Biomedical subjects

Jamie D Walls

Publications and source records attributed to Jamie D Walls.

7 recordsLinked to original sources

PubMed2006

Proton-observed carbon-edited NMR spectroscopy in strongly coupled second-order spin systems.

Proton-observed carbon-edited (POCE) NMR spectroscopy is commonly used to measure 13C labeling with higher sensitivity compared to direct 13C NMR spectroscopy, at the expense of spectral resolution. For weakly coupled first-order spin systems, the multiplet signal at a specific proton chemical shift in POCE spectra directly reflects 13C enrichment of the carbon attached to this proton. The present study demonstrates that this is not necessarily the case for strongly coupled second-order spin systems. In such cases NMR signals can be detected in the POCE spectra even at chemical shifts corresponding to protons bound to 12C. This effect is demonstrated theoretically with density matrix calculations and simulations, and experimentally with measured POCE spectra of [3-13C]glutamate.

Carbon Isotopes↗

PubMed2005

Constants of motion in NMR spectroscopy.

We present a general method for constructing a subset of the constants of motion in terms of products of spin operators. These operators are then used to give insight into the multi-spin orders comprising the quasi-equilibrium state formed under a Jeener-Broekaert sequence in small, dipolar-coupled, spin systems. We further show that constants of motion that represent single-quantum coherences are present due to the symmetry of the dipolar Hamiltonian under 180 degrees spin rotations, and that such coherences contribute a DC component to the FID which vanishes in the absence of the flip-flop terms and is only present for spin clusters with an odd number of spins.

Algorithms↗

PubMed2005

Isotropic proton-detected local-field nuclear magnetic resonance in solids.

A nuclear magnetic resonance method is presented which produces linear, isotropic proton-detected local-field spectra for INS spin systems in powdered samples. The method, heteronuclear isotropic evolution (HETIE), refocuses the anisotropic portion of the heteronuclear dipolar coupling frequencies by evolving the system under a series of specially designed Hamiltonians and evolution pathways. The theory behind HETIE is presented along with experimental studies conducted on a powdered sample of ferrocene, demonstrating the methodology outlined in this paper. Applications of HETIE for use in structure determination in the solid state are discussed.

Journal Article↗

PubMed2005

Imaging a single-electron quantum dot.

Images of a single-electron quantum dot were obtained in the Coulomb blockade regime at liquid He temperatures using a cooled scanning probe microscope (SPM). The charged SPM tip shifts the lowest energy level in the dot and creates a ring in the image corresponding to a peak in the Coulomb-blockade conductance. Fits to the line shape of the ring determine the tip-induced shift of the energy of the electron state in the dot. SPM manipulation of electrons in quantum dots promises to be useful in understanding, building, and manipulating circuits for quantum information processing.

Arsenicals↗

PubMed2004

Sizable concentration-dependent frequency shifts in solution NMR using sensitive probes.

With the growing use of high fields and ultrasensitive probes, radiation damping emerges as a significant feedback interaction in modern solution NMR. Motivated by recent observations of mysterious concentration-dependent frequency shifts, experiments carried out on a cryoprobe at 600 MHz have revealed a time-averaged frequency shift of up to +83/-81 Hz. The sizable frequency shifts arise from deviations in the phase of the radiation damping field from perfect orthogonality relative to the net transverse magnetization. The frequency shift is shown to depend on the longitudinal magnetization and probe tuning conditions through experiments and numerical simulations. Such unexpected shifts in the solvent precession frequency provide a physical explanation for the empirical practice of adjusting the irradiation frequency of the saturating B1 field in solvent presaturation to achieve optimal suppression. Additional applications of the radiation damping induced frequency shift to solvent suppression and NMR methodology are discussed.

Journal Article↗

PubMed2004

Signal irreproducibility in high-field solution magnetic resonance experiments caused by spin turbulence.

Turbulent spin dynamics arising from the joint action of radiation damping and the distant dipolar field are shown to generate irreproducible measurements in popular high-field, gradient-based magnetic resonance (MR) experiments, undermining the prevailing assumption of essentially predictable observables in MR. Sizeable fluctuations in echo amplitudes are reported and numerically simulated for pulsed gradient spin echo and stimulated echo diffusion measurements. The underlying microscopic dynamical instability is characterized by analysis of the finite-time Lyapunov exponents. Perturbations to the modulated magnetization are shown to render magic-angle gradients ineffective in suppressing signal fluctuations. Alternative approaches are suggested for cancelling out the feedback interactions leading to spin turbulence.

Artifacts↗

PubMedDate not supplied

Investigations of low-amplitude radio frequency pulses at and away from rotary resonance conditions for I = 5/2 nuclei.

Additional experimental evidence of rotary resonance effects for multiple-quantum coherence conversion in a spin-5/2 system is presented. Two-dimensional plots of the relative efficiency of MQ excitation and conversion are given as a function of radio frequency (rf) amplitude and pulse width. Data are presented for the excitation of five-quantum coherence (5QC), as well as for 5QC to three-quantum coherence (3QC) conversion, 5QC to IQC (the central transition coherence) conversion, and 3QC to IQC conversion. A two-fold increase in the signal-to-noise ratio is achieved by substituting low amplitude rf pulses in place of hard rf pulses for 5QC excitation and 5QC to 3QC conversion in a mixed multiple-quantum magic angle spinning (MAS) (MMQMAS) experiment. The anisotropic line shape for the low-amplitude rf pulse version of the MMQMAS experiment was observed to be distorted from the MAS line shape. The cause and implications of the distortion are discussed.

Anisotropy↗