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Berlin Brandenburg

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  • 1
    Language: English
    In: Journal of the American Chemical Society, 03 June 2015, Vol.137(21), pp.6965-71
    Description: The stereochemistry, kinetics, and mechanism of olefin polymerization catalyzed by a set of zirconium-based metallocenes was studied by NMR using dissolution dynamic nuclear polarization (DNP). Hyperpolarized 1-hexene was polymerized in situ with a C2 symmetric catalyst, [(EBI)ZrMe][B(C6F5)4] (EBI = rac-(C2H4(1-indenyl)2)), and a C2v symmetric catalyst, [(Cp)2ZrMe][B(C6F5)4] (Cp = cyclopentadienyl). Hyperpolarized (13)C NMR spectra were used to characterize product tacticity following initiation of the reaction. At the same time, a signal gain of 3 orders of magnitude from (13)C hyperpolarization enabled the real time observation of catalyst-polymeryl species and deactivation products, such as vinylidene and a Zr-allyl complex. The compounds appearing in the reaction provide evidence for the existence of β-hydride elimination and formation of a dormant site via a methane-generating mechanism. The presence of a deactivating mechanism was incorporated in a model used to determine kinetic parameters of the reaction. On this basis, rate constants were measured between 0.8 and 6.7 mol % of catalyst. The concentration dependence of the rate constants obtained indicates a second-order process for polymerization concomitant with a first-order process for deactivation. The simultaneous observation of both processes in the time evolution of (13)C NMR signals over the course of several seconds underlines the utility of hyperpolarized NMR for quantifying early events in polymerization reactions.
    Keywords: Polymerization – Research ; Catalysis – Research ; Zirconium Compounds – Chemical Properties ; Nuclear Magnetic Resonance Spectroscopy – Usage;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: The journal of physical chemistry. B, 29 December 2011, Vol.115(51), pp.15355-61
    Description: Short, secondary-structure-containing peptides are suitable models for the study of protein folding due to their relative simplicity. Here, we investigate thermal denaturation of the tryptophan zipper peptide, trpzip4, a peptide that forms a β-hairpin in solution. In order to monitor the thermal denaturation of peptides or small proteins, chemical shift values of H(α) or H(N) may be used. However, various factors other than secondary structure can influence chemical shift values, such as side-chain orientation of nearby aromatic residues. Nuclear Overhauser effect (NOE) intensity from backbone interproton cross peaks is an alternative way to study thermal denaturation, as long as various factors that give rise to a change in NOE intensity upon changing the temperature are considered. As a relative indicator for denaturation, we define a cutoff temperature, where half of the initial NOE intensity is lost for each backbone interproton cross peak. For trpzip4, this cutoff temperature is highest for residues in the central part of the structure and lowest for residues near the termini. These observations support the notion that the structure of the trpzip4 peptide is stabilized by a hydrophobic cluster formed by tryptophan residues located in the central region of the β-hairpin.
    Keywords: Peptides -- Chemistry ; Tryptophan -- Chemistry
    ISSN: 15206106
    E-ISSN: 1520-5207
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  • 3
    Language: English
    In: Journal of the American Chemical Society, 20 June 2012, Vol.134(24), pp.10039-46
    Description: The specific binding sites of Hofmeister ions with an uncharged 600-residue elastin-like polypeptide, (VPGVG)(120), were elucidated using a combination of NMR and thermodynamic measurements along with molecular dynamics simulations. It was found that the large soft anions such as SCN(-) and I(-) interact with the polypeptide backbone via a hybrid binding site that consists of the amide nitrogen and the adjacent α-carbon. The hydrocarbon groups at these sites bear a slight positive charge, which enhances anion binding without disrupting specific hydrogen bonds to water molecules. The hydrophobic side chains do not contribute significantly to anion binding or the corresponding salting-in behavior of the biopolymer. Cl(-) binds far more weakly to the amide nitrogen/α-carbon binding site, while SO(4)(2-) is repelled from both the backbone and hydrophobic side chains of the polypeptide. The Na(+) counterions are also repelled from the polypeptide. The identification of these molecular-level binding sites provides new insights into the mechanism of peptide-anion interactions.
