@article {401, title = {Reversal of radio-frequency-driven spin diffusion by reorientation of the sample spinning axis}, journal = {Journal of Chemical Physics}, volume = {108}, year = {1998}, note = {J Chem PhysYr359Times Cited:9Cited References Count:39}, month = {Jan 15}, pages = {826-829}, abstract = {

The dipolar Hamiltonian in a rapidly rotating sample is scaled by the second Legendre polynomial of the cosine of the angle between the rotation axis and the static magnetic field. It is, therefore, possible to refocus the spatial polarization-transfer process, often termed spin diffusion, in extended spin systems by reorienting the rotor axis such that the dipolar interaction Hamiltonian changes sign. We present experimental results which demonstrate that a rapid mechanical sample reorientation leads to a time reversal of the \"radio-frequency-driven\" spin diffusion among C-13 spins. (C) 1998 American Institute of Physics. [S0021-9606(98)03503-X].

}, keywords = {echoes}, isbn = {0021-9606}, doi = {Doi 10.1063/1.475494}, url = {://WOS:000071487500002}, author = {De Paul, S. M. and Tomaselli, M. and Pines, A. and Ernst, M. and Meier, B. H.} } @article {398, title = {Time reversal of cross-polarization in solid-state NMR}, journal = {Molecular Physics}, volume = {95}, year = {1998}, note = {Mol Phys146XYTimes Cited:4Cited References Count:21}, month = {Dec 10}, pages = {849-858}, abstract = {

Cross-polarization at the Hartmann-Hahn condition in solid-state NMR frequently is described in terms of thermodynamics. Spin temperatures characterizing the canonical density operator are assigned to the Zeeman reservoirs of the two spins and the cross-polarization process brings about a state of equilibrium of the two reservoirs with a common temperature. In such a model, cross-polarization from an initially polarized spin species (I spins) to another spin species (S spins) is inherently an irreversible process accompanied by an increase in the entropy of the system. However, a cross-polarization echo can be generated whereby the polarization transferred to the S spins returns to the I spins, restoring the initial density operator. Therefore a thermodynamic description should be applied with care even in samples where the build-up and the decay of the magnetization can be approximated well by multiexponential processes. Such cross-polarization echoes are formed by the consecutive application of two pulse trains that produce effective Hamiltonians differing in sign. The \&$\#$39;time reversal\&$\#$39; of cross-polarization is consistent with both the increase in Zeeman entropy during the approach to equilibrium and with the constraint of unitary quantum evolution.

}, keywords = {echoes}, isbn = {0026-8976}, doi = {Doi 10.1080/002689798166477}, url = {://WOS:000077460200014}, author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.} } @article {397, title = {Time-reversal of cross-polarization in nuclear magnetic resonance}, journal = {Journal of Chemical Physics}, volume = {108}, year = {1998}, note = {J Chem Phys108FUTimes Cited:11Cited References Count:17}, month = {Jun 15}, pages = {9611-9613}, abstract = {

It is demonstrated that the time evolution of the heteronuclear polarization-transfer process in a dipolar-coupled nuclear spin system can be reversed, leading to the observation of cross polarization echoes. The cross-polarization echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in sign, Cross-polarization echoes have been recorded for a powder sample of alanine. The time evolution of the spin;system is consistent with both unitary quantum dynamics and with spin thermodynamics of two systems approaching a common spin temperature. (C) 1998 American Institute of Physics.

}, keywords = {echoes}, isbn = {0021-9606}, doi = {Doi 10.1063/1.476435}, url = {://WOS:000075256100001}, author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.} } @article {421, title = {Cross-polarization from quadrupolar nuclei to silicon using low-radio-frequency amplitudes during magic-angle spinning}, journal = {Journal of Physical Chemistry B}, volume = {101}, year = {1997}, note = {J Phys Chem BWu630Times Cited:51Cited References Count:50}, month = {Apr 17}, pages = {3240-3249}, abstract = {

The dynamics of cross-polarization from the central transition of a quadrupolar nucleus (Al-27 or Na-23) to a spin-1/2 nucleus (Si-29) during magic-angle spinning and using low-radio-frequency Geld strengths are analyzed for the mineral low albite. Under these conditions additional complications in the spin-lock behavior of the quadrupolar nucleus and in the cross-polarization process were found experimentally and are examined in detail. A step-by-step procedure for optimizing cross-polarization from the central transition of a quadrupolar nucleus to a spin-1/2 nucleus is described. Significant enhancement of Si-29 NMR sensitivity and several applications are demonstrated.

