摘要:

AbstractExpressions for the transition amplitudes of the centerband and sidebands of magic angle spinning (MAS) NMR spectra are derived by applying the Floquet theory. Signal amplitudes are defined in terms of transition amplitude operators, which are linear combinations of the Floquet operators. Two examples of the utilization of the Floquet approach for the evaluation of MAS signals are presented. In the first, the REDOR experiment on heteronuclear spin systems is discussed. The effects of finite pulse lengths on the REDOR signals are also derived. The second example deals with the MAS spectrum of a spin coupled to a heteronuclear spin that is irradiated by an rf field. Recoupled spectra are examined with the help of the Floquet theory and decoupled spectra are evaluated. In all cases the methodology of the Floquet approach is emphasized.

ISSN:0021-2148    

DOI:10.1002/ijch.199200029

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractA simplified treatment of two‐dimensional homonuclear correlation spectroscopy (COSY) is proposed, based on a vector model and a consideration of the disturbance of energy level populations caused by the pulse sequence. It predicts the properties of diagonal and crosspeaks in the same manner as the more rigorous product operator treatment. Methods of studying the fine structure of COSY crosspeaks are discussed with a particular emphasis on the extraction of coupling constants when the splittings are poorly resolved. Three new schemes are described for the accurate measurement of couplings; they are called “J‐extension”, “J‐deconvolution”, and “J‐doubling”. All three rely on processing the experimental time‐domain signals with simple trigonometrical functions, followed by Fourier transformation. TheJ‐deconvolution and J‐doubling methods lend themselves to automated analysis by computer. These procedures can be applied to either active or passive couplings and yield an accuracy of ±0.04 Hz in the cases studied.J‐deconvolution may be implemented repetitively in order to reduce a COSY crosspeak to a single absorption peak at th

ISSN:0021-2148    

DOI:10.1002/ijch.199200030

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have used the elements of graph theory to describe NMR cross‐relaxation networks in macromolecules and to analyze different experiments used for topological editing. We propose a new experiment, block decoupled NOESY (B.D. NOESY), that splits the cross‐relaxation network of a macromolecular system into two noninteracting subdomains. Splitting is achieved by arranging for the effective z‐components of the magnetization in one subdomain, as viewed from the other subdomain, to be zero. The B.D. NOESY experiment, in favorable cases, can usefully simplify the analysis of cross‐relaxation spectra by removing certain ambiguities. We demonstrate the method by separating the aromatic resonances from the rest of the cross‐relaxation network of a small globular protein: turkey ovomucoid third domain, Mr6000. The resulting spectrum provides a clearer picture of cross‐relaxation pathways that involve only aromatic or only aliphatic spins. By comparing the B.D. NOESY spectrum with the normal NOESY spectrum, we were able to identify cross peaks that contain contributions from indirect magnetization transfer (spin diffusion) mediated by the aromatic side chains. In the terminology of graph theory, these experiments decompose topological networks of cross‐relaxation into two subgraphs, the join of which generates the o

ISSN:0021-2148    

DOI:10.1002/ijch.199200031

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractInterproton distances obtained from NMR experiments are used to determine the high‐resolution structure of biopolymers. The accuracy of internuclear distances extracted from proton homonuclear 2D NOE spectra can be improved by using a complete relaxation matrix analysis, taking into account all proton dipole‐dipole interactions as well as internal motions. The effect of internal motion on interproton distances, determined from the 2D NOE cross‐peak intensities, has been examined using a new version of themardigrasprogram, which iteratively determines interproton distances from experimental 2D NOE peak intensities using the complete relaxation matrix. The internal motion has been specified by the ‘model‐free’ approach in terms of an overall correlation time, local correlation times, and order parameters. Using a simulation for the 58‐residue protein bovine pancreatic trypsin inhibitor (BPTI), it is shown that if the local order parameter and correlation time for a given residue differ substantially (but within the experimentally determined range of values) from the average order parameter and correlation time, the error in the interproton distance may be as l

ISSN:0021-2148    

DOI:10.1002/ijch.199200032

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractVarious NMR methods have been applied to investigate the ring‐reversal process in the title compounds. In order to measure barriers an exact assignment of proton‐exchange connectivities was required. This assignment was made using phase‐sensitive NOESY and difference NOE and saturation transfer spectroscopies. The barriers for ring reversal were found to differ substantially between compounds1–3on one hand (9–10.5 kcal mol−1), and compound 4 on the other (20.2 kcal mol−1). This was rationalized in terms of the different geometries of the

