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Search Results: 1 - 10 of 252990 matches for " Peter R. Schreiner "
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Intramolecular hydroxycarbene C–H-insertion: The curious case of (o-methoxyphenyl)hydroxycarbene
Dennis Gerbig,David Ley,Hans Peter Reisenauer,Peter R. Schreiner
Beilstein Journal of Organic Chemistry , 2010, DOI: 10.3762/bjoc.6.121
Abstract: The first C–H insertion of a hydroxycarbene species in the gas phase has been observed experimentally by means of high vacuum flash pyrolysis (HVFP) and subsequent matrix isolation: (o-Methoxyphenyl)glyoxylic acid gives non-isolable (o-methoxyphenyl)hydroxycarbene upon pyrolysis at 600 °C, which rapidly inserts into the methyl C–H bond. The insertion product, 2,3-dihydrobenzofuran-3-ol, was trapped in an excess of Ar at 11 K and characterized by infrared spectroscopy. The insertion process kinetically outruns the alternative [1,2]H-tunneling reaction to o-anisaldehyde, a type of reaction observed for other hydroxycarbenes. Traces of the dehydration product, benzo[b]furan, were also detected. The potential energy hypersurface including the insertion and hydrogen migration processes was computed at the all-electron coupled-cluster level of theory encompassing single and double substitutions and perturbatively included triple excitations [AE-CCSD(T)] in conjunction with a correlation-consistent double-ζ basis set (cc-pVDZ) by utilizing density functional theory (DFT) optimized geometries (M06-2X/cc-pVDZ) with zero-point vibrational energy (ZPVE) corrections. Exchange of the methoxy for a trifluoromethoxy group successfully prevents insertion and (o-trifluoromethoxy)benzaldehyde is produced instead; however, the carbene cannot be observed under these conditions. Thermal decomposition of (o-methoxyphenyl)glyoxylic acid in refluxing xylenes does not give the insertion product but yields o-anisaldehyde. This unanticipated outcome can be rationalized by protonation of the hydroxycarbene intermediate leading to the tautomeric formyl group. Thermochemical computations at M06-2X/cc-pVDZ in conjunction with a self-consistent solvent reaction field model support this suggested reaction pathway.
Exploring covalently bonded diamondoid particles with valence photoelectron spectroscopy
Tobias Zimmermann,Robert Richter,Andre Knecht,Andrey A. Fokin,Tetyana V. Koso,Lesya V. Chernish,Pavel A. Gunchenko,Peter R. Schreiner,Thomas M?ller,Torbj?rn Rander
Physics , 2013, DOI: 10.1063/1.4818994
Abstract: We investigated the valence electronic structure of diamondoid particles in the gas phase, utilizing valence photoelectron spectroscopy. The samples were singly or doubly covalently bonded dimers or trimers of the lower diamondoids. Both the bond type and the combination of bonding partners are shown to affect the overall electronic structure. For singly bonded particles, we observe a small impact of the bond on the electronic structure, whereas for doubly bonded particles, the connecting bond determines the electronic structure of the highest occupied orbitals. In the singly bonded particles a superposition of the bonding partner orbitals determines the overall electronic structure. The experimental findings are supported by density functional theory computations at the M06-2X/cc-pVDZ level of theory.
Stereochemical errors and their implications for molecular dynamics simulations
Eduard Schreiner, Leonardo G Trabuco, Peter L Freddolino, Klaus Schulten
BMC Bioinformatics , 2011, DOI: 10.1186/1471-2105-12-190
Abstract: Here we illustrate the effects that chirality and peptide bond configuration flips may have on the secondary structure of proteins throughout a simulation. We also analyze the most common sources of stereochemical errors in biomolecular structures and present software tools to identify, correct, and prevent stereochemical errors in molecular dynamics simulations of biomolecules.Use of the tools presented here should become a standard step in the preparation of biomolecular simulations and in the generation of predicted structural models for proteins and nucleic acids.Biomolecules often feature asymmetries in stereochemistry. Many biologically active molecules are chiral, i.e., they exist in two forms, called enantiomers, which are non-superimposable mirror images of each other. Of particular relevance to biological compounds is the carbon atom as a chiral center: a carbon atom is chiral if it carries four nonequivalent substituents. Thus, all amino acids save glycine have at least one chiral center at Cα (see Figure 1A). Threonine and isoleucine have an additional chiral center at Cβ. Interestingly, only one of the two enantiomers is widely used in nature: according to the D-/L-naming convention, most naturally occurring amino acids are found in the L-configuration. Note, however, that D-amino acids do occur in biology, e.g., in cell walls of bacteria [1-3]. Nucleic acids also have chiral centers. For example, in DNA the atoms C1', C3', and C4' of the sugar moiety are chiral, while in RNA the presence of an additional OH group renders also C2' of the ribose chiral (see Figure 1B). Although the origin of the homochirality is not understood, the asymmetry due to preferential use of one enantiomer in biological systems has wide consequences. In particular, recognition processes of chiral molecules are impacted as can be demonstrated, e.g., by the different smell of D- and L-carvone or by the inhibition of proteases by D-amino acids [4]. On the level of protein structur
Clinical and Surgical Applications of the Ventral Intermediate Subnucleus
Larrarte, Guillermo A.,Piedimonte, Fabian C.,Azar Schreiner, Denise R.
Revista Argentina de Anatomia Online , 2011,
Abstract: The thalamus is a complex anatomical structure where most of the nerve pathways pass by. It is considered a diencephalic subcortical basal ganglia. It consists of a large number of nuclei and subnuclei, which perform varied functions. From the functional point of view it can be classified as: 1 - Motor Thalamus, 2 -Sensory Thalamus and 3- Limbic Thalamus.The motor thalamus is defined as the cytoarchitechtonic portion of the thalamus which links motor subcortical grey structures (basal ganglia and cerebellum) to cortical regions such as primary motor cortex, supplementary motor area and premotor cortex.The concept of motor thalamus is not related to the corticospinal tract but with classic extrapyramidal system.For this work, cadaveric material, histological preparations, stereotactic neurosurgery atlas and bibliography were used, as well as two videotapes about neurosurgical patients with tremor, with two different methodologies were filmed.Our goal is to determine the anatomical features of motor thalamus, specifically the ventral intermediate subnucleus constitent of ventral lateral nucleus, and their therapeutic relevance in neurosurgical treatment of tremor.
Hybrid Group IV Nanophotonic Structures Incorporating Diamond Silicon-Vacancy Color Centers
Jingyuan Linda Zhang,Hitoshi Ishiwata,Thomas M. Babinec,Marina Radulaski,Kai Müller,Konstantinos G. Lagoudakis,Constantin Dory,Jeremy Dahl,Robert Edgington,Veronique Soulière,Gabriel Ferro,Andrey A. Fokin,Peter R. Schreiner,Zhi-Xun Shen,Nicholas A. Melosh,Jelena Vu?kovi?
Physics , 2015,
Abstract: We demonstrate a new approach for engineering group IV semiconductor-based quantum photonic structures containing negatively charged silicon-vacancy (SiV$^-$) color centers in diamond as quantum emitters. Hybrid SiC/diamond structures are realized by combining the growth of nanoand micro-diamonds on silicon carbide (3C or 4H polytype) substrates, with the subsequent use of these diamond crystals as a hard mask for pattern transfer. SiV$^-$ color centers are incorporated in diamond during its synthesis from molecular diamond seeds (diamondoids), with no need for ionimplantation or annealing. We show that the same growth technique can be used to grow a diamond layer controllably doped with SiV$^-$ on top of a high purity bulk diamond, in which we subsequently fabricate nanopillar arrays containing high quality SiV$^-$ centers. Scanning confocal photoluminescence measurements reveal optically active SiV$^-$ lines both at room temperature and low temperature (5 K) from all fabricated structures, and, in particular, very narrow linewidths and small inhomogeneous broadening of SiV$^-$ lines from all-diamond nano-pillar arrays, which is a critical requirement for quantum computation. At low temperatures (5 K) we observe in these structures the signature typical of SiV$^-$ centers in bulk diamond, consistent with a double lambda. These results indicate that high quality color centers can be incorporated into nanophotonic structures synthetically with properties equivalent to those in bulk diamond, thereby opening opportunities for applications in classical and quantum information processing.
Gondang-Musik als überlieferungsgestalt Altv lkischer Lebensordnung
L. Schreiner
Bijdragen tot de Taal-, Land- en Volkenkunde , 1970,
Abstract:
Computer-Assisted Program Reasoning Based on a Relational Semantics of Programs
Wolfgang Schreiner
Electronic Proceedings in Theoretical Computer Science , 2012, DOI: 10.4204/eptcs.79.8
Abstract: We present an approach to program reasoning which inserts between a program and its verification conditions an additional layer, the denotation of the program expressed in a declarative form. The program is first translated into its denotation from which subsequently the verification conditions are generated. However, even before (and independently of) any verification attempt, one may investigate the denotation itself to get insight into the "semantic essence" of the program, in particular to see whether the denotation indeed gives reason to believe that the program has the expected behavior. Errors in the program and in the meta-information may thus be detected and fixed prior to actually performing the formal verification. More concretely, following the relational approach to program semantics, we model the effect of a program as a binary relation on program states. A formal calculus is devised to derive from a program a logic formula that describes this relation and is subject for inspection and manipulation. We have implemented this idea in a comprehensive form in the RISC ProgramExplorer, a new program reasoning environment for educational purposes which encompasses the previously developed RISC ProofNavigator as an interactive proving assistant.
