations to the protein superoxide dismutase 1 (SOD1), which hinder proper folding and result in the accumulation of toxic aggregates. We identified cyclic peptides that target precise epitopes on SOD1 through an emerging screening platform that furnishes high-affinity binders against regions of a protein independent of secondary and tertiary structure. Binding these epitopes both stabilizes the native state and accelerates folding. In this context, these small peptides...

  13. Noncovalent modulation of protien energy landscapes with targeted molecular binders

    Bunck, David; Atsavapranee, Beatriz; Museth, Katrine; Vander Velde, David; Heath, James
    Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease caused by the deterioration of motor neurons that abates essential biol. functions and exhibits survival times of 3 - 5 years after diagnosis. One driver of this disease derives from inherited mutations to the protein superoxide dismutase 1 (SOD1), which hinder proper folding and result in the accumulation of toxic aggregates. We identified cyclic peptides that target precise epitopes on SOD1 through an emerging screening platform that furnishes high-affinity binders against regions of a protein independent of secondary and tertiary structure. Binding these epitopes both stabilizes the native state and accelerates folding. In this context, these small peptides...

  14. DNA-mediated signaling among proteins with [4Fe4S] clusters

    Barton, Jacqueline
    Double helical DNA can serve as a conduit for efficient redox chem. over long mol. distances. We have now been exploring how this chem. may be used within the cell for long range signaling. Increasingly, [4Fe4S] clusters are being found in DNA-binding proteins involved in genome maintenance. These [4Fe4S] clusters, common redox cofactors, are assocd. not only with repair proteins but also DNA polymerases and primase. Studies are described to characterize DNA-mediated charge transport by these metalloproteins. Expts. indicate that this chem. is important in the context of oxidative damage and may provide a first step in how DNA repair...

  15. DNA-mediated signaling among proteins with [4Fe4S] clusters

    Barton, Jacqueline
    Double helical DNA can serve as a conduit for efficient redox chem. over long mol. distances. We have now been exploring how this chem. may be used within the cell for long range signaling. Increasingly, [4Fe4S] clusters are being found in DNA-binding proteins involved in genome maintenance. These [4Fe4S] clusters, common redox cofactors, are assocd. not only with repair proteins but also DNA polymerases and primase. Studies are described to characterize DNA-mediated charge transport by these metalloproteins. Expts. indicate that this chem. is important in the context of oxidative damage and may provide a first step in how DNA repair...

  16. Sensors using DNA charge transport

    Barton, Jacqueline
    Many expts. have now shown that double helical DNA can serve as a conduit for efficient charge transport over long mol. distances. This chem. is exquisitely sensitive to perturbations in the DNA base stack, such as arise with base mismatches, lesions, and protein binding. We describe how this chem. may be applied in constructing DNA-based electrochem. sensors for DNA-binding proteins and lesions. For example, using a two electrode multiplexed platform, human DNMT1 methyltransferase activity is easily detected from crude cell lysates. The effect of magnetic fields on DNA charge transport chem. has also been explored. These results point to sensitive...

  17. Sensors using DNA charge transport

    Barton, Jacqueline
    Many expts. have now shown that double helical DNA can serve as a conduit for efficient charge transport over long mol. distances. This chem. is exquisitely sensitive to perturbations in the DNA base stack, such as arise with base mismatches, lesions, and protein binding. We describe how this chem. may be applied in constructing DNA-based electrochem. sensors for DNA-binding proteins and lesions. For example, using a two electrode multiplexed platform, human DNMT1 methyltransferase activity is easily detected from crude cell lysates. The effect of magnetic fields on DNA charge transport chem. has also been explored. These results point to sensitive...

  18. Recent developments in stereoselective iridium-catalyzed allylic alkylation methodologies

    Shockley, Samantha; Hethcox, Caleb; Stoltz, Brian
    We report our ongoing studies in the field of iridium-catalyzed allylic alkylation chem. The first regio-, diastereo-, and enantioselective transition-metal-catalyzed allylic alkylation reaction forming vicinal tertiary and all-carbon quaternary stereocenters between prochiral enolates and an aliph.-substituted electrophile is disclosed. Crit. to the success of this new reaction is the identity and ubiquity of the chloride counterion in addn. to the use of proton sponge, the combination of which affords excellent regio- and enantioselectivities along with good yields and diastereoselectivities. Addnl., a no. of transformations were carried out on the alkylation products to demonstrate the value of this method in rapidly accessing highly functionalized, stereochem....

  19. Recent developments in stereoselective iridium-catalyzed allylic alkylation methodologies

    Shockley, Samantha; Hethcox, Caleb; Stoltz, Brian
    We report our ongoing studies in the field of iridium-catalyzed allylic alkylation chem. The first regio-, diastereo-, and enantioselective transition-metal-catalyzed allylic alkylation reaction forming vicinal tertiary and all-carbon quaternary stereocenters between prochiral enolates and an aliph.-substituted electrophile is disclosed. Crit. to the success of this new reaction is the identity and ubiquity of the chloride counterion in addn. to the use of proton sponge, the combination of which affords excellent regio- and enantioselectivities along with good yields and diastereoselectivities. Addnl., a no. of transformations were carried out on the alkylation products to demonstrate the value of this method in rapidly accessing highly functionalized, stereochem....

  20. Catalytic asymmetric synthesis of alkylsilanes enabled by stereoconvergent nickel-catalyzed cross-coupling

    Schwarzwalder, Gregg; Fu, Gregory
    Over the years, the Fu group has demonstrated enantioconvergent cross-coupling reactions to be powerful tools for the synthesis of chiral mols. Most often, racemic secondary alkyl halide electrophiles are coupled with achiral nucleophiles to produce tertiary stereogenic centers in high yield and enantiomeric excess. In this talk, I will discuss our recent application of enantioconvergent nickel catalysis towards the prepn. of unactivated chiral alkyl silanes. The scope and limitations of this approach will be discussed, as well as some further applications of the enantioenriched products.

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