Thermal averaging of Hamiltonians in NMR revisited.

In this Note, we reexamine an earlier alternative proposal for averaging NMR spectroscopic parameters in the fast-exchange regime [D. H. Jones, N.

Structure, Dynamics, and Reactivity of Encapsulated Molecules in Restricted Spaces: Arylazoisoxazoles within an Octa Acid Capsule.

In this study, a well-defined organic capsule assembled from two octa acid (OA) molecules acting as host and select arylazoisoxazoles (AAIO) acting as guests were employed to demonstrate that confined molecules have restricted freedom that translates into reaction selectivity in both ground and excited states. The behavior of these AAIO guests in confined capsules was found to be different from that found in both crystals, where there is very little freedom, and in isotropic solvents, where there is complete freedom. Through one-dimensional (1D) and two-dimensional (2D) H NMR spectroscopic experiments, we have established a relationship between structure, dynamics and reactivity of molecules confined in an OA capsule.

Reverse-dialysis can be misleading for drug release studies in emulsions as demonstrated by NMR dilution experiments.

Emulsions are an important class of carriers for the delivery of hydrophobic drugs. While knowledge of drug release kinetics is critical to optimizing drug carrying emulsions, there remain many open questions about the validity of standard characterization methods such as the commonly used reverse-dialysis. In this paper, the kinetic parameters of isoflurane release in perfluorotributylamine emulsions determined from both reverse-dialysis and nuclear magnetic resonance (NMR) dilution experiments are compared.

Determining chemical exchange rate constants in nanoemulsions using nuclear magnetic resonance.

In this work, the second-order kinetics of molecules exchanging between chemically distinct microenvironments, such as those found in nanoemulsions, is studied using nuclear magnetic resonance (NMR). A unique aspect of NMR exchange studies in nanoemulsions is that the difference in molecular resonance frequencies between the two phases, which determines whether the exchange is fast, intermediate, or slow on the NMR timescale, can depend upon the emulsion droplet composition, which is also determined by the kinetic exchange constants themselves. Within the fast-exchange regime, changes in resonance frequencies and line widths with dilution were used to extract the exchange rate constants from the NMR spectra in a manner analogous to determining the kinetic parameters in NMR ligand binding experiments.

In vivo H MR spectroscopy with J-refocusing.

Purpose: The goal of this study was to propose a novel localized proton MR spectroscopy (MRS) sequence that reduces signal loss due to J-modulation in the rat brain in vivo.

Methods: Sprague-Dawley rats were studied at 9.4 T.

Diffusion Selective Pulses.

The self-diffusion coefficient, , provides important chemical and physical information about a molecular species and its environment, and can be routinely measured under equilibrium conditions using nuclear magnetic resonance (NMR). Differences in diffusion coefficients can also be exploited in NMR to suppress signals from fast diffusing species relative to slow diffusing species. To date, no method for selectively suppressing signals only from species with a particular diffusion coefficient has been presented.

Breakdown of linear response theory under low-power excitation in NMR. II. The case of "long-lived" signals in homogeneously broadened dipolar spin systems.

In this work, the previous linear response theory developed to describe low-power, radiofrequency (RF) excitation in inhomogeneously broadened spin systems [Z. Gong and J. D.

Enhancing the detection of edges and non-differentiable points in an NMR spectrum using delayed-acquisition.

Delayed-acquisition, which is a common technique for improving spectral resolution in Fourier transform based spectroscopies, typically relies upon differences in T relaxation rates that are often due to underlying differences in dynamics and/or complexities of the spin systems being studied. After an acquisition delay, the broad signals from fast T-relaxing species are more suppressed relative to the sharp signals from slow T-relaxing species. In this paper, an alternative source of differential "dephasing" under delayed-acquisition is demonstrated that is based solely upon the mathematical properties of the line shape and is independent of the underlying spin dynamics and/or complexity.

Breakdown of linear response theory under low-power excitation in NMR. I. The case of long-lived signals in inhomogeneously broadened spin systems.

In this work, we examine the application of linear response theory to the problem of low-power excitation in inhomogeneously broadened spin systems when the strength of the radiofrequency (RF) pulse, ν, is smaller than the inhomogeneous linewidth. Even for small overall excitations [Θ = 2πνT ≪ 1 where T is the RF pulse length], linear response theory is shown to break down for spins with resonance frequencies that are on the order of ν, which is due to the fact that the RF interaction cannot be treated as a small perturbation in this case. This breakdown in linear response theory can be partially corrected for by enforcing unitarity in the linear response.

(1)H NMR studies distinguish the water soluble metabolomic profiles of untransformed and RAS-transformed cells.

Metabolomic profiling is an increasingly important method for identifying potential biomarkers in cancer cells with a view towards improved diagnosis and treatment. Nuclear magnetic resonance (NMR) provides a potentially noninvasive means to accurately characterize differences in the metabolomic profiles of cells. In this work, we use (1)H NMR to measure the metabolomic profiles of water soluble metabolites extracted from isogenic control and oncogenic HRAS-, KRAS-, and NRAS-transduced BEAS2B lung epithelial cells to determine the robustness of NMR metabolomic profiling in detecting differences between the transformed cells and their untransformed counterparts as well as differences among the RAS-transformed cells.

The Talbot Effect for two-dimensional massless Dirac fermions.

A monochromatic beam of wavelength λ transmitted through a periodic one-dimensional diffraction grating with lattice constant d will be spatially refocused at distances from the grating that are integer multiples of . This self-refocusing phenomena, commonly referred to as the Talbot effect, has been experimentally demonstrated in a variety of systems ranging from optical to matter waves. Theoretical predictions suggest that the Talbot effect should exist in the case of relativistic Dirac fermions with nonzero mass.

Suppressing Klein tunneling in graphene using a one-dimensional array of localized scatterers.

