1,721,018 research outputs found
J -Driven dynamic nuclear polarization for sensitizing high field solution state NMR
No full textDynamic nuclear polarization (DNP) is widely used to enhance solid state nuclear magnetic resonance (NMR) sensitivity. Its efficiency as a generic signal-enhancing approach for liquid state NMR, however, decays rapidly with magnetic field
B
0, unless mediated by scalar interactions arising only in exceptional cases. This has prevented a more widespread use of DNP in structural and dynamical solution NMR analyses. This study introduces a potential solution to this problem, relying on biradicals with exchange couplings
J
ex of the order of the electron Larmor frequency
ω
E. Numerical and analytical calculations show that in such
J
ex ≈ ±
ω
E cases a phenomenon akin to that occurring in chemically induced DNP (CIDNP) happens, leading to different relaxation rates for the biradical singlet and triplet states which are hyperfine-coupled to the nuclear
α or
β states. Microwave irradiation can then generate a transient nuclear polarization build-up with high efficiency, at all magnetic fields that are relevant in contemporary NMR, and for all rotational diffusion correlation times that occur in small- and medium-sized molecules in conventional solvents.
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High‐field solution state DNP using cross‐correlations
No full textAt the magnetic fields of common NMR instruments, electron Zeeman frequencies are too high for efficient electron-nuclear dipolar cross-relaxation to occur in solution. The rate of that process fades with the electron Zeeman frequency as ω
-2 – in the absence of isotropic hyperfine couplings, liquid state dynamic nuclear polarisation (DNP) in high-field magnets is therefore impractical. However, contact coupling and dipolar cross-relaxation are not the only mechanisms that can move electron magnetisation to nuclei in liquids: multiple cross-correlated (CC) relaxation processes also exist, involving various combinations of interaction tensor anisotropies. The rates of some of those processes have more favourable high-field behaviour than dipolar cross-relaxation, but due to the difficulty of their numerical – and particularly analytical – treatment, they remain largely uncharted. In this communication, we report analytical evaluation of every rotationally driven relaxation process in liquid state for 1e1n and 2e1n spin systems, as well as numerical optimisations of the steady-state DNP with respect to spin Hamiltonian parameters. A previously unreported cross-correlated DNP (CCDNP) mechanism was identified for the 2e1n system, involving multiple relaxation interference effects and inter-electron exchange coupling. Using simulations, we found realistic spin Hamiltonian parameters that yield stronger nuclear polarisation at high magnetic fields than dipolar cross-relaxation.
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La resonancia magnética nuclear en fase sólida : su uso en el análisis de estructuras y equilibrios dinámicos en móleculas orgánicas
No full textEl presente Trabajo de Tesis presenta una serie de estudios de resonanciamagnética nuclear (RHN) llevados a cabo para obtener información sobre lanaturaleza de diversos sólidos orgánicos cristalinos. En el Capítulo 1 seintroducen las bases teóricas de las técnicas que fueron utilizadas a lo largodel presente Trabajo. El Capítulo 2 expone brevemente los aspectos prácticos dedichas técnicas, prestando especial atencion a las características que debeposeer un sistema de observación de RHN en fase sólida. El Capitulo 3 describelos efectos que son introducidos en los espectros de RHN por la presencia deprocesos de intercambio químico. Así, se describen en él el fenómeno deafinamiento de las resonancias de los espines abundantes; el fenomeno derelajación; el fenomeno de la coalescencia de las resonancias isotropicas; loscambios introducidos por los movimientos sobre los espectros de espinessometidos a interacciones tensoriales; y los fenómenos de interferencia entrelos movimientos moleculares y la rotación de los nucleos en el espacio real y enel espacio de los espines. Estos métodos de deteccion de movimientos en sólidosson ejemplificados con el caso de una droga muy comun (la sulfanilamida). El Capítulo 4 presenta una serie de estudios de RMN que se realizaron sobre unafamilia de compuestos: las porfirinas simetricamente sustituidas. Estos estudiospermitieron determinar la presencia en estas moléculas de un proceso detautomerismo N-H que tiene lugar no solo en solución sino tambien en la fasesolida. La fuerte dependencia de los parámetros de activación respecto alentorno cristalino tambien permitió obtener conclusiones acerca del mecanismodel proceso. El Capitulo 5 discute el caso del compuesto paterno de estafamilia, la porfina; para el cual RHN detecta la presencia de