Monday, September 28, 2009

Science, v325

Some nice 27Al SSNMR spectra can be seen here:

Science, v325, page 1670

Coordinatively Unsaturated Al3+ Centers as Binding Sites for Active Catalyst Phases of Platinum on -Al2O3

Ja Hun Kwak,1,* Jianzhi Hu,1 Donghai Mei,1 Cheol-Woo Yi,2 Do Heui Kim,1 Charles H. F. Peden,1,* Lawrence F. Allard,3 Janos Szanyi1,*

In many heterogeneous catalysts, the interaction of metal particles with their oxide support can alter the electronic properties of the metal and can play a critical role in determining particle morphology and maintaining dispersion. We used a combination of ultrahigh magnetic field, solid-state magic-angle spinning nuclear magnetic resonance spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy coupled with density functional theory calculations to reveal the nature of anchoring sites of a catalytically active phase of platinum on the surface of a -Al2O3 catalyst support material. The results obtained show that coordinatively unsaturated pentacoordinate Al3+ (Al3+penta) centers present on the (100) facets of the -Al2O3 surface are anchoring Pt. At low loadings, the active catalytic phase is atomically dispersed on the support surface (Pt/Al3+penta = 1), whereas two-dimensional Pt rafts form at higher coverages.
1 Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Post Office Box 999, MSIN K8-87, Richland, WA 99352, USA.2 Department of Chemistry and Institute of Basic Science, Sungshin Women’s University, Seoul 136-742, Repulic of Korea.3 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.

Thursday, September 03, 2009

Single molecule, one million times smaller than a grain of sand, pictured for first time
By Claire Bates
Last updated at 11:45 AM on 31st August 2009
Comments (330) Add to My Stories It may look like a piece of honeycomb, but this lattice-shaped image is the first ever close-up view of a single molecule.
Scientists from IBM used an atomic force microscope (AFM) to reveal the chemical bonds within a molecule.
'This is the first time that all the atoms in a molecule have been imaged,' lead researcher Leo Gross said.
The delicate inner structure of a pentacene molecule has been imaged with an atomic force microscope
The researchers focused on a single molecule of pentacene, which is commonly used in solar cells. The rectangular-shaped organic molecule is made up of 22 carbon atoms and 14 hydrogen atoms.

In the image above the hexagonal shapes of the five carbon rings are clear and even the positions of the hydrogen atoms around the carbon rings can be seen.
To give some perspective, the space between the carbon rings is only 0.14 nanometers across, which is roughly one million times smaller than the diameter of a grain of sand.
Textbook model: A computer-generated image of how we're used to seeing a molecule represented with balls and sticks
'If you think about how a doctor uses an X-ray to image bones and organs inside the human body, we are using the atomic force microscope to image the atomic structures that are the backbones of individual molecules,' said IBM researcher Gerhard Meyer.
A 3D view showing how a single carbon monoxide molecule was used to create the image using a 'tuning fork' effect
The team from IBM Research Zurich said the results could have a huge impact of the field of nanotechnology, which seeks to understand and control some of the smallest objects known to mankind.
The AFM uses a sharp metal tip that acts like a tuning fork to measure the tiny forces between the tip and the molecule. This requires great precision as the tip moves within a nanometer of the sample.

'Above the skeleton of the molecular backbone (of the pentacene) you get a different detuning than above the surface the molecule is lying on,' Mr Gross said.
This detuning is then measured and converted into an image.
To stop the tip from absorbing the pentacene molecule, the researchers replaced the metal with a single molecule of carbon monoxide. This was found to be more stable and created weaker electrostatic attractions with the pentacene, creating a higher resolution image.
Enlarge IBM researchers Nikolaj Moll, Reto Schlittler, Gerhard Meyer, Fabian Mohn and Leo Gross (l-r) stand behind an atomic force microscope Photo taken by Michael Lowry Image courtesy of IBM Research - Zurich
The experiment was also performed inside a high vacuum at the extremely cold temperature of -268C to avoid stray gas molecules or atomic vibrations from affecting the measurements.
'Eventually we want to investigate using molecules for molecular electronics,' Mr Gross said.
'We want to use molecules as wires

Read more:

Tuesday, September 01, 2009

J. Am. Chem. Soc., 2009, 131 (31), pp 11062–11079

Searching for Microporous, Strongly Basic Catalysts: Experimental and Calculated 29Si NMR Spectra of Heavily Nitrogen-Doped Y Zeolites

Fulya Dogan†, Karl D. Hammond‡, Geoffrey A. Tompsett‡, Hua Huo†, W. Curtis Conner, Jr.‡, Scott M. Auerbach‡§ and Clare P. Grey*†

Nitrogen substituted zeolites with high crystallinity and microporosity are obtained by nitrogen substitution for oxygen in zeolite Y. The substitution reaction is performed under ammonia flow by varying the temperature and reaction time. We examine the effect of aluminum content and charge-compensating cation (H+/Na+/NH4+) on the degree of nitrogen substitution and on the preference for substitution of Si−O−Al vs Si−O−Si linkages in the FAU zeolite structure. Silicon-29 magic angle spinning (MAS) nuclear magnetic resonance (NMR) and 1H/29Si cross-polarization MAS NMR spectroscopy have been used to probe the different local environments of the nitrogen-substituted zeolites. Experimental data are compared to simulated NMR spectra obtained by constructing a compendium (>100) of zeolite clusters with and without nitrogen, and by performing quantum calculations of chemical shifts for the NMR-active nuclei in each cluster. The simulated NMR spectra, which assume peak intensities predicted by statistical analysis, agree remarkably well with the experimental data. The results show that high levels of nitrogen substitution can be achieved while maintaining porosity, particularly for NaY and low-aluminum HY materials, without significant loss in crystallinity. Experiments performed at lower temperatures (750−800 °C) show a preference for substitution at Si−OH−Al sites. No preference is seen for reactions performed at higher temperatures and longer reaction times (e.g., 850 °C and 48 h).

J. Am. Chem. Soc., 2009, 131 (31), pp 10834–10835

Observation of NMR Signals from Proteins Introduced into Living Mammalian Cells by Reversible Membrane Permeabilization Using a Pore-Forming Toxin, Streptolysin O

Shinji Ogino†, Satoshi Kubo†, Ryo Umemoto†, Shuxian Huang†, Noritaka Nishida† and Ichio Shimada*†‡

We have developed a new in-cell NMR method that is applicable to any type of cell and does not require target protein modification or specialized equipment. The stable-isotope-labeled target protein, thymosin β4 (Tβ4), was delivered to 293F cells, which were permeabilized by a pore-forming toxin, streptolysin O, and resealed by Ca2+ after Tβ4 uptake. As a result, we successfully observed 1H−15N HSQC signals originating from the Tβ4, including those from the N-terminal acetylation, which had occurred inside the cell as a post-translational modification.

J. Am. Chem. Soc., 2009, 131 (31), pp 10832–10833

Measuring the Signs of 1Hα Chemical Shift Differences Between Ground and Excited Protein States by Off-Resonance Spin-Lock R1ρ NMR Spectroscopy

Renate Auer†‡, Philipp Neudecker‡, D. Ranjith Muhandiram‡, Patrik Lundstrm‡§, D. Flemming Hansen‡, Robert Konrat† and Lewis E. Kay*‡

