1,721,018 research outputs found
Steady-state in magnetic resonance pulse experiments
No full textRelaxation during multiple-pulse magnetic resonance experiments is treated by an average Liouvillian technique. The finite lattice temperature is taken into account by means of phenomenological correction terms. Both the transient and long-term response of the spin system are readily treated. We predict and verify a novel steady state of correlated spin polarizations in a spin-pair system under a periodic sequence of strong pi pulses
Orientational Sampling Schemes Based on Four Dimensional Polytopes
No full textThe vertices of regular four-dimensional polytopes are used to generate sets of uniformly distributed three-dimensional rotations, which are provided as tables of Euler angles. The spherical moments of these orientational sampling schemes are treated using group theory. The orientational sampling sets may be used in the numerical computation of solid-state nuclear magnetic resonance spectra, and in spherical tensor analysis procedures
Anisotropic nuclear spin interactions in H2O@C60 determined by solid-state NMR
No full textWe report a solid-state NMR study of the anisotropic nuclear spin interactions in H2O@C60 at room temperature. We find evidence of significant dipole- dipole interactions between the water protons, and also a proton chemical shift anisotropy (CSA) interaction. The principal axes of these interaction tensors are found to be perpendicular. The magnitude of the CSA is too large to be explained by a model in which the water molecules are partially aligned with respect to an external axis. The evidence indicates that the observed CSA is caused by a distortion of the geometry or electronic structure of the fullerene cages, in response to the presence of the endohedral water. © 2013 The Author(s) Published by the Royal Society. All rights reserved
Quenching spin diffusion in selective measurement of transient Overhauser effects in nuclear magnetic resonance. Applications to nucleotides
No full textIn high-resolution nuclear magnetic resonance (NMR), the transfer of longitudinal magnetization from one spin to another (A squiggly arrow pointing right X) under the effect of cross relaxation (nuclear Overhauser effect) is often complicated by spin-diffusion pathways through other spins K in the vicinity (e.g., A squiggly arrow pointing right K squiggly arrow pointing right X). It is shown how these undesirable pathways can be quenched by manipulating the magnetization of the two sites A and X with doubly selective inversion pulses. At the beginning of the experiment, after selective inversion of the ''source'' spin A, the longitudinal magnetization tends to migrate not only to the 'target' nucleus X but also to various other 'clandestine'' nuclei K, K, ... ([I(z)A] squiggly arrow pointing right [I(z)K], [I(z)K'], ...). In the middle of the interval tau(m), the longitudinal magnetization components of both A and X are inverted simultaneously, without affecting the spins K, K', .... The direct flow of magnetization from A to X is not perturbed by this manipulation, but the indirect flow via K, K', ... is reversed in sign and almost perfectly canceled at the end of the relaxation interval tau(m). If the signal of the target spin X overlaps with other resonances, the polarization [I(z)x] may be monitored indirectly by a doubly selective magnetization transfer to a 'spy'' proton M through a scalar coupling J(MX). The methods are illustrated by applications to Overhauser effects in the palindromic deoxyribonucleic acid d(CGCGAATTCGCG)2, Which forms a B-type double helix
Versatile magnetic resonance singlet tags compatible with biological conditions
No full textThe long lifetime of nuclear singlet states holds promise for the development of molecular tracers to study motional processes in proteins with increased precision or to act as imaging contrast agents. We introduce a singlet tag (STAG) based on bromoacrylate that is readily attached to a variety of biomolecules, compatible with biological conditions, and is relatively insensitive to the presence of molecular oxygen. The generality of this approach is demonstrated by tagging buildings blocks for biomolecules using a simple labelling route based on commercially available starting materials. Additionally, we demonstrate a singlet NMR experiment on the endogenous molecule phosphoenolpyruvate
Nuclear hyperpolarization of (1-13C)-pyruvate in aqueous solution by proton-relayed side-Arm hydrogenation