}, keywords = {locking}, isbn = {1089-5647}, doi = {Doi 10.1021/Jp9623711}, url = {://WOS:A1997WU63000031}, author = {DePaul, S. M. and Ernst, M. and Shore, J. S. and Stebbins, J. F. and Pines, A.} } @article {415, title = {A novel detection-estimation scheme for noisy NMR signals: Applications to delayed acquisition data}, journal = {Journal of Magnetic Resonance}, volume = {128}, year = {1997}, note = {J Magn ResonYc696Times Cited:47Cited References Count:59}, month = {Sep}, pages = {30-41}, abstract = {

Many potentially interesting and useful classes of NMR experiments generate data for which conventional spectral estimation and quantification aa the Fourier transform are unsatisfactory. In particular, recently introduced solid-state NMR experiments which involve long delays before data acquisition fall into this category, as the free induction decays are heavily \&$\#$39;\&$\#$39;truncated\&$\#$39;\&$\#$39; and have low signal-to-noise ratios. A novel detection-estimation scheme is introduced in order to analyze data from such experiments and others where the sensitivity is low and/or the data record is strongly damped or truncated, Based on the assumption of exponential data modeling, the number of signals present is first detected using criteria derived from information theory and the spectral parameters are then estimated using the matrix pencil method, Monte Carlo simulations and experimental. applications are carried out to demonstrate its superior statistical and computational performances and its general applicability to delayed acquisition data. Over the range of note levels investigated, it is found that this approach is essentially near-optimal in the sense that the estimated spectral parameters have biases almost equal to zero and variances very close to their theoretical Cramer-Rao lower bounds. Compared to the popular method of linear prediction with singular value decomposition, this method not only improves the estimation accuracy (by a factor of 2-4) with a lower \&$\#$39;\&$\#$39;break-down\&$\#$39;\&$\#$39; signal-to-noise threshold (approximate to 1.5 dB), but also reduces the computational cost by about an order of magnitude, It also holds great promise in effectively reducing truncation artifacts. It is concluded that this approach not only facilitates the analysis of delayed acquisition data, but can also become a valuable tool in the routine quantification of general NMR spectra, ia listing of programs is also included in the Appendix. (C) 1997 Academic Press.

}, keywords = {resolution}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.1997.1215}, url = {://WOS:A1997YC69600004}, author = {Lin, Y. Y. and Hodgkinson, P. and Ernst, M. and Pines, A.} } @article {428, title = {Application of rotational resonance to inhomogeneously broadened systems}, journal = {Chemical Physics Letters}, volume = {251}, year = {1996}, note = {Chem Phys LettUd924Times Cited:17Cited References Count:28}, month = {Mar 22}, pages = {223-229}, abstract = {

A protocol is presented for the determination of internuclear distances using rotational-resonance magnetization-exchange NMR in systems with inhomogeneously broadened lines. Non-linear least-square fitting procedures are used to obtain the distance, the inhomogeneous broadening, the zero-quantum relaxation time, and error estimates for these parameters. We apply this procedure to a biological system of unknown structure.

}, keywords = {nmr}, isbn = {0009-2614}, doi = {Doi 10.1016/0009-2614(96)00098-X}, url = {://WOS:A1996UD92400015}, author = {Heller, J. and Larsen, R. and Ernst, M. and Kolbert, A. C. and Baldwin, M. and Prusiner, S. B. and Wemmer, D. E. and Pines, A.} } @article {430, title = {Isotropic second-order dipolar shifts in the rotating frame}, journal = {Journal of Chemical Physics}, volume = {104}, year = {1996}, note = {J Chem PhysUm909Times Cited:9Cited References Count:44}, month = {Jun 1}, pages = {8258-8268}, abstract = {

An experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio-frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation-dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field-dependent chemical shift in solids due to the second-order dipolar perturbation observed in magic-angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one-bond C-H coupling compared to a shift of a few Hertz in the laboratory-frame experiment. In addition to the isotropic shift, a line broadening due to the P-4(cos beta) terms is observed when the experiment is carried out under magic-angle sample spinning (MAS) conditions, leading to the requirement of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation of CH, CH2, and CH3 groups in a 2D spectrum under MAS is demonstrated. Implemented under DOR it could be used as a technique to select carbon atoms according to the number of directly attached protons. (C) 1996 American Institute of Physics.