    Keywords: Ions -- Chemistry ; Peptides -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 4
    Language: English
    In: Journal of the American Chemical Society, 24 October 2012, Vol.134(42), pp.17448-51
    Description: Fluorine NMR spectroscopy is widely used for detection of protein-ligand interactions in drug discovery because of the simplicity of fluorine spectra combined with a relatively high likelihood for a drug molecule to include at least one fluorine atom. In general, an important limitation of NMR spectroscopy in drug discovery is its sensitivity, which results in the need for unphysiologically high protein concentrations and large ligand:protein ratios. An enhancement in the (19)F signal of several thousand fold by dynamic nuclear polarization allows for the detection of submicromolar concentrations of fluorinated small molecules. Techniques for exploiting this gain in signal to detect ligands in the strong-, intermediate-, and weak-binding regimes are presented. Similar to conventional NMR analysis, dissociation constants are determined. However, the ability to use a low ligand concentration permits the detection of ligands in slow exchange that are not easily amenable to drug screening by traditional NMR methods. The relative speed and additional information gained may make the hyperpolarization-based approach an interesting alternative for use in drug discovery.
    Keywords: Fluorine -- Chemistry ; Trypsin -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 5
    Language: English
    In: Angewandte Chemie International Edition, 26 August 2013, Vol.52(35), pp.9192-9195
    Description: : A method is presented for the measurement of protein folding by nuclear magnetic resonance. Denatured polypeptide is hyperpolarized using dissolution dynamic nuclear polarization, yielding a substantial signal enhancement that allows real‐time C NMR spectroscopy of the refolding process after a rapid pH jump. The resulting spectra indicate global and site‐specific changes in the protein.
    Keywords: Hyperpolarization ; Nmr Spectroscopy ; Protein Folding
    ISSN: 1433-7851
    E-ISSN: 1521-3773
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  • 6
    Language: English
    In: Journal of the American Chemical Society, 27 March 2013, Vol.135(12), pp.4636-9
    Description: Intermediates during the anionic polymerization of styrene were observed using hyperpolarized NMR. Dissolution dynamic nuclear polarization (DNP) of monomers provides a sufficient signal-to-noise ratio for detection of (13)C NMR signals in real time as the reaction progresses. Because of its large chemical shift dispersion, (13)C is well-suited to distinguish and characterize the chemical species that arise during the reaction. At the same time, incorporation of hyperpolarized small-molecule monomers is a unique way to generate polymers that exhibit a transient signal enhancement at the active site. This strategy is applicable despite the decay of the hyperpolarization of the polymer due to rapid spin-lattice relaxation. Real-time measurements on polymerization reactions provide both mechanistic and kinetic information without the need for stable isotope labeling of the molecules of interest. These capabilities are orthogonal to currently established methods that separate synthesis and analysis into two steps, making dissolution DNP an attractive method to study polymerization reactions.
    Keywords: Chemical Kinetics -- Analysis ; Nuclear Magnetic Resonance Spectroscopy -- Usage ; Polymerization -- Analysis ; Signal To Noise Ratio -- Usage;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 7
    Language: English
    In: Journal of Chemical Education, July 2010, Vol.87(7), p.747
    Description: Nuclear magnetic resonance (NMR) poses significant challenges for teaching in the context of an undergraduate laboratory, foremost because of high equipment cost. Current off-the-shelf data-acquisition hardware, however, is sufficiently powerful to constitute the core of a fully digital NMR...
    Keywords: Chemistry ; Undergraduate Study ; Laboratory Experiments ; Laboratory Equipment ; Computer Software ; Cost Effectiveness ; Spectroscopy ; Data Collection ; Inquiry ; Education ; Chemistry
    ISSN: 0021-9584
    E-ISSN: 1938-1328
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  • 8
    Language: English
    In: Analytical chemistry, 20 January 2015, Vol.87(2), pp.1004-8
    Description: The use of a blind source separation (BSS) algorithm is demonstrated for the analysis of time series of nuclear magnetic resonance (NMR) spectra. This type of data is obtained commonly from experiments, where analytes are hyperpolarized using dissolution dynamic nuclear polarization (D-DNP), both in in vivo and in vitro contexts. High signal gains in D-DNP enable rapid measurement of data sets characterizing the time evolution of chemical or metabolic processes. BSS is based on an algorithm that can be applied to separate the different components contributing to the NMR signal and determine the time dependence of the signals from these components. This algorithm requires minimal prior knowledge of the data, notably, no reference spectra need to be provided, and can therefore be applied rapidly. In a time-resolved measurement of the enzymatic conversion of hyperpolarized oxaloacetate to malate, the two signal components are separated into computed source spectra that closely resemble the spectra of the individual compounds. An improvement in the signal-to-noise ratio of the computed source spectra is found compared to the original spectra, presumably resulting from the presence of each signal more than once in the time series. The reconstruction of the original spectra yields the time evolution of the contributions from the two sources, which also corresponds closely to the time evolution of integrated signal intensities from the original spectra. BSS may therefore be an approach for the efficient identification of components and estimation of kinetics in D-DNP experiments, which can be applied at a high level of automation.
