1,720,960 research outputs found
Enhancing NMR Signals in Liquids by Fluorine‐19 Overhauser Dynamic Nuclear Polarization (DNP) and Hyperpolarization Transfer to Carbon‐13
No full textAbstract Nuclear magnetic resonance (NMR) plays an essential role in life and material science but suffers from low sensitivity due to the small energy differences it aims to detect. Therefore, techniques that can increase spin polarization, and thus sensitivity, in NMR are gaining tremendous attention. Overhauser effect dynamic nuclear polarization (OE‐DNP) can increase multidimensional NMR signals of small molecules in liquid solutions; however, at high magnetic fields, the direct mechanism to 13 C is site selective, while it is poorly efficient for 1 H. Here, we report OE‐DNP hyperpolarization of fluorine‐19 ( 19 F) nuclei at 9.4 T, thus expanding the methodological toolbox of 19 F NMR. Signal enhancements up to ∼20 under steady‐state conditions can be used to either rapidly detect 19 F resonances or to enhance through‐bond J‐ coupled 13 C nuclei by two orders of magnitude. The latter corresponds to a 10 4 gain in signal acquisition. We demonstrate this with 19 F→ 13 C INEPT (insensitive nuclei enhanced by polarization transfer) spectra of the drug flutrimazole at 13 C natural abundance, where quaternary carbons become readily detectable, opening perspectives for accelerated NMR on fluorinated drugs or small molecules. The results are corroborated by a theoretical analysis of 19 F OE‐DNP coupling factors.European Research Council https://doi.org/10.13039/50110000078
Enhancing NMR Signals in Liquids by Fluorine‐19 Overhauser Dynamic Nuclear Polarization (DNP) and Hyperpolarization Transfer to Carbon‐13
No full textEuropean Research Council https://doi.org/10.13039/50110000078
Nanoparticle-assisted dynamic nuclear polarization in liquids
No full textGold nanoparticles (AuNPs) protected with self-assembled organic monolayers represent versatile nanostructures with applications in catalysis, molecular sensing, and recognition. The ordered arrangement of ligands within the monolayer promotes specific orientations and proximities of functional groups, facilitating weak yet selective interactions with small molecules in solution. In the context of nuclear magnetic resonance (NMR), AuNPs can act as magnetization reservoirs, allowing selective transfer to interacting molecules and making them promising chemosensors. While “NMR chemosensing” typically exploits nucleus–nucleus magnetization transfer mechanisms, we explored the potential of electron–nucleus interactions to allow selectively enhanced Overhauser effect dynamic nuclear polarization (OE-DNP). To this aim, we designed AuNPs simultaneously functionalized with two different thiols: a radical-bearing thiol as a polarizing agent for OE-DNP and an organic thiol tailored with different head groups to impart selective recognition. To evaluate both the polarization transfer efficiency and the selectivity, we performed OE-DNP experiments on a mixture of chloroform and methanol. Our results show sizeable enhancements on chloroform, possibly limited only by the mobility of the radical. On the other hand, the affinity for methanol does not correspond to greater enhancements on this target molecule. Although the system, as designed, does not provide efficient DNP on methanol as expected, our results confirm that AuNPs may be used as polarizing agents at magnetic fields up to 9.4 T. We foresee that future modifications of the nanoparticles’ monolayer will allow the combination of spin polarization transfer efficiency with molecular recognition, offering a novel strategy to boost hyperpolarization techniques and expanding their application towards NMR detection of specific analytes
Overhauser enhanced liquid state nuclear magnetic resonance spectroscopy in one and two dimensions
No full textAbstract Nuclear magnetic resonance (NMR) is fundamental in the natural sciences, from chemical analysis and structural biology, to medicine and physics. Despite its enormous achievements, one of its most severe limitations is the low sensitivity, which arises from the small population difference of nuclear spin states. Methods such as dissolution dynamic nuclear polarization and parahydrogen induced hyperpolarization can enhance the NMR signal by several orders of magnitude, however, their intrinsic limitations render multidimensional hyperpolarized liquid-state NMR a challenge. Here, we report an instrumental design for 9.4 Tesla liquid-state dynamic nuclear polarization that enabled enhanced high-resolution NMR spectra in one and two-dimensions for small molecules, including drugs and metabolites. Achieved enhancements of up to two orders of magnitude translate to signal acquisition gains up to a factor of 10,000. We show that hyperpolarization can be transferred between nuclei, allowing DNP-enhanced two-dimensional 13 C– 13 C correlation experiments at 13 C natural abundance. The enhanced sensitivity opens up perspectives for structural determination of natural products or characterization of drugs, available in small quantities. The results provide a starting point for a broader implementation of DNP in liquid-state NMR
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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