Friday, February 22, 2008

Hiyam's Journal Update

Low-lying excitations at the rare-earth site due to the rattling motion in the filled skutterudite LaOs4Sb12 revealed by 139La NMR and 121/123Sb NQR
Yusuke Nakai,1 Kenji Ishida,1 Hitoshi Sugawara,2 Daisuke Kikuchi,3 and Hideyuki Sato3

We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the Sb site in the filled skutterudite LaOs4Sb12. We found that the nuclear spin-lattice relaxation rate divided by temperature 1/T1T at the La site exhibits a different temperature dependence from that at the Sb site. Although 1/T1T at the Sb site is explained by the Korringa mechanism, 1/T1T at the La site exhibits a broad maximum around 50 K, showing the presence of an additional contribution at the La site. The additional low-lying excitations observed at the La site can be understood with the relaxation from anharmonic phonons due to the rattling motion of the La atoms.

Phys. Rev. B 77, 014412 (2008) (10 pages)

17O and 51V NMR for the zigzag spin-1 chain compound CaV2O4
X. Zong,1,2 B. J. Suh,1 A. Niazi,1 J. Q. Yan,1 D. L. Schlagel,3 T. A. Lograsso,3 and D. C. Johnston1,2
1Ames Laboratory, Ames, Iowa 50011, USA

51V NMR studies on CaV2O4 single crystals and 17O NMR studies on 17O-enriched powder samples are reported. The temperature dependences of the 17O NMR linewidth and nuclear spin-lattice relaxation rate give strong evidence for a long-range antiferromagnetic transition at TN=78 K in the powder. Magnetic susceptibility measurements show that TN=69 K in the crystals. A zero-field 51V NMR signal was observed at low temperatures (f237 MHz at 4.2 K) in the crystals. The field-swept spectra with the field in different directions suggest the presence of two antiferromagnetic substructures. Each substructure is collinear, with the easy axes of the two substructures separated by an angle of 19(1)°, and with their average direction pointing approximately along the b axis of the crystal structure. The two spin substructures contain equal numbers of spins. The temperature dependence of the ordered moment, measured up to 45 K, shows the presence of an energy gap EG in the antiferromagnetic spin wave excitation spectrum. Antiferromagnetic spin wave theory suggests that EG/kB lies between 64 and 98 K.

J. Am. Chem. Soc., ASAP Article 10.1021/ja0778803 S0002-7863(07)07880-8 Web Release

Solid-State Deuterium NMR Studies Reveal s-ns Motions in the HIV-1 Transactivation Response RNA Recognition Site
Greg L. Olsen, Dorothy C. Echodu, Zahra Shajani, Michael F. Bardaro, Jr., Gabriele Varani, and Gary P. Drobny*

Solution and solid-state NMR measurements were used together to examine motion in three sites in the HIV-1 TAR RNA. We wished to investigate the dynamics facilitating the conformational rearrangements the TAR RNA must undergo for Tat binding and in particular to characterize the full range of motional time scales accessible to this RNA. Our results demonstrate that the dynamics in TAR involving residues essential to Tat binding include not only the faster motions detected by solution relaxation measurements but also a significant component in the s-ns time scale.

J. Am. Chem. Soc., 130 (8), 2667 -2675, 2008. 10.1021/ja078337p S0002-7863(07)08337-0 Web Release Date: February 1, 2008 Copyright © 2008 American Chemical Society

Probing Chemical Shifts of Invisible States of Proteins with Relaxation Dispersion NMR Spectroscopy: How Well Can We Do?
D. Flemming Hansen, Pramodh Vallurupalli, Patrik Lundström, Philipp Neudecker, and Lewis E. Kay*

Carr-Purcell-Meiboom-Gill relaxation dispersion NMR spectroscopy has evolved into a powerful approach for the study of low populated, invisible conformations of biological molecules. One of the powerful features of the experiment is that chemical shift differences between the exchanging conformers can be obtained, providing structural information about invisible excited states. Through the development of new labeling approaches and NMR experiments it is now possible to measure backbone 13C and 13CO relaxation dispersion profiles in proteins without complications from 13C-13C couplings. Such measurements are presented here, along with those that probe exchange using 15N and 1HN nuclei. A key experimental design has been the choice of an exchanging system where excited-state chemical shifts were known from independent measurement. Thus it is possible to evaluate quantitatively the accuracy of chemical shift differences obtained in dispersion experiments and to establish that in general very accurate values can be obtained. The experimental work is supplemented by computations that suggest that similarly accurate shifts can be measured in many cases for systems with exchange rates and populations that fall within the range of those that can be quantified by relaxation dispersion. The accuracy of the extracted chemical shifts opens up the possibility of obtaining quantitative structural information of invisible states of the sort that is now available from chemical shifts recorded on ground states of proteins.