Analysis of Carr−Purcell−Meiboom−Gill (CPMG) relaxation dispersion NMR profiles provides the kinetics and thermodynamics of millisecond-time-scale exchange processes involving the interconversion of populated ground and invisible excited states. In addition, the absolute values of chemical shift differences between NMR probes in the exchanging states, |Δ|, are also extracted. Herein, we present a simple experiment for obtaining the sign of 1Hα Δ values by measuring off-resonance 1Hα decay rates, R1ρ, using weak proton spin-lock fields. A pair of R1ρ values is measured with a spin-lock field applied |Δω| downfield and upfield of the major-state peak. In many cases, these two relaxation rates differ substantially, with the larger one corresponding to the case where the spin-lock field coincides with the resonance frequency of the probe in the minor state. The utility of the methodology is demonstrated first on a system involving protein ligand exchange and subsequently on an SH3 domain exchanging between a folded state and its on-pathway folding intermediate. With this experiment, it thus becomes possible to determine 1Hα chemical shifts of the invisible excited state, which can be used as powerful restraints in defining the structural properties of these elusive conformers.

J. Am. Chem. Soc., 2009, 131 (31), pp 10830–10831

High Resolution Heteronuclear Correlation NMR Spectroscopy of an Antimicrobial Peptide in Aligned Lipid Bilayers: Peptide−Water Interactions at the Water−Bilayer Interface

Riqiang Fu*†, Eric D. Gordon‡, Daniel J. Hibbard‡ and Myriam Cotten*§

High-resolution two-dimensional (2D) 1H−15N heteronuclear correlation (HETCOR) spectroscopy has been used to characterize the structure and dynamics of 15N-backbone labeled antimicrobial piscidin 1 (p1) oriented in “native-like” hydrated lipid bilayers. Piscidin belongs to a family of amphipatic cationic antimicrobial peptides, which are membrane-active and have broad spectrum antimicrobial activity on bacteria. When the 1H chemical shifts are encoded by the 1H−15N dipolar couplings, 2D dipolar-encoded HETCOR (i.e., de-HETCOR) solid-state NMR spectra yield high resolution 1H and 15N chemical shifts as well as 1H−15N heteronuclear dipolar couplings. Several advantages of HETCOR and de-HETCOR techniques that emerge from our investigations could facilitate the atomic-level investigations of structure−function relationships in membrane-active peptides and membrane-bound species. First, the de-HETCOR NMR spectrum of a ten-site 15N-labeled sample of p1 aligned in hydrated lipid bilayers can resolve resonances that are overlapped in the standard HETCOR spectrum. Second, the resolution in de-HETCOR spectra of p1 improves significantly at higher magnetic field due to an enhanced alignment that improves spectrum definition uniformly. Third, the HETCOR spectrum of 15N−K14 p1 oriented in hydrated lipid bilayers displays not only the expected crosscorrelation between the chemical shifts of bonded amide1H and 15N spins but also a cross peak between the 1H chemical shift from bulk water and the 15N chemical shift from the labeled amide nitrogen. This information provides new insights into the intermolecular interactions of an amphipathic antimicrobial peptide optimized to partition at the water-bilayer interface and may have implications at the biological level.

J. Am. Chem. Soc., 2009, 131 (31), pp 10816–10817

Transverse-Dephasing Optimized Homonuclear J-Decoupling in Solid-State NMR Spectroscopy of Uniformly 13C-Labeled Proteins

Sgolne Laage†, Anne Lesage†, Lyndon Emsley†, Ivano Bertini‡, Isabella C. Felli‡, Roberta Pierattelli‡ and Guido Pintacuda*†

A transverse-dephasing optimized S3E (spin-state selective excitation) method is implemented in solid-state NMR experiments of uniformly labeled protein samples, and it is shown to provide a simultaneous significant gain in both resolution (up to a factor of 2.2) and sensitivity (up to a factor of 1.4). This is illustrated with high-resolution NCO and NCA correlations of a microcrystalline sample of the oxidized form of the 153 residue human Cu(II)Zn(II) superoxide dismutase (SOD), a dimeric paramagnetic enzyme of 32 kDa. This method allows the resolution of 145 signals in the highly crowded carbonyl region in the NCO correlation spectrum.