ISSN:0021-2148    

DOI:10.1002/ijch.199200033

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractRf pulses with frequency responses which correspond to the rows of the 8th‐order Hadamard matrix are developed. These pulses can be used in theB1insensitive Hadamard Spectroscopic Imaging multi‐volume localization technique. It is shown how any adiabatic rf pulse, which inverts spin population at a well defined frequency band, can be modified to invert populations at several frequency bands simultaneously. Conditions on the frequency and amplitude functions which allow such modifications are defined. For increasing amplitude the interaction between the bands produces nonlinear irradiation at several frequencies. By using an integrated form of the Floquet formalism and perturbation theory, the interaction between the bands is described analytically by an effective Hamiltonian. From this description the conditions which guarantee minimization of the unwanted irradiation are defined. The experimental results agree very well with the theoretical predicati

ISSN:0021-2148    

DOI:10.1002/ijch.199200034

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractDiffusion barrier effects on diffusion and flow mapping by NMR microscopy were simulated for the general case of an interface between two compartments with different relaxation properties and diffusion coefficients. The extreme case of a completely impermeable barrier (i.e. a glass wall) shows substantial changes in signal intensity and phase at distances of a few micrometers from the surface. Permeable boundaries show varying degrees of intensity and phase changes that can be used for deducing permeability properties of barriers that would otherwise be below the true spatial resolution of the image. The simulations were done for the case of a solid, water‐imbibing polymer immersed in water and for water diffusion between intra‐ and extracellular compartments across a biological membrane. These specific boundary effects have substantial implications for the determination of image resolution and for the separation and measurement of coherent flow and random diffus

ISSN:0021-2148    

DOI:10.1002/ijch.199200035

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper reports the results of31P and1H NMR studies on alterations of glycerophosphocholine phosphodiesterase (EC 3.1.4.2) activity in Friend leukemia cells, upon in vitro induced erythroid differentiation, in which cells undergo a transition from a highly proliferative (proerythroblastic) to a more mature (normoblastic) state. Experiments on cell homogenates incubated in the presence of added glycerophosphocholine (GroPCho) demonstrated that in differentiated cells the activity of GroPCho phosphodiesterase was at least three times higher than in undifferentiated control cells. Activation of the same enzyme was also observed in previous studies on FLC tumors implanted in syngeneic mice and treated in vivo with antitumor cytokines. These results, together with the decrease of GroPCho reported also in other cell systems undergoing cell transitions to more mature forms, suggest the hypothesis that an activation of GroPCho phosphodiesterase might play a role in cell maturation and, perhaps, also in tumor cell proliferation. An interpretation is then proposed of the alterations observed in the levels of phospholipid precursors and/or catabolites in FLC in vitro and in vivo, in light of possible correlations between phospholipid turnover, cell stimulation, and signal transduction.

ISSN:0021-2148    

DOI:10.1002/ijch.199200036

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractThe two components T2fandT2sof the transverse magnetization decay curve of23Na in dog erythrocytes were measured using the triple quantum filtered spectra. It was shown that this technique, in spite of its inferior sensitivity, may be used for systems whereT2fandT2sare not very different. Thus it was possible to measure the effect of 2, 3‐diphosphoglycerate (2, 3 DPG) depletion on the intracellular23Na relaxation rates. The effect found indicates some contribution of hemoglobin‐2, 3‐DPG complexes to the23Na relaxation in intact erythrocytes.39K relaxation times were measured in human and chicken erythrocytes. The results indicate binding similar to that of23Na. Finally, a decrease of the cellular volume of erythrocytes was found to shorten23Na relaxation times. The result was interpreted as due to an increase in intracellular concentration of pro

ISSN:0021-2148    

DOI:10.1002/ijch.199200037

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY

摘要:

AbstractA new class of pulsed EPR experiments, based on FID‐detected hole‐burning, is introduced. A transient spectral hole burnt into an inhomogeneously broadened EPR line by means of a selective microwave pulse is shifted or broadened by different types of perturbations and is subsequently recorded in a single experiment via an FID following a nonselective microwave pulse. The proposed approach often exceeds corresponding electron spin‐echo experiments with respect to sensitivity and resolution. A number of applications of the FID‐detected hole‐burning technique are discussed and the predicted features of the new experiments are verified by model calculations and ex

ISSN:0021-2148    

DOI:10.1002/ijch.199200038

出版商:WILEY‐VCH Verlag    

年代:1992

数据来源: WILEY