Repository of works by Caltech published authors.
Type = Article
Mostrando recursos 1 - 20 de 30.575
The autodestruction of pepsin in relation to its ionization - Goulding, Arthur M.; Borsook, Henry; Wasteneys, Hardolph
1. Evidence is presented that pepsin is a univalent acid with a value for pK of 6.85 (or a base, with pK 7.39).
2. The autodestruction of the pepsin is shown to be dependent in part upon an instantaneous irreversible change occurring in the ionized form of the enzyme (if it be an acid) or in the unionized form (if it be a base).
3. A further progressive autodestruction of pepsin at any given hydrogen ion concentration and temperature is defined by the mass law equation for a monomolecular reaction
4. The velocity of autodestruction of pepsin is directly proportional to...
A Chemical Epitope-Targeting Strategy for Protein Capture Agents: The Serine 474 Epitope of the Kinase Akt2 - Nag, Arundhati; Das, Samir; Yu, Mary Beth; Deyle, Kaycie M.; Millward, Steven W.; Heath, James R.
Target and click: Peptide ligands targeted to the C-terminal motif of the kinase Akt2 were obtained by combining phosphate recognition of a dinuclear zinc(II) complex with in situ click chemistry to target this epitope. The peptide ligands (shown as XXXXX) selectively bind the C-terminal polypeptide of Akt2, and are selective for Akt2 relative to the Akt1 and Akt3 isoforms. The ligands differentially modulate Akt2 activity.
Local Relative Density Modulates Failure and Strength in Vertically Aligned Carbon Nanotubes - Pathak, Siddhartha; Mohan, Nisha; Decolvenaere, Elizabeth; Needleman, Alan; Bedewy, Mostafa; Hart, A. John; Greer, Julia R.
Micromechanical experiments, image analysis, and theoretical modeling revealed that local failure events and compressive stresses of vertically aligned carbon nanotubes (VACNTs) were uniquely linked to relative density gradients. Edge detection analysis of systematically obtained scanning electron micrographs was used to quantify a microstructural figure-of-merit related to relative local density along VACNT heights. Sequential bottom-to-top buckling and hardening in stress–strain response were observed in samples with smaller relative density at the bottom. When density gradient was insubstantial or reversed, bottom regions always buckled last, and a flat stress plateau was obtained. These findings were consistent with predictions of a 2D material...
Intrachain Contact Dynamics in Unfolded Cytochrome cb_(562) - Bouley Ford, Nicole D.; Shin, Dong-Woo; Gray, Harry B.; Winkler, Jay R.
We have investigated intrachain contact dynamics in unfolded cytochrome cb_(562) by monitoring heme quenching of excited ruthenium photosensitizers covalently bound to residues along the polypeptide. Intrachain diffusion for chemically denatured proteins proceeds on the microsecond time scale with an upper limit of 0.1 μs. The rate constants exhibit a power-law dependence on the number of peptide bonds between the heme and Ru complex. The power-law exponent of −1.5 is consistent with theoretical models for freely jointed Gaussian chains, but its magnitude is smaller than that reported for several synthetic polypeptides. Contact formation within a stable loop was examined in a...
A negative-stiffness phase in elastic composites can produce stable extreme effective dynamic but not static stiffness - Wojnar, Charles S.; Kochmann, Dennis M.
We investigate the effective elastic properties and overall stability of four specific two-phase elastic composite systems having a non-positive-definite phase (often referred to as a negative-stiffness phase) to determine whether or not the presence of the negative-stiffness phase can lead to stable extreme overall stiffness. We start with an instructive spring-mass model to illustrate the underlying physical mechanisms before proceeding to the two- and three-dimensional two-phase solids of coated cylindrical and coated spherical inclusions, and we finally study a general particle-matrix composite. For all examples, we correlate effective stiffness with overall stability to demonstrate that the static effective stiffness measures...
Electron Flow through Metalloproteins - Winkler, Jay R.; Gray, Harry B.
Electron flow through proteins and protein assemblies in the
photosynthetic and respiratory machinery commonly occurs
between metal centers or other redox cofactors that are
separated by relatively large molecular distances, often in the
10−20 Å range. To inorganic chemists, such long-distance
electron transfer was a mystery for many years, as we were
accustomed to close-contact models for the transition states of
simple self-exchange reactions between metal ions in aqueous
solution. One of our favorite reactions was the ferrous/ferric
self-exchange, which had been investigated thoroughly by
Richard (Dick) Dodson at the Brookhaven National Laboratory
in the 1940s (and published in Journal of the American Chemical
Society in 1950). Dodson showed that the half-time...
Ortho-to-Para Ratio in Interstellar Water on the Sight line toward Sagittarius B2(N) - Lis, Dariusz C.; Bergin, Edwin A.; Schilke, Peter; van Dishoeck, Ewine F.
