Friday, September 28, 2007

Sept 28, 2007 - Journal Updates

Aluminum Siting in Silicon-Rich Zeolite Frameworks: A Combined High-Resolution 27Al NMR Spectroscopy and Quantum Mechanics / Molecular Mechanics Study of ZSM-5
Stepan Sklenak, Jií Ddeek, Chengbin Li, Blanka Wichterlová, Vendula Gábová, Marek Sierka, Joachim Sauer
Angewandte (2007)46, 7286.
Finding Al: The distribution of catalytically active sites over crystallographic positions in zeolites is not random but controlled by synthesis. For differently synthesized ZSM-5 samples, distinct 27Al resonances have been identified by NMR spectroscopy. Shift calculations by DFT show that the observed resonances belong to Al in different crystallographic sites.

Structural and dynamical properties of guest molecules confined in mesoporous silica materials revealed by NMR
Gerd Buntkowsky, Hergen Breitzke, Anna Adamczyk, Frank Roelofs, Thomas Emmler, Egbert Gedat, Bob Grünberg, Yeping Xu, Hans-Heinrich Limbach, Ilja Shenderovich, Anastasia Vyalikh and Gerhard Findenegg
In the last fifteen years several novel porous silica materials, which are periodically structured on the mesoscopic length scale, have been synthesized. They are of broad interest for fundamental studies of surface–substrate interactions, for studies of the dynamics of guest molecules in confinement and for studies of the effect of confinement on the structural and thermophysical properties of fluids. Examples of such confinement effects include the change of the freezing and melting points or glass transitions of the confined liquids. These effects are studied by combinations of several NMR techniques, such as 15N- and 2H-solid-state NMR line shape analysis, MAS NMR and NMR diffusometry with physico-chemical characterization techniques such as nitrogen adsorption and small angle diffraction of neutrons or X-rays. This combination does not require crystalline samples or special clean and well defined surfaces such as conventional surface science techniques, but can work with typical ill-defined real world systems. The review discusses, after a short introduction, the salient features of these materials and the applied NMR experiments to give the reader a basic knowledge of the systems and the experiments. The rest of the review then focuses on the structural and dynamical properties of guest molecules confined in the mesoporous silica. It is shown that the confinement into the pores leads to fascinating new features of the guests, which are often not known for their bulk phases. These features depend strongly on the interplay of the their interactions with the silica surface and their mutual interactions.

Solid-state NMR opf endoheral hydrogen-fullerene complexes
M. Carravetta, A. Danquigny, S. Mamone, F. Cuda, O. G. Johannessen, I. Heinmaa, K. Panesar, R. Stern, M. C. Grossel, A. J. Horsewill, A. Samoson, M. Murata, Y. Murata, K. Komatsu and M. H. Levitt
We present an overview of solid-state NMR studies of endohedral H2-fullerene complexes, including 1H and 13C NMR spectra, 1H and 13C spin relaxation studies, and the results of 1H dipole–dipole recoupling experiments. The available data involves three different endohedral H2-fullerene complexes, studied over a wide range of temperatures and applied magnetic fields. The symmetry of the cage influences strongly the motionally-averaged nuclear spin interactions of the endohedral H2 species, as well as its spin relaxation behaviour. In addition, the non-bonding interactions between fullerene cages are influenced by the presence of endohedral hydrogen molecules. The review also presents several pieces of experimental data which are not yet understood, one example being the structured 1H NMR lineshapes of endohedral H2 molecules trapped in highly symmetric cages at cryogenic temperatures. This review demonstrates the richness of NMR phenomena displayed by H2-fullerene complexes, especially in the cryogenic regime.

Friday, September 21, 2007

Hiyam's Journal Update

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

Chemical Shift Correlation NMR Spectroscopy with Indirect Detection in Fast Rotating Solids: Studies of Organically Functionalized Mesoporous Silicas
Jerzy W. Wiench, Charles E. Bronnimann, Victor S.-Y Lin, and Marek Pruski*

The first indirectly detected 13C-1H solid-state NMR correlation spectra of species bound to a surface are reported. The experiments were performed at 14.1 T under fast (40 kHz) MAS. The sensitivity gain was demonstrated on a mesoporous silica sample containing approximately 300 g of covalently bound allyl groups, in the absence of templating molecules and without isotope enrichment. A well-resolved two-dimensional spectrum was acquired in 15 min, which is more than 10 times faster than using direct detection of 13C nuclei.

Wednesday, September 19, 2007

Hiyam's Journal Update

Proton-Detected Solid-State NMR Spectroscopy of Fully Protonated Proteins at 40 kHz Magic-Angle Spinning
Donghua H. Zhou, Gautam Shah, Mircea Cormos, Charles Mullen, Dennis Sandoz, and Chad M. Rienstra

Remarkable progress in solid-state NMR has enabled complete structure determination of uniformly labeled proteins in the size range of 5-10 kDa. Expanding these applications to larger or mass-limited systems requires further improvements in spectral sensitivity, for which inverse detection of 13C and 15N signals with 1H is one promising approach. Proton detection has previously been demonstrated to offer sensitivity benefits in the limit of sparse protonation or with ~30 kHz magic-angle spinning (MAS). Here we focus on experimental schemes for proteins with ~100% protonation. Full protonation simplifies sample preparation and permits more complete chemical shift information to be obtained from a single sample. We demonstrate experimental schemes using the fully protonated, uniformly 13C,15N-labeled protein GB1 at 40 kHz MAS rate with 1.6-mm rotors. At 500 MHz proton frequency, 1-ppm proton line widths were observed (500 ± 150 Hz), and the sensitivity was enhanced by 3 and 4 times, respectively, versus direct 13C and 15N detection. The enhanced sensitivity enabled a family of 3D experiments for spectral assignment to be performed in a time-efficient manner with less than a micromole of protein. CANH, CONH, and NCAH 3D spectra provided sufficient resolution and sensitivity to make full backbone and partial side-chain proton assignments. At 750 MHz proton frequency and 40 kHz MAS rate, proton line widths improve further in an absolute sense (360 ± 115 Hz). Sensitivity and resolution increase in a better than linear manner with increasing magnetic field, resulting in 14 times greater sensitivity for 1H detection relative to that of 15N detection.

Tuesday, September 18, 2007

interesting article

September 17, 2007
Volume 85, Number 38 , p. 10
Nuclear Chemistry

Exotic Molecule

Dipositronium molecule contains only electrons and positrons

Wednesday, September 12, 2007

Hiyam's Journal Update

Solid-State NMR Structural Measurements on the Membrane-Associated Influenza Fusion Protein Ectodomain
Jaime Curtis-Fisk, Casey Preston, Zhaoxiong Zheng, R. Mark Worden, and David P. Weliky*

The 185 residue "FHA2" domain of the influenza virus hemagglutinin protein lies outside the virus and plays a significant role in catalyzing fusion between viral and host cell membranes. FHA2 was expressed in Escherichia coli with a yield of 3 mg/L of fermentation and after purification was shown to be folded in detergent and to rapidly catalyze vesicle fusion. More than half of the FHA2 residues are the first residue in a unique sequential pair, and for a particular pair, 13CO and 15N amino acid type labeling was, respectively, done for the first and second residues in the pair. For membrane-associated FHA2, NMR filtering allowed selective observation of the 13CO signal of the first residue, and the 13CO chemical shift was correlated with local secondary structure. Such shift/conformational measurements were made at critical glycine residues in the functionally important "fusion peptide" region of FHA2 and at a more C-terminal leucine. This should be a general approach to mapping the conformation of membrane-associated FHA2 and for testing FHA2 structure-function models. The approach will also be applicable to other viral fusion proteins and other large membrane proteins.

J. Am. Chem. Soc., 129 (37), 11438 -11446, 2007.

Phosphate-Mediated Arginine Insertion into Lipid Membranes and Pore Formation by a Cationic Membrane Peptide from Solid-State NMR
Ming Tang, Alan J. Waring, and Mei Hong*

The insertion of charged amino acid residues into the hydrophobic part of lipid bilayers is energetically unfavorable yet found in many cationic membrane peptides and protein domains. To understand the mechanism of this translocation, we measured the 13C-31P distances for an Arg-rich -hairpin antimicrobial peptide, PG-1, in the lipid membrane using solid-state NMR. Four residues, including two Arg's, scattered through the peptide were chosen for the distance measurements. Surprisingly, all residues show short distances to the lipid 31P: 4.0-6.5 Å in anionic POPE/POPG membranes and 6.5-8.0 Å in zwitterionic POPC membranes. The shortest distance of 4.0 Å, found for a guanidinium C at the -turn, suggests N-H···O-P hydrogen bond formation. Torsion angle measurements of the two Arg's quantitatively confirm that the peptide adopts a -hairpin conformation in the lipid bilayer, and gel-phase 1H spin diffusion from water to the peptide indicates that PG-1 remains transmembrane in the gel phase of the membrane. For this transmembrane -hairpin peptide to have short 13C-31P distances for multiple residues in the molecule, some phosphate groups must be embedded in the hydrophobic part of the membrane, with the local 31P plane parallel to the -strand. This provides direct evidence for toroidal pores, where some lipid molecules change their orientation to merge the two monolayers. We propose that the driving force for this toroidal pore formation is guanidinium-phosphate complexation, where the cationic Arg residues drag the anionic phosphate groups along as they insert into the hydrophobic part of the membrane. This phosphate-mediated translocation of guanidinium ions may underlie the activity of other Arg-rich antimocrobial peptides and may be common among cationic membrane proteins.

Wednesday, September 05, 2007

Hiyam's Journal Update

J. Am. Chem. Soc., 129 (36), 11161 -11171, 2007.
Brnsted/Lewis Acid Synergy in Dealuminated HY Zeolite: A Combined Solid-State NMR and Theoretical Calculation Study
Shenhui Li, Anmin Zheng, Yongchao Su, Hailu Zhang, Lei Chen, Jun Yang, Chaohui Ye, and Feng Deng*

The Brnsted/Lewis acid synergy in dealuminated HY zeolite has been studied using solid-state NMR and density function theory (DFT) calculation. The 1H double quantum magic-angle spinning (DQ-MAS) NMR results have revealed, for the first time, the detailed spatial proximities of Lewis and Brnsted acid sites. The results from 13C NMR of adsorbed acetone as well as DFT calculation demonstrated that the Brnsted/Lewis acid synergy considerably enhanced the Brnsted acid strength of dealuminated HY zeolite. Two types of Brnsted acid sites (with enhanced acidity) in close proximity to extra-framework aluminum (EFAL) species were identified in the dealuminated HY zeolite. The NMR and DFT calculation results further revealed the detailed structures of EFAL species and the mechanism of Brnsted/Lewis acid synergy. Extra-framework Al(OH)3 and Al(OH)2+ species in the supercage cage and Al(OH)2+ species in the sodalite cage are the preferred Lewis acid sites. Moreover, it is the coordination of the EFAL species to the oxygen atom nearest the framework aluminum that leads to the enhanced acidity of dealuminated HY zeolite though there is no direct interaction (such as the hydrogen-bonding) between the EFAL species and the Brnsted acid sites. All these findings are expected to be important in understanding the roles of Lewis acid and its synergy with the Brnsted acid in numerous zeolite-mediated hydrocarbon reactions.