un tautomerismorápido para las moléculas en el cristal mientras que su estructura de difracciónlas muestra como un sólido carente de todo tipo de movimientos. Para reconciliarestas dos observaciones se propone la existencia de un segundo proceso dinámicoen la fase sólida; cuya existencia es comprobada y cuyos parámetros deactivacion son determinados. El Capitulo 6 investiga el tautomerismo de valenciade los 1,6-metano[10]anulenos en la fase sólida; que consiste en una migraciónde los electrones que permite a estos compuestos poseer característicasaromáticas u olefínicas. Se comprueba que en el caso de dos derivados de estafamilia el proceso tautomérico tiene lugar en la fase sólida, pero conparámetros de activación totalmente distintos a los que los compuestos mostrabanen solución. Finalmente, el Capítulo 7 concluye el presente Trabajo colocandosus resultados en un marco químico mas general. Para eso, se introduce unaaproximación que permite relacionar los datos de RMN en fase sólida con los dedifracción por rayos x; y se la aplica tanto a los casos estudiados en los Capítulos anteriores del presente Trabajo como a estudios realizados por otrosgrupos de investigación. De esta manera, es posible obtener conclusionesquímicamente relevantes sobre diversos procesos tautoméricos.Fil: Frydman, Lucio. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Internal gradient distributions: A susceptibility-derived tensor delivering morphologies by magnetic resonance
Get PDFNuclear magnetic resonance is a powerful tool for probing the structures of chemical and biological systems. Combined with field gradients it leads to NMR imaging (MRI), a widespread tool in non-invasive examinations. Sensitivity usually limits MRI's spatial resolution to tens of micrometers, but other sources of information like those delivered by constrained diffusion processes, enable one extract morphological information down to micron and sub-micron scales. We report here on a new method that also exploits diffusion-isotropic or anisotropic-to sense morphological parameters in the nm-mm range, based on distributions of susceptibility-induced magnetic field gradients. A theoretical framework is developed to define this source of information, leading to the proposition of internal gradient-distribution tensors. Gradient-based spin-echo sequences are designed to measure these new observables. These methods can be used to map orientations even when dealing with unconstrained diffusion, as is here demonstrated with studies of structured systems, including tissues.Fil: Alvarez, Gonzalo Agustin. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Weizmann Institute Of Science Israel; Israel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Shemesh, Noam. Champalimaud Neuroscience Programme At The Champalimaud Centre For The Unknown; Portugal. Weizmann Institute Of Science Israel; IsraelFil: Frydman, Lucio. Weizmann Institute Of Science Israel; Israe
Heteronuclear transfers from labile protons in biomolecular NMR: Cross polarization, revisited
No full textINEPT- and HMQC-based pulse sequences are widely used to transfer polarization between heteronuclei, particularly in biomolecular spectroscopy: they are easy to setup and involve low power deposition. Still, these short-pulse polarization transfers schemes are challenged by fast solvent chemical exchange. An alternative to improve these heteronuclear transfers is J-driven cross polarization (J-CP), which transfers polarization by spin-locking the coupled spins under Hartmann-Hahn conditions. J-CP provides certain immunity against chemical exchange and other T2-like relaxation effects, a behavior that is here examined in depth by both Liouville-space numerical and analytical derivations describing the transfer efficiency. While superior to INEPT-based transfers, fast exchange may also slow down these J-CP transfers, hurting their efficiency. This study therefore explores the potential of repeated projective operations to improve 1H→15N and 1H→15N→13C J-CP transfers in the presence of fast solvent chemical exchanges. It is found that while repeating J-CP provides little 1H→15N transfer advantages over a prolonged CP, multiple contacts that keep both the water and the labile protons effectively spin-locked can improve 1H→15N→13C transfers in the presence of chemical exchange. The ensuing Looped, Concatenated Cross Polarization (L-CCP) compensates for single J-CP losses by relying on the 13C’s longer lifetimes, leading to a kind of “algorithmic cooling” that can provide high polarization for the 15N as well as carbonyl and alpha 13Cs. This can facilitate certain experiments, as demonstrated with triple resonance experiments on intrinsically disordered proteins involving labile, chemically exchanging protons
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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