}, keywords = {spins}, isbn = {0021-9606}, doi = {Doi 10.1063/1.471579}, url = {://WOS:A1996UM90900008}, author = {Ernst, M. and Kolbert, A. C. and Schmidt-Rohr, K. and Pines, A.} } @article {431, title = {Second-order recoupling of chemical-shielding and dipolar-coupling tensors under spin decoupling in solid-state NMR}, journal = {Journal of Chemical Physics}, volume = {105}, year = {1996}, note = {J Chem PhysVd768Times Cited:55Cited References Count:53}, month = {Sep 1}, pages = {3387-3397}, abstract = {

The source of the residual line broadening in continuous-wave (cw) decoupled spectra under magic-angle sample spinning conditions is reexamined. It is shown that an important contribution to the line broadening comes from a second-order recoupling of the heteronuclear dipolar-coupling tensor and the chemical-shielding tensor of the irradiated spin. Such an interference between the two tensors leads to a sum of a zeroth-rank, a second-rank, and a fourth-rank tensor component in the Hamiltonian. The zeroth-rank and the fourth-rank tensor components are not averaged out under magic-angle sample spinning (MAS) conditions, requiring the use of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. This broadening is distinctly different from off-resonance decoupling effects which transform as a second-rank tensor and can be averaged out by MAS. The properties of this second-order recoupling as a source of structural information are explored, and the conditions for removing the broadening in systems with weak homonuclear dipolar-coupling networks are discussed. (C) 1996 American Institute of Physics.

}, keywords = {waltz-16}, isbn = {0021-9606}, doi = {Doi 10.1063/1.472224}, url = {://WOS:A1996VD76800005}, author = {Ernst, M. and Bush, S. and Kolbert, A. C. and Pines, A.} } @article {429, title = {Solid-state NMR studies of the prion protein H1 fragment}, journal = {Protein Science}, volume = {5}, year = {1996}, note = {Protein SciVa138Times Cited:74Cited References Count:53}, month = {Aug}, pages = {1655-1661}, abstract = {

Conformational changes in the prion protein (PrP) seem to be responsible for prion diseases. We have used conformation-dependent chemical-shift measurements and rotational-resonance distance measurements to analyze the conformation of solid-state peptides lacking long-range order, corresponding to a region of PrP designated H1. This region is predicted to undergo a transformation of secondary structure in generating the infectious form of the protein. Solid-state NMR spectra of specifically C-13-enrrched samples of H1, residues 109-122 (MKHMAGAAAAGAVV) of Syrian hamster PrP, have been acquired under cross-polarization and magic-angle spinning conditions. Samples lyophilized from 50\% acetonitrile/50\% water show chemical shifts characteristic of a beta-sheet conformation in the region corresponding to residues 112-121, whereas samples lyophilized from hexafluoroisopropanol display shifts indicative of alpha-helical secondary structure in the region corresponding to residues 113-117. Complete conversion to the helical conformation was not observed and conversion from alpha-helix back to beta-sheet, as inferred from the solid-state NMR spectra, occurred when samples were exposed to water. Rotational-resonance experiments were performed on seven doubly C-13-labeled H1 samples dried from water. Measured distances suggest that the peptide is in an extended, possibly beta-strand, conformation. These results are consistent with the experimental observation that PrP can exist in different conformational states and with structural predictions based on biological data and theoretical modeling that suggest that H1 may play a key role in the conformational transition involved in the development of prion diseases.

}, keywords = {alpha-helices}, isbn = {0961-8368}, url = {://WOS:A1996VA13800019}, author = {Heller, J. and Kolbert, A. C. and Larsen, R. and Ernst, M. and Bekker, T. and Baldwin, M. and Prusiner, S. B. and Pines, A. and Wemmer, D. E.} } @article {444, title = {Low-Power Decoupling Sequences for High-Resolution Chemical-Shift and Local-Field Nmr-Spectra of Liquid-Crystals}, journal = {Journal of Magnetic Resonance Series A}, volume = {113}, year = {1995}, note = {J Magn Reson Ser AQw158Times Cited:11Cited References Count:29}, month = {Apr}, pages = {169-176}, abstract = {

Various new and well-established multiple-pulse sequences that provide homonuclear dipolar decoupling are compared on a high-resolution spectrometer/probe configuration using low RF-irradiation powers. Proton-proton decoupled carbon-13 spectra of singly C-13-labeled benzene dissolved in a nematic liquid crystal and of the liquid crystal 152 are presented. Two classes of multiple-pulse sequences are studied that produce either broadband decoupled carbon spectra or separated local-held spectra. Several new approaches for the design of windowless sequences are demonstrated to be valuable in these heteronuclear experiments. They include implementations of the iterative MLEV scheme and the use of 270 degrees pulses. Furthermore, noncyclic propagators have been found that exhibit advantages over closely related cyclic analogues in both classes of decoupling sequences. (C) 1995 Academic Press, Inc.

}, keywords = {dipolar}, isbn = {1064-1858}, doi = {Doi 10.1006/Jmra.1995.1077}, url = {://WOS:A1995QW15800005}, author = {Nanz, D. and Ernst, M. and Hong, M. and Ziegeweid, M. A. and Schmidt-Rohr, K. and Pines, A.} }