Thursday, April 02, 2009

J Phys Chem C and B, vol. 113, Issue 13 and 14

Designing Amphotropic Smectic Liquid Crystals Based on Phosphonium Salts for Partial Ordering of Solutes as Monitored by NMR Spectroscopy

Astghik A. Shahkhatuni, Kefeng Ma and Richard G. Weiss*
Department of Chemistry, Georgetown University, Washington, D.C. 20057-1227
J. Phys. Chem. B, 2009, 113 (13), pp 4209–4217

Abstract: The ordering parameters of selected solutes from NMR spectroscopic measurements have been assessed in the thermotropic and amphotropic smectic liquid-crystalline phases of a wide structural range of phosphonium salts with three equivalent long n-alkyl chains, one shorter chain, and various anions. The nature of the added liquids that convert the salts to amphotropic phasesalcohols and other small organic moleculesand their concentrations have been determine. These factors are correlated with the NMR-derived parameters in order to understand how the phases can be optimized to maximize information about the solutes. The various salts cover a range of liquid crystallinity from −40 to 100 °C. The phosphonium salts are easily aligned in the strong magnetic fields of the spectrometers. In several of the systems, a coexistence of isotropic and anisotropic phases is observed over a wide range of temperatures. The order parameters of the amphotropic liquid-crystalline phases vary from high to very low values, and some of the systems provide good spectral resolution for the solute molecules. Also, structural and orientational parameters of a model molecule, 13C-enriched acetonitrile, have been calculated in various systems in order to evaluate more precisely the applicability of the host systems for determining solute structures by NMR. The results, in toto, indicate that several of the phosphonium salts are very promising as hosts to determine solute structures.

51V NMR Chemical Shifts Calculated from QM/MM Models of Peroxo Forms of Vanadium Haloperoxidases

K. R. Geethalakshmi, Mark P. Waller, Walter Thiel and Michael Bhl*
Max-Planck-Institut fr Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mlheim an der Ruhr, Germany, and School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, U.K.
J. Phys. Chem. B, 2009, 113 (13), pp 4456–4465

Abstract: QM/MM models of the peroxo forms of vanadium-containing haloperoxidases (VHPOs) are critically assessed in terms of active site geometries, hydrogen bonds within the active site, isotropic and anisotropic 51V NMR chemical shifts, and TD-DFT excitation energies. The geometric stability within the active site of the protein is comparable to the respective native forms, as indicated by low standard deviations in bond lengths across a number of local minima sampled along MD trajectories. There is a significant calculated upfield shift in δ(51V) upon formation of the peroxo from the respective native forms for both the vanadium-containing chloroperoxidase (VCPO) and vanadium-containing bromoperoxidase (VBPO) models, which is in qualitative agreement with 51V NMR experiments of VBPO in solution. The models show appreciable differences between the anisotropic chemical shifts of the different protonation states of the peroxo form of VHPO. The most likely candidates for the peroxo forms of the VHPO enzymes appear to be unprotonated or have a single proton on either of the equatorial oxygen ligands, based on QM/MM modeling in combination with X-ray, 51V NMR, and UV−vis data.

Calcium-Induced Formation of Subdomains in Phosphatidylethanolamine−Phosphatidylglycerol Bilayers: A Combined DSC, 31P NMR, and AFM Study

Laura Picas, M. Teresa Montero, Antoni Morros§, Miquel E. Cabaas, Bastien Seantier#, Pierre-Emmanuel Milhiet# and Jordi Hernndez-Borrell*
J. Phys. Chem. B, 2009, 113 (14), pp 4648–4655

Abstract: We study the effect of Ca2+ on the lateral segregation of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) (3:1, mol/mol). Supported lipid bilayers (SLBs) were observed by atomic force microscopy (AFM). Since SLBs are formed from liposomes of POPE:POPG, we examined the effect of calcium on these suspensions by differential scanning calorimetry (DSC) and 31P nuclear magnetic resonance spectroscopy (31P NMR). AFM images revealed the existence of two separated phases, the higher showing a region with protruding subdomains. Force spectroscopy (FS) was applied to clarify the nature of each phase. The values of breakthrough force (Fy), adhesion force (Fadh), and height extracted from the force curves were assigned to the corresponding gel (Lβ) and fluid (Lα) phase. The endotherms obtained by DSC suggest that, in the presence of Ca2+, phase separation already exists in the suspensions of POPE:POPG used to form SLBs. Due to the temperature changes applied during preparation of SLBs a 31P NMR study was performed to assess the lamellar nature of the samples before spreading them onto mica. With in situ AFM experiments we showed that the binding of Ca2+ to POPG-enriched domains only induces the formation of subdomains in the Lβ phase.

Alumina-Promoted Sulfated Mesoporous Zirconia Catalysts

Chi-Chau Hwang and Chung-Yuan Mou*
J. Phys. Chem. C, 2009, 113 (13), pp 5212–5221
Abstract: Mesoporous zirconia, hydrothermally synthesized from surfactant templating, was directly impregnated with aluminum sulfate to give the acidic Al-promoted sulfated mesoporous zirconia (AS/MP-ZrO2). A series of AS/MP-ZrO2 catalysts were characterized by Brunauer−Emmett−Teller and X-ray diffraction for their texture properties and crystalline phases. The catalytic behavior for n-butane isomerization was found to be strongly promoted at relatively low temperature by the addition of a proper amount of alumina as a promoter. 27Al S.S. magic-angle spinning nuclear magnetic resonance results indicated that Zr atoms were partially substituted by Al, giving a considerable increased concentration of Brønsted acids. X-ray photoelectron spectroscopy and diffuse-reflectance infrared Fourier-transformed spectra (DRIFT) analysis were then employed to identify and relatively quantify properties of acid sites on catalyst surface. A balanced distribution of acid sites strength was proven to prevent a catalyst from deactivating rapidly due to coke formation on the catalyst surface. A small concentration of olefins formed by oxidation of n-butane and proven to be key intermediates during n-butane isomerization on sulfated zirconia was found by the Baeyer test. Electron paramagnetic resonance and in situ DRIFT results show that this occurs via oxidative dehydrogenation of butane by the sulfate groups to form butene which leads to butyl carbenium species for skeleton isomerization. A modified biomolecular mechanism for the isomerization of butane is examined to explain the catalysis results.

No comments: