Thursday, December 02, 2010

J. Phys. Chem. B and C, vol. 114, Issues 37-42

Accurate Determination of Interstrand Distances and Alignment in Amyloid Fibrils by Magic Angle Spinning NMR

Marc A. Caporini†§, Vikram S. Bajaj†, Mikhail Veshtort†, Anthony Fitzpatrick‡, Cait E. MacPhee‡, Michele Vendruscolo‡, Christopher M. Dobson‡, and Robert G. Griffin*†

J. Phys. Chem. B, 2010, 114 (42), pp 13555–13561
DOI: 10.1021/jp106675h
Publication Date (Web): October 6, 2010
Copyright © 2010 American Chemical Society

Abstract: Amyloid fibrils are structurally ordered aggregates of proteins whose formation is associated with many neurodegenerative and other diseases. For that reason, their high-resolution structures are of considerable interest and have been studied using a wide range of techniques, notably electron microscopy, X-ray diffraction, and magic angle spinning (MAS) NMR. Because of the excellent resolution in the spectra, MAS NMR is uniquely capable of delivering site-specific, atomic resolution information about all levels of amyloid structure: (1) the monomer, which packs into several (2) protofilaments that in turn associate to form a (3) fibril. Building upon our high-resolution structure of the monomer of an amyloid-forming peptide from transthyretin (TTR105−115), we introduce single 1-13C labeled amino acids at seven different sites in the peptide and measure intermolecular carbonyl−carbonyl distances with an accuracy of 0.11 A. Our results conclusively establish a parallel, in register, topology for the packing of this peptide into a β-sheet and provide constraints essential for the determination of an atomic resolution structure of the fibril. Furthermore, the approach we employ, based on a combination of a double-quantum filtered variant of the DRAWS recoupling sequence and multispin numerical simulations in SPINEVOLUTION, is general and should be applicable to a wide range of systems.

Solid-State NMR Study of Cysteine on Gold Nanoparticles

Anuji Abraham, Eugene Mihaliuk, Bharath Kumar, Justin Legleiter, and Terry Gullion*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
J. Phys. Chem. C, 2010, 114 (42), pp 18109–18114
DOI: 10.1021/jp107112b
Publication Date (Web): September 30, 2010
Copyright © 2010 American Chemical Society

Abstract: Solid-state NMR spectroscopy is used to characterize the interaction of l-cysteine with gold nanoparticles. The experiments show that there are two types of cysteine in the gold−cysteine complex, with nearly equal populations. We postulate that cysteine forms a two-layer boundary around the gold nanoparticles. The first layer is made of cysteine molecules forming a thiolate bond with the gold surface and having its charged amino and carboxyl groups oriented away from the gold surface. The second layer has its amino and carboxyl groups oriented toward the first layer and its sulfur group oriented away from the gold particles.

Pore Size Distribution Analysis of Mesoporous TiO2 Spheres by 1H Nuclear Magnetic Resonance (NMR) Cryoporometry

Su-Yeol Ryu, Dong Suk Kim, Jae-Deok Jeon, and Seung-Yeop Kwak*

J. Phys. Chem. C, 2010, 114 (41), pp 17440–17445
DOI: 10.1021/jp105496h
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract: Mesoporous TiO2 spheres with various pore sizes were prepared by varying the calcination temperature in the range of 300−700 °C. Increasing calcination temperature was found to increase the crystal size, decrease the surface area, and increase the pore size. The morphologies of mesoporous TiO2 spheres consist of well-defined spherical shapes of monodisperse sizes near 0.8 μm. To determine the pore size distributions (PSDs) of these mesoporous TiO2 spheres, 1H nuclear magnetic resonance (NMR) cryoporometry and Barrett−Joyner−Halenda (BJH) analysis were conducted. NMR cryoporometry is based on the theory of the melting point depression (MPD) of a probe molecule confined within a pore, which is dependent on the pore diameter. MPD was determined by analyzing the variation of the NMR spin−echo intensity with temperature. From the resulting spin−echo intensity versus temperature (I−T) curves, it was found that the maximum MPD of a probe molecule confined within the pores of mesoporous TiO2 decreases with increasing calcination temperature; that is, the pore size increases with increasing calcination temperature. Because mesoporous TiO2 spheres consist of aggregates of nanocrystallite TiO2 and mesopores located at intercrystallites, an increase in the calcination temperature induces an increase in the crystallite size and, thus, in the pore size because the small pores collapse and the large pores increase in size. We also confirmed by BJH analysis that the pore size of mesoporous TiO2 increases with increasing calcination temperature. This trend is in agreement with our 1H NMR cryoporometry results. Overall, these findings indicate that NMR cryoporometry is a very effective method for determining the PSDs of mesoporous TiO2 spheres.

Defect Functionalization of Hexagonal Boron Nitride Nanosheets

Yi Lin*†, Tiffany V. Williams‡, Wei Cao§, Hani E. Elsayed-Ali§, and John W. Connell‡

J. Phys. Chem. C, 2010, 114 (41), pp 17434–17439
DOI: 10.1021/jp105454w
Publication Date (Web): September 21, 2010
Copyright © 2010 American Chemical Society

Abstract:A pristine hexagonal boron nitride (h-BN) powder sample with layered crystalline sheetlike particles of 1−10 μm in lateral sizes and a few hundred nanometers in thicknesses was mechanically treated using a ball-mill to intentionally introduce defect sites. The h-BN was ball-milled for various times and subsequently was functionalized with a long alkyl chain amine via Lewis acid−base interactions between the amino groups and the boron atoms of h-BN to obtain soluble amine-attached nanosheet samples as the products. The functionalized h-BN nanosheet samples were characterized via various microscopic and spectroscopic techniques. The results strongly support a direct correlation between increasing defect site concentrations of the h-BN nanosheet samples and improved reaction efficiency with the amine. This suggests the enhanced reactivity of defect boron atoms in comparison to conjugated ones on an unperturbed h-BN plane. NMR investigations provided the strongest evidence supporting the hypothesis that the amino groups reacted with the h-BN at specific defect sites induced by ball-milling. The mechanistic implications are discussed.