The determination of the water ortho-to-para ratio (OPR) is of great interest for studies of the formation and thermal history of water ices in the interstellar medium and protoplanetary disk environments. We present new Herschel observations of the fundamental rotational transitions of ortho- and para-water on the sightline toward Sagittarius B2(N), which allow improved estimates of the measurement uncertainties due to instrumental effects and assumptions about the excitation of water molecules. These new measurements, suggesting a spin temperature of 24–32 K, confirm the earlier findings of an OPR below the high-temperature value on the nearby sightline toward Sagittarius B2(M). The...
Electroflotation clarifier to enhance nitrogen removal in a two-stage alternating aeration bioreactor - Cho, Kangwoo; Chung, Chong Min; Kim, Yun Jung; Hoffmann, Michael R.; Chung, Tai Hak
Stringent water treatment criteria and rapidly growing pollutant loads provoke the demand for retrofitting wastewater treatment plants towards a higher capacity. In this study, we assess a two stage alternating aeration (AA) bioreactor equipped with electroflotation (EF) clarifier, for nitrogen removal within a short hydraulic reTENTion time (HRT). The EF under steady solids loading required a minimum unit height and gas:solids ratio of 0.006 for efficient clarification. The separated sludge blanket was further thickened with retaining stability when the cyclic solids loading was smaller than 1.0 kg m^(−2). In the continuous operation of the bioreactor, the returned activated sludge concentration...
Epitaxial Growth of DNA-Assembled Nanoparticle Superlattices on Patterned Substrates - Hellstrom, Sondra L.; Kim, Youngeun; Fakonas, James S.; Senesi, Andrew J.; Macfarlane, Robert J.; Mirkin, Chad A.; Atwater, Harry A.
DNA-functionalized nanoparticles, including plasmonic nanoparticles, can be assembled into a wide range of crystalline arrays via synthetically programmable DNA hybridization interactions. Here we demonstrate that such assemblies can be grown epitaxially on lithographically patterned templates, eliminating grain boundaries and enabling fine control over orientation and size of assemblies up to thousands of square micrometers. We also demonstrate that this epitaxial growth allows for orientational control, systematic introduction of strain, and designed defects, which extend the range of structures that can be made using superlattice assembly. Ultimately, this will open the door to integrating self-assembled plasmonic nanoparticle materials into on-chip optical...
Planetary Imaging in Powers of Ten: A Multiscale, Multipurpose Astrobiological Imager - Fink, Wolfgang; Sun, Henry J.; Mahaney, William C.; Kuhlman, Kimberly R.; Schulze-Makuch, Dirk
Contextual, multiscale astrobiological imaging is necessary to discover, map, and image patchy microbial colonization in extreme environments on planetary surfaces. The large difference in scale—several orders of magnitude—between search environment and microorganisms or microbial communities represents a challenge, which to date no single imaging instrument is able to overcome. In support of future planetary reconnaissance missions, we introduce an adapter-based imager, built from an off-the-shelf consumer digital camera, that offers scalable imaging ranging from macroscopic (meters per pixel) to microscopic (micrometers per pixel) imaging, that is, spanning at least 6 orders of magnitude. Magnification in digital cameras is governed by...
Luminescence of [Ru(bpy)_2(dppz)]^(2+) Bound to RNA Mismatches - McConnell, Anna J.; Song, Hang; Barton, Jacqueline K.
The luminescence of rac-[Ru(bpy)_2(dppz)]^(2+) (bpy = 2,2′-bipyridine and dppz = dipyrido[3,2-a:2′,3′-c]phenazine) was explored in the presence of RNA oligonucleotides containing a single RNA mismatch (CA and GG) in order to develop a probe for RNA mismatches. While there is minimal luminescence of [Ru(bpy)_2(dppz)]^(2+) in the presence of matched RNA due to weak binding, the luminescence is significantly enhanced in the presence of a single CA mismatch. The luminescence differential between CA mismatched and matched RNA is substantially higher compared to the DNA analogue, and therefore, [Ru(bpy)_2(dppz)]^(2+) appears to be also a sensitive light switch probe for a CA mismatch in...
Substituent Effects on Energetics of Peptide-Carboxylate Hydrogen Bonds as Studied by ^1H NMR Spectroscopy: Implications for Enzyme Catalysis - Emenike, Bright U.; Liu, Albert Tianxiang; Naveo, Elsy P.; Roberts, John D.
Substituent effects in N–H···O hydrogen bonds were estimated by comparing the acidities of two series of model compounds: N-benzoylanthranilic acids (A) and 4-benzoylamidobenzoic acids (B). Intramolecular N–H···O hydrogen bonds were found to be present in the A series of compounds, while B acids were used as control models. The respective pK_a values for A and B acids were determined experimentally in DMSO solution using proton NMR spectroscopy. With X = H, the pK_a for A and B acids were observed to be 7.6 and 11.6, respectively, a difference of 4.0 units (ΔpK_a). However, with X = p-NO_2, the ΔpK_a value...
Prometastatic GPCR CD97 Is a Direct Target of Tumor Suppressor microRNA-126 - Lu, Ying Y.; Sweredoski, Michael J.; Huss, David; Lansford, Rusty; Hess, Sonja; Tirrell, David A.