J. Am. Chem. Soc., 130 (8), 2432 -2433, 2008. 10.1021/ja710477h S0002-7863(71)00477-3 Web Release Date: February 5, 2008 Copyright © 2008 American Chemical Society
Accurately Probing Slow Motions on Millisecond Timescales with a Robust NMR Relaxation Experiment
Dong Long, Maili Liu, and Daiwen Yang*

A new pulse scheme is proposed for the accurate measurement of relaxation dispersion, which cycles the phases of CPMG pulses. Numerical simulations show that systematic errors in the measured relaxation rates mainly result from off-resonance and radio frequency inhomogeneity effects and they can be significantly suppressed with the method proposed here. The method has been demonstrated on human liver fatty acid binding protein. It allows the reliable identification of residues undergoing conformational exchange on millisecond timescales and accurate extraction of kinetics parameters. The relaxation dispersion data indicate that human liver fatty acid binding protein is highly flexible on millisecond timescales.

J. Am. Chem. Soc., 130 (8), 2412 -2413, 2008. 10.1021/ja710557t S0002-7863(71)00557-2 Web Release Date: February 1, 2008 Copyright © 2008 American Chemical Society
A High-Resolution 43Ca Solid-State NMR Study of the Calcium Sites of Hydroxyapatite

Danielle Laurencin, Alan Wong, John V. Hanna, Ray Dupree, and Mark E. Smith*

High resolution 43Ca solid-state NMR studies of hydroxyapatite (Ca10(PO4)6(OH)2) were performed at 14.1 T. The two crystallographically distinct calcium sites were unequivocally resolved by a triple-quantum magic angle spinning experiment, and the unambiguous assignment of the signals was possible using 1H-43Ca rotational echo double resonance and 1H-43Ca cross polarization magic angle spinning experiments.

J. Am. Chem. Soc., ASAP Article 10.1021/

Probing Local Structure in Zeolite Frameworks: Ultrahigh-Field NMR Measurements and Accurate First-Principles Calculations of Zeolite 29Si Magnetic Shielding Tensors
Darren H. Brouwer* and Gary D. Enright

The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.

ASAP Chem. Mater., ASAP Article, 10.1021/cm0717763 Web Release Date: February 16, 2008 Copyright © 2008 American Chemical Society
A Solid-State NMR, X-ray Diffraction, and Ab Initio Investigation into the Structures of Novel Tantalum Oxyfluoride Clusters

Todd M. Alam,*† Jacalyn S. Clawson,†‡ François Bonhomme,† Steven G. Thoma,† Mark A. Rodriguez,† Shaohui Zheng,§ and Jochen Autschbach§

A series of tantalum oxyfluoride materials containing the [Ta4F16O4]4− and [Ta8F24O12]8− anion clusters have been synthesized and characterized using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (SSNMR) spectroscopy. The structure of both tantalum oxyfluoride materials display octahedrally bonded tantalum atoms with bridging oxygen and terminal fluoride atoms. The [Ta4F16O4]4− cluster is an eight-membered ring, whereas the [Ta8F24O12]8- cluster forms a cagelike structure. Solid-state dynamics of these clusters were explored by monitoring the impact of temperature on the one-dimensional (1D) 19F magic angle spinning (MAS) NMR, 13C cross-polarization (CP) MAS NMR, and two-dimensional (2D) double quantum (DQ) 19F MAS NMR spectra. The DQ 19F NMR correlation experiments allowed the through space connectivity between the different resolved fluorine environments to be determined, thus aiding in the spectral assignment and structural refinement of these materials. Ab initio 19F NMR chemical shift calculations were used to assist in the interpretation of the 19F NMR spectra. The influence of scalar relativistic and Ta−F spin–orbit coupling on the 19F NMR shielding calculation arising from bonding to tantalum atoms is also addressed.

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