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Magnetic Resonance Experiments (magnetic + resonance_experiment)
Selected AbstractsArtifact due to B0 fluctuations in fMRI: Correction using the k- space central lineMAGNETIC RESONANCE IN MEDICINE, Issue 1 2001Emmanuel Durand Abstract Magnetic resonance experiments require the main magnetic field, B0, to remain very stable. Several external sources, such as moving ferromagnetic objects and/or changing electromagnetic fields, can significantly change the value of B0 over time. This work describes an apparent displacement along the phase-encoding axis caused by a variation in B0. This artifact was observed in fMRI images acquired with EPI. The effect was characterized and tested using an immobile phantom. The image displacement motion along the phase-encoding axis closely followed the changes in B0. The phase of the central line in the Fourier space was successfully used to correct this artifact. Fluctuations in B0 may result in artifacts that mimic subject head motion, and must be appropriately corrected. Magn Reson Med 46:198,201, 2001. © 2001 Wiley-Liss, Inc. [source] Increased cerebral activity in Parkinson's disease patients carrying the DRD2 TaqIA A1 allele during a demanding motor task: a compensatory mechanism?GENES, BRAIN AND BEHAVIOR, Issue 6 2007D. Bartrés-Faz Previous studies suggest that neuroimaging techniques are useful for detecting the effects of functional genetic polymorphisms on brain function in healthy subjects or in patients presenting with psychiatric or neurodegenerative conditions. Former evidence showed that individuals carrying risk alleles displayed broader patterns of brain activity during behavioural and cognitive tasks, despite being clinically comparable to non-carriers. This suggests the presence of compensatory brain mechanisms. In the present study, we investigated this effect in Parkinson's disease (PD) patients carrying the DRD2 TaqIA A1 allelic variant. This variant may confer an increased risk of developing the disease and/or influence the clinical presentation. During a complex sequential motor task, we evidenced by functional magnetic resonance imaging that A1 allele carriers activated a larger network of bilateral cerebral areas than non-carriers, including cerebellar and premotor regions. Both groups had similar clinical and demographic measures. In addition, their motor performance during the functional magnetic resonance experiment was comparable. Therefore, our conclusions, pending replication in a larger sample, seem to reflect the recruitment of compensatory cerebral resources during motor processing in PD patients carrying the A1 allele. [source] An accessible two-dimensional solution nuclear magnetic resonance experiment on human ubiquitin,BIOCHEMISTRY AND MOLECULAR BIOLOGY EDUCATION, Issue 2 2005David Rovnyak Abstract Solution-state nuclear magnetic resonance (NMR) is an invaluable tool in structural and molecular biology research, but may be underutilized in undergraduate laboratories because instrumentation for performing structural studies of macromolecules in aqueous solutions is not yet widely available for use in undergraduate laboratories. We have implemented an experiment that is ideal for more commonly available 4.8,7.0 Tesla, double-channel NMR instruments that would not usually be used for biomolecular NMR work. We analyzed a commercially available, 15N-enriched human ubiquitin sample with a two-dimensional correlation experiment using indirect 1H evolution and direct 15N detection, which produced spectra with high resolution on a spectrometer operating at 7.0 Tesla (300 MHz 1H resonance frequency). The simplicity of the experiment makes it possible to be configured by undergraduate students with minimal supervision from the instructor. Students gain experience in acquiring multidimensional biomolecular NMR experiments, confirm that ubiquitin is stably folded, and observe the correspondence between specific signals and individual amino acids in ubiquitin. [source] Predicting the tautomeric equilibrium of acetylacetone in solution.JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2010Abstract This study investigates how the various components (method, basis set, and treatment of solvent effects) of a theoretical approach influence the relative energies between keto and enol forms of acetylacetone, which is an important model system to study the solvent effects on chemical equilibria from experiment and theory. The computations show that the most popular density functional theory (DFT) approaches, such as B3LYP overestimate the