PubMed Health⌕ Search

Biomedical subjects

L Emsley

Publications and source records attributed to L Emsley.

At least 19 recordsLinked to original sources
PubMed2003

Transverse dephasing optimized solid-state NMR spectroscopy.

It is shown how coherence lifetimes in solid-state NMR experiments can be controlled. New decoupling schemes are introduced which actively optimize dephasing times, providing increases of up to a factor of 2 with respect to the best existing schemes. The new schemes are implemented in transverse-dephasing-optimized (TDOP) NMR experiments for the disorded solid cellulose, and for a microcrystalline protein, where sensitivity improvements of up to a factor of 5 are obtained.

Bacillus subtilis↗
PubMed2001

Improved resolution in proton NMR spectroscopy of powdered solids.

We present a new solid-state nuclear magnetic resonance experiment that yields, under CRAMPS decoupling conditions, a significant reduction in proton line widths for powdered organic solids. This experiment which relies on a constant-time acquisition of the proton transverse magnetization, removes the contribution of nonrefocusable broadening from the proton line widths. Although this new technique suffers from relatively low sensitivity, we demonstrate in this paper its feasibility on two model samples, L-alanine and the dipeptide Ala-Asp. In both cases a factor of between 2 and 3 in line width reduction is obtained for most of the proton resonances.

Alanine↗
PubMed2001

Synthesis of deuterium-labeled cryptophane-A and investigation of Xe@cryptophane complexation dynamics by 1D-EXSY-NMR experiments.

We present the synthesis of a series of deuterated cryptophanes 2-6 by a slightly modified procedure used for cryptophane-A. We show that for [Xe@cryptophane] complexes the use of variable-temperature one-dimensional 129Xe magnetization transfer (1D-EX-SY) allows the measurement of exchange rates. From these data the decomplexation activation energy Ea has been estimated to be 37.5+/-2 kJ mol(-1). The decomplexation activation enthalpy, deltaH(++) = 35.5+/-2 kJ mol(-1), and entropy, deltaS(++) = -60+/-5 J mol(-1) K(-1), have also been calculated. The calculated negative activation entropy suggests that the activated complex associated with decomplexation is conformationally more strained than the complex in its ground state.

Journal Article↗
PubMed2001

Through-bond heteronuclear single-quantum correlation spectroscopy in solid-state NMR, and comparison to other through-bond and through-space experiments.

A new through-bond carbon-proton correlation technique, the MAS-J-HSQC experiment, is described for solid-state NMR. This new pulse scheme is compared experimentally with the previously proposed MAS-J-HMQC experiment in terms of proton resolution on a model sample of powdered L-alanine. We show that for natural abundance compounds, the MAS-J-HMQC and MAS-J-HSQC experiments give about the same proton resolution, whereas, for (13)C-labeled materials, narrower proton linewidths are obtained with the MAS-J-HSQC experiment. In addition we show that in scalar as well as in dipolar heteronuclear shift correlation experiments, when the proton chemical shift is encoded by the evolution of a single-quantum coherence, the proton resolution can be enhanced by simply adding a 180 degrees carbon pulse in the middle of the t(1) evolution time.

Journal Article↗
PubMed2001

Enhanced sensitivity in RIACT/MQ-MAS NMR experiments using rotor assisted population transfer.

The rotor assisted population transfer (RAPT) sequence is used to enhance the sensitivity of the RIACT(II) experiment for spin-3/2 quadrupolar nuclei. A detailed theoretical analysis of the polarizations that contribute to different types of MQ-MAS experiments is provided. In particular, two polarization pathways are distinguished for the creation of triple-quantum coherence. The existence of these pathways is experimentally demonstrated by comparing the sensitivities of different sequences with and without RAPT preparation.

Journal Article↗
PubMed2001

Spectral editing in solid-state NMR using scalar multiple quantum filters.

In this paper we describe the use of heteronuclear scalar couplings in solid-state NMR in order to generate multiple-quantum filtering (MQF) pulse sequences. These sequences can be used to edit CP/MAS spectra according to carbon multiplicity. Analytic expressions for the intensity of the MQF signals are obtained using the standard product operator formalism. Experiments that demonstrate the technique are shown in powder samples of camphor and a tripeptide.

Journal Article↗
PubMed2000

Sample restriction using magnetic field gradients in high-resolution solid-state NMR

Many solid-state NMR experiments are sensitive to inhomogeneity in the radiofrequency field. We propose a method to restrict the sample volume, in magic angle spinning experiments, using a static magnetic field gradient and a selective pulse. The position of the gradient is calculated for our experimental configuration and we have simulated the effects of selective pulses to determine the excited volume. The resulting sequences are applied to a sample of sodium acetate using frequency-switched Lee-Goldburg proton-proton homonuclear dipolar decoupling. A gain of a factor of 2 on the carbon resolution is experimentally observed. Copyright 2000 Academic Press.

Journal Article↗
PubMed1999

The accuracy of distance measurements in solid-state NMR.

The accuracy with which distances can be measured using dipolar recoupling experiments in solid-state NMR is investigated. The relative precision of experiments in a three spin system versus an isolated spin pair is found to depend very strongly on the nature of the coupling Hamiltonian. The accuracy of distances measured in even the simplified three spin system is seen to be very poor for existing homonuclear recoupling Hamiltonians. This suggests that it would be difficult to exploit broadband homonuclear recoupling to measure geometrical information reliably in complex spin systems. These conclusions apply equally to both single-crystal studies and powder samples. In contrast, the presence of additional spins has marginal impact on the accuracy when the coupling Hamiltonians commute with each other, as in the case of heteronuclear recoupling. The possibility of creating such a Hamiltonian for homonuclear recoupling using a suitable rotor-synchronized pulse sequence is discussed.

Magnetic Resonance Spectroscopy↗
PubMed1998

Unidirectional steady state rates of central metabolism enzymes measured simultaneously in a living plant tissue.

The unidirectional steady state reaction rates of several enzymes and metabolic fluxes of distinct processes were measured simultaneously in hypoxic maize root tips using two-dimensional phosphorus NMR exchange spectroscopy. A single spectrum monitors ATP synthesis and hydrolysis as well as the activities of four enzymes involved in key pathways of central metabolism: UDP-glucose pyrophosphorylase, phosphoglucomutase, hexose-phosphate isomerase, and enolase. The corresponding unidirectional reaction rates and net metabolic fluxes were calculated from spectral intensities. This method provides a unique picture, at enzyme resolution, of how metabolism reacts in a concerted fashion to changes in external parameters such as temperature and oxygen concentration. By increasing hypoxia via an increase in temperature, we measured the expected increase in glycolysis through enolase activity while total ATP synthesis settled. At the same time, we observed a net flux through phosphoglucomutase and UDP-glucose pyrophosphorylase toward carbohydrate synthesis. This result is discussed in relation to the current hypothesis on the turnover of cell walls and sucrose. This reaction also produces a net flux of pyrophosphate, which is needed by pyrophosphate:fructose-6-phosphate 1-phosphotransferase to work as a glycolytic enzyme.

Adenosine Triphosphate↗
PubMed1998

Intrinsic asymmetry in multidimensional solid-state NMR correlation spectra.

Solid-state MAS correlation spectra involving cross polarization will generally provide spectra which are asymmetric about the main diagonal. A quantitative treatment of this effect is provided and is demonstrated experimentally with examples of two-dimensional spin-diffusion spectra. The effect, which is due to the nonuniform nonequilibrium initial coherent state induced by cross polarization, is usually pronounced, and cannot be eliminated simply. It must be taken into account when quantifying solid-state CPMAS correlation spectra.

Carbon Isotopes↗
PubMed1997

Two-dimensional spin-exchange solid-state NMR studies of 13 C-enriched wood.

Two-dimensional high-resolution solid-state NMR has been used to study 13C-enriched wood. Wood is a complex material containing three major polymers: cellulose, hemicelluloses and lignin. The use of an enriched 13C-compound allows the observation of intra-molecular spin-diffusion driven by dipolar couplings. Correlations between spins at progressively longer distances have been obtained as the mixing time is increased, corresponding, for each of the separate polymer chains, to intra-unit and then to inter-unit interactions, and in the case of cellulose to inter-chain interactions. A straightforward qualitative analysis of the spin diffusion spectra is shown to yield the assignment of the carbon-13 spectrum. The cellulose resonances can all be sequentially assigned using the spin diffusion experiment. Using the experiments it is shown that, at least on a distance scale of several nanometres explored by the spin diffusion process, the three main components of wood occur in separate phases. Also, a question concerning the structure of the hemicellulose units is resolved by locating the O-acetyl group at the 2-position of the xylan chains.

Carbon Isotopes↗
PubMed1994

Methods for reconstructing phase sensitive slice profiles in magnetic resonance imaging.

The experimental determination of slice profiles excited by applying radiofrequency pulses in the presence of a gradient generally results in magnitude profiles. The conditions necessary to obtain a phase-sensitive picture of the profile of a slice are discussed. A distinction is made between the "excitation profile" (distribution of the transverse magnetization immediately after the RF pulse) and the "slice profile" (distribution after refocusing by gradient reversal and/or imperfect gradient switching). Methods are presented that allow one to obtain either the excitation profile or the slice profile. It is shown that phase encoding along the direction of the slice selection gradient provides a convenient protocol for obtaining the distribution of both the real and imaginary parts of the slice profile. The phase sensitive excitation profile can be obtained by frequency encoding. These methods were used to evaluate the performance of various shaped pulses.

Algorithms↗
PubMed1994

Assessment of MR image deformation for stereotactic neurosurgery using a tagging sequence.

A tagging sequence is used to assess MR image deformations before a stereotactic neurosurgical procedure, in a test model and in two patients. This pulse sequence super-imposes narrow parallel orthogonal tag lines on an image, which can be used as an internal reference frame. Image deformation is directly related to surface area variations in the squares produced by the tagging-sequence pulses. Small spatial deformations of the tags can be detected on the images used for measuring stereotactic-target spatial coordinates. A threshold of 2 SD guarantees that the distortion is smaller than one pixel.

Biopsy↗
PubMed1991

NMR studies of an oligoproline-containing peptide analogue that binds specifically to the H-2Kd histocompatibility molecule.

T lymphocytes expressing variable cell surface antigen receptors recognize "processed" forms of antigen, presented on the surface of other cells by molecules of the major histocompatibility complex (MHC). Naturally processed antigenic peptides can be replaced by synthetic ones. The synthetic peptide AYPPPPPTLA (P5) is an active competitor to the antigenic peptide HLA A24 170-182 (sequence RYLENGKETLQRA) that is recognized by A24 specific T cells in association with the H-2Kd class I MHC molecule. In P5 the five prolines were designed to play the role of a rigid spacer between the residue Y and the T-L unit, so as to mimic the role of Y171, T178, and L179 in the HLA A24 antigenic peptide, since these residues have proven to be the most important with respect to the binding of the HLA A24 peptide with the H-2Kd MHC molecule. Nuclear magnetic resonance studies allow us to demonstrate that in aqueous solution P5 adopts at least three long-lived conformations that can be classified with respect to the Y2-P3-P4 amide bonds as trans-trans, cis-trans, and cis-cis. Among these, the trans-trans form is present in 67% of the molecules while the two others share the remaining 33%.

Amino Acid Sequence↗