SPOC1-Mediated Antiviral Host Cell Response Is Antagonized Early in Human Adenovirus Type 5 Infection
Sabrina Schreiner,Sarah Kinkley,Carolin Bürck,Andreas Mund,Peter Wimmer,Tobias Schubert,Peter Groitl,Hans Will,Thomas Dobner
PLOS Pathogens , 2013, DOI: 10.1371/journal.ppat.1003775
Abstract: Little is known about immediate phases after viral infection and how an incoming viral genome complex counteracts host cell defenses, before the start of viral gene expression. Adenovirus (Ad) serves as an ideal model, since entry and onset of gene expression are rapid and highly efficient, and mechanisms used 24–48 hours post infection to counteract host antiviral and DNA repair factors (e.g. p53, Mre11, Daxx) are well studied. Here, we identify an even earlier host cell target for Ad, the chromatin-associated factor and epigenetic reader, SPOC1, recently found recruited to double strand breaks, and playing a role in DNA damage response. SPOC1 co-localized with viral replication centers in the host cell nucleus, interacted with Ad DNA, and repressed viral gene expression at the transcriptional level. We discovered that this SPOC1-mediated restriction imposed upon Ad growth is relieved by its functional association with the Ad major core protein pVII that enters with the viral genome, followed by E1B-55K/E4orf6-dependent proteasomal degradation of SPOC1. Mimicking removal of SPOC1 in the cell, knock down of this cellular restriction factor using RNAi techniques resulted in significantly increased Ad replication, including enhanced viral gene expression. However, depletion of SPOC1 also reduced the efficiency of E1B-55K transcriptional repression of cellular promoters, with possible implications for viral transformation. Intriguingly, not exclusive to Ad infection, other human pathogenic viruses (HSV-1, HSV-2, HIV-1, and HCV) also depleted SPOC1 in infected cells. Our findings provide a general model for how pathogenic human viruses antagonize intrinsic SPOC1-mediated antiviral responses in their host cells. A better understanding of viral entry and early restrictive functions in host cells should provide new perspectives for developing antiviral agents and therapies. Conversely, for Ad vectors used in gene therapy, counteracting mechanisms eradicating incoming viral DNA would increase Ad vector efficacy and safety for the patient.
Relationship between resistivity and specific heat in a canonical non-magnetic heavy fermion alloy system: UPt_5-xAu_x
B. Andraka,R. Pietri,S. G. Thomas,G. R. Stewart,E. W. Scheidt,T. Schreiner
Physics , 1998, DOI: 10.1007/s100510050976
Abstract: UPt_(5-x)Au_x alloys form in a single crystal structure, cubic AuBe_5-type, over a wide range of concentrations from x = 0 to at least x = 2.5. All investigated alloys, with an exception for x = 2.5, were non-magnetic. Their electronic specific heat coefficient $\gamma$ varies from about 60 (x = 2) to about 700 mJ/mol K^2 (x = 1). The electrical resistivity for all alloys has a Fermi-liquid-like temperature variation, \rho = \rho_o + AT^2, in the limit of T -> 0 K. The coefficient A is strongly enhanced in the heavy-fermion regime in comparison with normal and transition metals. It changes from about 0.01 (x = 0) to over 2 micro-ohm cm/K^2 (x = 1). A/\gamma^2, which has been postulated to have a universal value for heavy-fermions, varies from about 10^-6 (x = 0, 0.5) to 10^-5 micro-ohm cm (mol K/mJ)^2 (x > 1.1), thus from a value typical of transition metals to that found for some other heavy-fermion metals. This ratio is unaffected, or only weakly affected, by chemical or crystallographic disorder. It correlates with the paramagnetic Curie-Weiss temperature of the high temperature magnetic susceptibility.
Structural and chemical properties of ZnSe-Fe electrodeposited granular films
Moraes A.R. de,Mosca D.H.,Schreiner W.H.,Mattoso N.
Brazilian Journal of Physics , 2002,
Abstract: Potentiostatic electrodeposition methods have been used to grow polycrystalline ZnSe and ZnSe-Fe granular thin films on stainless steel substrates from aqueous solutions. These films were characterized by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy and Raman spectroscopy.
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