stability of the enol form with respect to the keto form by ,10 kJ mol,1, whereas the very promising SCS-MP2 approach is underestimating it. MP2 calculations indicate that in particular the basis set size is crucial. The Dunning Huzinaga double , basis (D95z(d,p)) used in previous studies overestimates the stability of the keto form considerably as does the popular split-valence plus polarization (SVP) basis. Bulk properties of the solvent included by continuum approaches strongly stabilize the keto form, but they are not sufficient to reproduce the reversal in stabilities measured by low-temperature nuclear magnetic resonance experiments in freonic solvents. Enthalpic and entropic effects further stabilize the keto form, however, the reversal is only obtained if also molecular effects are taken into account. Such molecular effects seem to influence only the energy difference between the keto and the enol forms. Trends arising due to variation in the dielectric constant of the solvent result from bulk properties of the solvent, i.e., are already nicely described by continuum approaches. As such this study delivers a deep insight into the abilities of various approaches to describe solvent effects on chemical equilibria. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source] Principles of carbopeptoid folding: a molecular dynamics simulation studyJOURNAL OF PEPTIDE SCIENCE, Issue 2 2005Riccardo Baron Abstract The conformational spaces of five oligomers of tetrahydrofuran-based carbopeptoids in chloroform and dimethyl sulfoxide were investigated through nine molecular dynamics simulations. Prompted by nuclear magnetic resonance experiments that indicated various stable folds for some but not all of these carbopeptoids, their folding behaviour was investigated as a function of stereochemistry, chain length and solvent. The conformational distributions of these molecules were analysed in terms of occurrence of hydrogen bonds, backbone torsional-angle distributions, conformational clustering and solute configurational entropy. While a cis -linkage across the tetrahydrofuran ring favours right-handed helical structures, a trans -linkage results in a larger conformational variability. Intra-solute hydrogen bonding is reduced with increasing chain length and with increasing solvent polarity. Solute configurational entropies confirm the picture obtained: they are smaller for cis - than for trans -linked peptides, for chloroform than for dimethyl sulfoxide as solvent and for shorter peptide chains. The simulations provide an atomic picture of molecular conformational variability that is consistent with the available experimental data. Copyright © 2004 European Peptide Society and John Wiley & Sons, Ltd. [source] High-field optically detected EPR and ENDOR of semiconductor defects using W-band microwave Fabry,Pérot resonators,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005J.-M. Spaeth Abstract The designs of W-band (,95 GHz) Fabry,Pérot microwave resonators for optically detected EPR and ENDOR using the magnetic circular dichroism of the optical absorption (MCDA) as well as for photo-luminescence-detected EPR are briefly described. We report on the first MCDA-detected high-field EPR/ENDOR investigation of the paramagnetic EL2+ defect in semi-insulating GaAs. The higher-order effects, which prevented the unambiguous analysis of previous MCDA-detected K-band EPR/ENDOR experiments could be suppressed in W-band. The analysis of the ENDOR spectra showed that an extremely precise alignment of the samples is necessary. The paramagnetic El2+ defect turned out to be an As antisite defect, which has four almost equivalent nearest 75As neighbours differing less than 1.5% in the superhyperfine interactions suggestive of an isolated As antisite, while the third 75As shell (fifth neighbour shell) is clearly of lower symmetry than expected for an isolated As antisite. We discuss as a possible solution to this paradoxical situation that EL2+ is an isolated antisite at room temperature, which at low temperature, where all magnetic resonance experiments are performed, associates itself with shallow acceptors such as ZnGa, more than two nearest neighbour distances away. According to recent theoretical calculations, such ,loose' complexes with binding energies between 0.01 eV and 0.05 eV and disturb the equivalence of the nearest neighbour superhyperfine (shf) interactions less than 1.5%. Also, W-band EPR was measured using the photo-luminescence for detection to investigate P dopants in 6H-SiC. Copyright © 2005 John Wiley & Sons, Ltd. [source] | ||||||||||