Graphene's unique physical and chemical properties make it an attractive platform for use in micro- and nanoelectronic devices. However, electrostatically controlling the flow of electrons in graphene can be challenging as a result of Klein tunneling, where electrons normally incident to a one-dimensional potential barrier of height V are perfectly transmitted even as V → ∞. In this study, theoretical and numerical calculations predict that the transmission probability for an electron wave normally incident to a one-dimensional array of localized scatterers can be significantly less than unity when the electron wavelength is smaller than the spacing between scatterers.

Conditional rotations of heteronuclear coupled spins.

We present a new pulse sequence that conditionally excites I spin magnetization only in the presence of a nonzero heteronuclear coupling to an S spin. The pulse sequence, referred to as the reverse INEPT pathway selective pulse or RIPSP, generates a pure I spin rotation by an angle that depends upon the heteronuclear coupling constant in InS spin systems. Experimental demonstrations are shown in (13)C labeled chloroform, dichloromethane, and toluene samples and in unlabeled 2,3-dibromopropionic acid and brucine samples.

Measuring chirality in NMR in the presence of a time-dependent electric field.

Traditional nuclear magnetic resonance (NMR) experiments are "blind" to chirality since the spectra for left and right handed enantiomers are identical in an achiral medium. However, theoretical arguments have suggested that the effective Hamiltonian for spin-1/2 nuclei in the presence of electric and magnetic fields can be different for left and right handed enantiomers, thereby enabling NMR to be used to spectroscopically detect chirality even in an achiral medium. However, most proposals to detect the chiral NMR signature require measuring signals that are equivalent to picomolar concentrations for (1)H nuclei, which are outside current NMR detection limits.

Relaxation selective pulses in fast relaxing systems.

In this work, the selectivity or sharpness of the saturation profiles for relaxation selective pulses (R^rsps) that suppress magnetization possessing relaxation times of T2=T2(rsp) and T1=αT2 for α∈12,∞ was optimized. Along with sharpening the selectivity of the R^rsps, the selective saturation of these pulses was also optimized to be robust to both B0 and B1 inhomogeneities. Frequency-swept hyperbolic secant and adiabatic time-optimal saturation pulse inputs were found to work best in the optimizations, and the pulse lengths required to selectivity saturate the magnetization were always found to be less than the inversion recovery delay, T1ln(2).

Spatially encoded multiple-quantum excitation.

In this work, we present a simple method to spatially encode the transition frequencies of nuclear spin transitions and to read out these frequencies within a single scan. The experiment works by combining pulsed field gradients with an excitation sequence that selectively excites spin transitions within certain sample regions. After the initial excitation, imaging the resulting ẑ-magnetization is used to determine the locations where the excitations occurred, from which the corresponding transition frequencies are determined.

Fluorescent stilbazolium dyes as probes of the norepinephrine transporter: structural insights into substrate binding.

We report the synthesis, binding kinetics, optical spectroscopy and predicted binding modes of a series of sterically demanding, fluorescent norepinephrine transporter (NET) ligands. A series of bulky stilbazolium dyes, including six newly synthesized compounds, were evaluated to determine the effect of extending the molecular probes' 'heads' or 'tails'. Taking advantage of the dyes' characteristic 'turn-on' emission, the kinetic binding parameters, k(on) and k(off) were determined revealing that extension of the molecules' tails is well tolerated while expansion of the head is not.

Pseudorandom selective excitation in NMR.

In this work, average Hamiltonian theory is used to study selective excitation under a series of small flip-angle θ-pulses [θ≪π/3] applied either periodically [corresponding to the DANTE pulse sequence] or aperiodically to a spin-1/2 system. First, an average Hamiltonian description of the DANTE pulse sequence is developed that is valid for frequencies either at or very far from integer multiples of 1τ, where τ is the interpulse delay. For aperiodic excitation, a single resonance, νsel, can be selectively excited if the θ-pulse phases are modulated in concert with the interpulse delays.

Measuring chirality in NMR in the presence of a static electric field.

The scalar Hamiltonian of nuclear spins in the presence of a static electric field supports chirality. However, the eigenvalues of the Hamiltonian are not chiral; hence, chirality is not manifested in the usual NMR experiment. In this work, we show that the magnetization response to certain radio frequency pulse sequences exhibits chirality as well as handedness.

Spin-orbit coupling induced interference in quantum corrals.

Lack of inversion symmetry at a metallic surface can lead to an observable spin-orbit interaction. For certain metal surfaces, such as the Au(111) surface, the experimentally observed spin-orbit coupling results in spin rotation lengths on the order of tens of nanometers, which is the typical length scale associated with quantum corral structures formed on metal surfaces. In this work, multiple scattering theory is used to calculate the local density of states (LDOS) of quantum corral structures composed of nonmagnetic adatoms in the presence of spin-orbit coupling.

Spin amplification in solution magnetic resonance using radiation damping.

The sensitive detection of dilute solute spins is critical to biomolecular NMR. In this work, a spin amplifier for detecting dilute solute magnetization is developed using the radiation damping interaction in solution magnetic resonance. The evolution of the solvent magnetization, initially placed along the unstable -z direction, is triggered by the radiation damping field generated by the dilute solute magnetization.

Excitation of magnetization using a modulated radiation damping field.

In this work, pulsed-field gradients are used to modulate the radiation damping field generated by the detection coil in an NMR experiment in order that spins with significantly different chemical shifts can affect one another via the radiation damping field. Experiments performed on solutions of acetone/water and acetone/DMSO/water demonstrate that spins with chemical shift differences much greater than the effective radiation damping field strength can still be coupled by modulating the radiation damping field. Implications for applications in high-field NMR and for developing sensitive magnetization detectors are discussed.

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.

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.

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.