    Keywords: Algorithms ; Magnetic Resonance Spectroscopy -- Methods ; Malates -- Analysis ; Oxaloacetic Acid -- Analysis
    ISSN: 00032700
    E-ISSN: 1520-6882
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  • 9
    Language: English
    In: Analytical chemistry, 16 January 2018, Vol.90(2), pp.1217-1222
    Description: Hyperpolarization by dissolution dynamic nuclear polarization (D-DNP) has emerged as a technique for enhancing NMR signals by several orders of magnitude, thereby facilitating the characterization of metabolic pathways both in vivo and in vitro. Following the introduction of an externally hyperpolarized compound, real-time NMR enables the measurement of metabolic flux in the corresponding pathway. Spin relaxation however limits the maximum experimental time and prevents the use of this method with compounds exhibiting slow membrane transport rates. Here, we demonstrate that on-line electroporation can serve as a method for membrane permeabilization for use with D-DNP in cell cultures. An electroporation apparatus hyphenated with stopped-flow sample injection permits the introduction of the hyperpolarized metabolite within 3 s after the electrical pulse. In yeast cells that do not readily take up pyruvate, the addition of the electroporation pulse to the D-DNP experiment increases the signals of the downstream metabolic products CO and HCO, which otherwise are near the detection limit, by 8.2- and 8.6-fold. Modeling of the time dependence of these signals then permits the determination of the respective kinetic rate constants. The observed conversion rate from pyruvate to CO normalized for cell density was found to increase by a factor of 12 due to the alleviation of the membrane transport limitation. The use of electroporation therefore extends the applicability of D-DNP to in vitro studies with a wider range of metabolites and at the same time reduces the influence of membrane transport on the observed conversion rates.
    Keywords: Cells ; Enzymes ; Nuclear Magnetic Resonance–NMR ; Permeability ; Transport ; Electroporation ; Conversion ; Cell Density ; Pyruvic Acid ; Metabolic Pathways ; Carbon Dioxide ; Electroporation ; Metabolic Flux ; Metabolites ; Hyperpolarization ; Rate Constants ; Cultures ; Yeast ; Metabolites ; Carbon Dioxide ; Carbon Dioxide ; Metabolites ; Nuclear Magnetic Resonance–NMR ; Nuclear Magnetic Resonance–NMR;
    ISSN: 00032700
    E-ISSN: 1520-6882
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  • 10
    Language: English
    In: Analytical chemistry, 06 August 2013, Vol.85(15), pp.7385-90
    Description: The emergence of the dissolution dynamic nuclear polarization (D-DNP) technique provides an important breakthrough to overcome inherent sensitivity limitations in nuclear magnetic resonance (NMR) experiments. In dissolution DNP, only a small amount of frozen sample is polarized, dissolved, and injected into an NMR spectrometer. Although substantially enhanced NMR signals can be obtained, the single scan nature of this technique a priori impedes the use of correlation experiments, which represent some of the most powerful applications of NMR spectroscopy. Here, an alternative method for multiscan spectroscopy from D-DNP samples utilizing a flow NMR probe is described. Multiple hyperpolarized segments of sample are sequentially injected using a purpose designed device. Hadamard spectroscopy can then be applied for obtaining chemical shift correlation information even from a small number of scans. This capability is demonstrated with a four-scan data set for obtaining the [(13)C,(1)H] correlations in the test molecule 1-butanol. Because of the effects of spin-lattice relaxation and concentration gradients in the D-DNP experiment, the subtractive process for Hadamard reconstruction requires an additional step of intensity scaling. For this purpose, a reconstruction procedure was developed that uses entropy maximization and is robust with respect to noise and signal overlap. In a broader sense, the multiscan NMR as described here is amenable to various correlation NMR experiments, and increases the versatility of D-DNP in small-molecule characterization.
    Keywords: Magnetic Resonance Spectroscopy -- Methods
    ISSN: 00032700
    E-ISSN: 1520-6882
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