Tumor suppressor microRNA-126 (miR-126) is often down-regulated in cancer cells, and its overexpression is found to inhibit cancer metastasis. To elucidate the mechanism of tumor suppression by miR-126, we analyzed the proteomic response to miR-126 overexpression in the human metastatic breast cancer cell line MDA-MB-231. To acquire quantitative, time-resolved information, we combined two complementary proteomic methods, BONCAT and SILAC. We discovered a new direct target of miR-126: CD97, a pro-metastatic G-protein-coupled receptor (GPCR) that has been reported to promote tumor cell invasion, endothelial cell migration, and tumor angiogenesis. This discovery establishes a link between down-regulation of miR-126 and overexpression of...
Observational Insights into Aerosol Formation from Isoprene - Worton, David R.; Beaver, Melinda; St. Clair, Jason M.; Crounse, John; Wennberg, Paul; Seinfeld, John H.
Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO_2) and hydroperoxyl (HO_2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very...
Modeling curvature effects in diffusion flames using a laminar flamelet model - Xuan, Yuan; Blanquart, Guillaume; Mueller, Michael E.
The goal of this paper is to investigate the effects of curvature of mixture fraction iso-surfaces on the
transport of species in diffusion flames. A general flamelet formulation is derived mathematically considering
both curvature effects and differential diffusion effects. These theoretical results suggest that curvature
does not play a role in the transport process irrespective of the flame curvature if species transport
is described with a unity Lewis number. On the other hand, a curvature-induced term becomes explicit
when differential diffusion effects are considered, and it acts as a convective term in mixture fraction
space. It is found that this term needs to be taken into...
Involving Students in a Collaborative Project To Help Discover
Inexpensive, Stable Materials for Solar Photoelectrolysis - Anunson, Paige N.; Winkler, Gates R.; Winkler, Jay R.; Parkinson, Bruce A.; Christus, Jennifer D. Schuttlefield
In general, laboratory experiments focus on traditional chemical disciplines. While this approach allows students the ability to learn and explore fundamental concepts in a specific area, it does not always encourage students to explore interdisciplinary science. Often little transfer of knowledge from one area to another is observed, as students are given step-by-step instructions on how to complete their task with little involvement or problem solving. Herein, we provide an example of a real-time research laboratory experiment that is aimed at individual’s exploration and development, with the scientific goal of discovering inexpensive, stable oxide semiconductors that can efficiently photoelectrolyze water...
Catechol 2,3-dioxygenase and other meta-cleavage catabolic pathway genes in the “anaerobic” termite gut spirochete Treponema primitia - Lucey, Kaitlyn S.; Leadbetter, Jared R.
Microorganisms have evolved a spectacular diversity of metabolisms, some of which allow them to overcome environmental constraints, utilize abundant but inaccessible resources, and drive nutrient cycling in various ecosystems. The termite hindgut microbial community is optimized to metabolize wood and in recent years, the in situ physiological and ecological functions of community members have been researched. Spirochetes are abundant in the termite gut and herein, putative aromatic meta-cleavage pathway genes typical of aerobic pseudomonads were located in genomes of homoacetogenic termite hindgut “anaerobes,” Treponema primitia str. ZAS-1 and ZAS-2. Phylogenetic analyses suggest the T. primitia catechol 2,3-dioxygenase and several other...
Bonding and interfacial reaction between Ni foil and n-type PbTe thermoelectric materials for thermoelectric module applications - Xia, Haiyang; Drymiotis, Fivos; Chen, Cheng-Lung; Wu, Aiping; Snyder, G. Jeffrey
Integration of next generation thermoelectric materials in thermoelectric modules requires a novel or alternative approach for mating the brittle semiconducting thermoelectric materials and the ductile metal interconnects. In this study, pure Ni foil was directly bonded to PbTe-based thermoelectric materials using a rapid hot-press. The materials were sintered at 600 and 650 °C, under a pressure of 40 MPa and for various holding times. The resulting interfacial microstructures of the Ni/PbTe joints were investigated. Additionally, the distributions of elements and the phases formed at the Ni/PbTe interface were analyzed. The β_2 phase (Ni_(3±x) Te_2, 38.8–41 at.% Te) was identified at...
Pathological wave dynamics: A postulate for sudden cardiac death in athletes - Pahlevan, Niema M.; Gharib, Morteza
Sudden death (SD) in young athletes is a shocking and disturbing event with significant societal impact. Previous
studies have demonstrated that sudden cardiac death (SCD) is the leading medical cause of SD in athletes.
Various structural and pathological cardiovascular abnormalities have identified as the underlying
causes of SCD in young athletes. However, there have been reported cases of SCD in athletes with no structural
or pathological cardiovascular disorders. Our proposed hypothesis in this article is that abnormalities
in aortic wave dynamics and coronary wave dynamics may be responsible for SCD in these athletes. These
abnormal waves—pathological waves—can act as a trigger toward cardiac death in the...