Get PDFWe employ Parahydrogen Induced Polarization with Side-Arm Hydrogenation (PHIP-SAH) to polarize (1-13C)-pyruvate. We introduce a new method called proton-relayed side-Arm hydrogenation (PR-SAH) in which an intermediate proton is used to transfer polarization from the side-Arm to the 13C-labelled site of the pyruvate before hydrolysis. This significantly reduces the cost and effort needed to prepare the precursor for radio-frequency transfer experiments while still maintaining acceptable polarization transfer efficiency. Experimentally we have attained on average 4.33% 13C polarization in an aqueous solution of (1-13C)-pyruvate after about 10 seconds of cleavage and extraction. PR-SAH is a promising pulsed NMR method for hyperpolarizing 13C-labelled metabolites in solution, conducted entirely in high magnetic field
Synthesis and characterisation of an open-cage fullerene encapsulating hydrogen fluoride
No full textThe first encapsulation of hydrogen fluoride in an open-cage fullerene is reported. Solution and solid-state NMR spectra of the novel open-cage endofullerene are described
Symmetry-breaking in the H2@C60 endofullerene revealed by inelastic neutron scattering at low temperature
Get PDFThe fine structure of the rotational ground state of molecular ortho-hydrogen confined inside the fullerene cage C60 is investigated by inelastic neutron scattering (INS). The INS line corresponding to transitions between the three sub-levels comprising the ortho ground state to the non-degenerate para ground state was studied as a function of temperature down to 60 mK in neutron energy gain. The experiments show that at ambient pressure the three ortho sub-levels are split into a low energy non-degenerate level and a high energy doubly degenerate level separated by 0.135 ± 0.010 meV. This observation is consistent with hydrogen molecules being located at sites with axial symmetry superseding the icosahedral symmetry of isolated rigid C60 cages in the solid phase. To gain insight into the role of inter-cage interactions in determining the symmetry breaking potential, the effects of hydrostatic pressure on the fine structure of the line was also investigated. The analysis of the INS spectra shows that the potential and the energy levels of H2 are sensitive to the orientation of neighbouring cages, consistent with the low-temperature crystalline phase of C60
Magic-Angle Spinning NMR of Cold Samples
No full textM agic-angle-spinning solid-state NMR provides site-resolved structural and chemical information about molecules that complements many other physical techniques. Recent technical advances have made it possible to perform magic-angle-spinning NMR experiments at low temperatures, allowing researchers to trap reaction intermediates and to perform site-resolved studies of low-temperature physical phenomena such as quantum rotations, quantum tunneling, ortho-para conversion between spin isomers, and super- conductivity. In examining biological molecules, the improved sensitivity provided by cryogenic NMR facilitates the study of protein assembly or membrane proteins. The combination of low-temperatures with dynamic nuclear polarization has the potential to boost sensitivity even further. Many research groups, including ours, have addressed the technical challenges and developed hardware for magic-angle-spinning of samples cooled down to a few tens of degrees Kelvin.
In this Account, we briefly describe these hardware developments and review several recent activities of our group which
involve low-temperature magic-angle-spinning NMR. Low-temperature operation allows us to trap intermediates that cannot be studied under ambient conditions by NMR because of their short lifetime. We have used low-temperature NMR to study the electronic structure of bathorhodopsin, the primary photoproduct of the light-sensitive membrane protein, rhodopsin. This project used a custom-built NMR probe that allows low-temperature NMR in the presence of illumination (the image shows the illuminated spinner module).
We have also used this technique to study the behavior of molecules within a restricted environment. Small-molecule endofullerenes are interesting molecular systems in which molecular rotors are confined to a well-insulated, well-defined, and highly symmetric environment. We discuss how cryogenic solid state NMR can give information on the dynamics of ortho-water confined in a fullerene cage.
Molecular motions are often connected with fundamental chemical properties; therefore, an understanding of molecular dynamics can be important in fields ranging from material science to biochemistry. We present the case of ibuprofen sodium salt which exhibits different degrees of conformational freedom in different parts of the same molecule, leading to a range of line broadening and line narrowing phenomena as a function of temperature
- …