Access to Well-Defined Ruthenium Mononuclear Species Grafted via a Si−Ru Bond on Silane Functionalized Silica†

Fernando Rascn‡, Romain Berthoud‡, Raphal Wischert‡, Wayne Lukens§, and Christophe Copret*‡

J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp1064962
Publication Date (Web): September 20, 2010
Copyright © 2010 American Chemical Society

Abstract: A functionalized silica with T3 silane surface groups (i.e., (≡SiO)3Si−H) was prepared and interacted with Ru(cod)(cot), resulting in the formation of monometallic surface species attached to the surface via a Si−Ru bond, according to EXAFS spectroscopy, infrared spectroscopy, and solid-state NMR.

Time-Resolved and Site-Specific Insights into Migration Pathways of Li+ in α-Li3VF6 by 6Li 2D Exchange MAS NMR

M. Wilkening*†, E. E. Romanova†‡, S. Nakhal§, D. Weber§, M. Lerch§, and P. Heitjans†

J. Phys. Chem. C, 2010, 114 (44), pp 19083–19088
DOI: 10.1021/jp103433h
Publication Date (Web): September 14, 2010
Copyright © 2010 American Chemical Society

Abstract: Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6Li NMR spectra on α-Li3VF6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ−1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.

Adsorbate Effect on AlO4(OH)2 Centers in the Metal−Organic Framework MIL-53 Investigated by Solid-State NMR Spectroscopy

Christian Lieder, Sabine Opelt, Michael Dyballa, Harald Henning, Elias Klemm, and Michael Hunger*

J. Phys. Chem. C, 2010, 114 (39), pp 16596–16602
DOI: 10.1021/jp105700b
Publication Date (Web): September 10, 2010
Copyright © 2010 American Chemical Society

Abstract:1H and 27Al MAS NMR spectroscopies have been applied for studying the effect of water molecules, nitrogen bases, and o-xylene on the hydroxyl protons of bridging AlOH groups and framework aluminum atoms in the metal−organic framework (MOF) MIL-53. For water molecules adsorbed on the low-temperature form MIL-53lt, two 1H MAS NMR signals were found indicating the formation of different O−H···O hydrogen bonds to neighboring oxygen atoms, such as to carboxylic oxygens. Upon adsorption of the nitrogen bases acetonitrile, ammonia, and pyridine, a linear increase of the quadrupole coupling constant, CQ, of the framework aluminum atoms in dehydrated MIL-53 from CQ = 8.5 MHz (unloaded material) to maximum 10.8 MHz (pyridine-loaded material) as a function of the proton affinity of the adsorbates was observed. Adsorption of o-xylene led to three stepwise changes of the quadrupole coupling constants, CQ, of framework aluminum atoms in dehydrated MIL-53. While the first two stepwise changes of the CQ values (CQ = 8.0 and 8.7 MHz) occur for o-xylene loadings of lower than 4 molecules per unit cell and for all AlO4(OH)2 centers, the third change of the CQ value to 9.4 MHz was observed for o-xylene loadings higher than 4 o-xylene molecules per unit cell and for maximum 50% of the framework aluminum atoms. This third adsorbate-induced change of the CQ value of framework aluminum atoms in MIL-53 is accompanied by a significant decrease of the adsorbate mobility

Impact of Controlling the Site Distribution of Al Atoms on Catalytic Properties in Ferrierite-Type Zeolites†

Yuriy Romn-Leshkov, Manuel Moliner, and Mark E. Davis*
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
J. Phys. Chem. C, Article ASAP
DOI: 10.1021/jp106247g
Publication Date (Web): September 9, 2010
Copyright © 2010 American Chemical Society

Abstract: Zeolites with the ferrierite (FER) topology are synthesized using a combination of tetramethylammonium (TMA) cations with differently sized cyclic amines (pyrrolidine (Pyr), hexamethyleneimine (HMI), and 1,4-diazabicyclo[2.2.2]octane (DAB)). Using these organic structure-directing agents (SDAs), low Si/Al ratios and concentrated synthesis mixtures favor the crystallization of FER materials. Increasing the size of the cyclic amine or decreasing the aluminum content leads to the crystallization of other phases or the creation of excessive amounts of connectivity defects. TMA cations play a decisive role in the synthesis of the FER materials, and their presence allows the use of HMI to synthesize FER. Proton MAS NMR is used to quantify the accessibility of pyridine to acid sites in these FER samples, where it is found that the FER+HMI+TMA sample contains only 27% acid sites in the 8-MR channels, whereas FER+Pyr and FER+Pyr+TMA contain 89% and 84%, respectively. The constraint index (CI) test and the carbonylation of dimethyl ether (DME) with carbon monoxide are used as probe reactions to evaluate how changes in the aluminum distribution in these FER samples affect their catalytic behavior. Results show that the use of Pyr as an SDA results in the selective population of acid sites in the 8-MR channels, whereas the use of HMI generates FER zeolites with an increased concentration of acid sites in the 10-MR channels.

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