Thursday, February 19, 2009

J Phys Chem B, Vol. 113, Issue 7

NMR Measure of Translational Diffusion and Fractal Dimension. Application to Molecular Mass Measurement
Sophie Aug
, Pierre-Olivier Schmit, Christopher A. Crutchfield, Mohammad T. Islam, Douglas J. Harris, Emmanuelle Durand§, Martin Clemancey, Anne-Agathe Quoineaud§, Jean-Marc Lancelin, Yann Prigent, Francis Taulelle and Marc-Andr Delsuc*#

J. Phys. Chem. B, 2009, 113 (7), pp 1914–1918
Abstract: Experimental NMR diffusion measure on polymers and on globular proteins are presented. These results, complemented with results found in the literature, enable a general description of effective fractal dimension for objects such as small organic molecules, sugars, polymers, DNA, and proteins. Results are compared to computational simulations as well as to theoretical values. A global picture of the diffusion phenomenon emerges from this description. A power law relating molecular mass with diffusion coefficients is described and found to be valid over 4 orders of magnitude. From this law, the fractal dimension of the molecular family can be measured, with experimental values ranging from 1.41 to 2.56 in full agreement with theoretical approaches. Finally, a method for evaluating the molecular mass of unknown solutes is described and implemented as a Web page.

A Comparative 13C NMR Study of Local Ordering in a Homologous Series of Bent-core Liquid Crystals
Ronald Y. Dong

J. Phys. Chem. B, 2009, 113 (7), pp 1933–1939
Abstract:A 13C NMR study is carried out in a number of banana-shaped molecules belonging to a homologous series. The derivatives contain chlorine or bromine substituent(s) in the center ring of 1,3-phenylene bis[4′-alkenyloxy biphenyl]-4-carboxylate (nPBBC), and different terminal chain lengths are systematically compared in terms of their local order parameters and their ability to form an aligned nematic phase in the NMR magnet. The chemical shift anisotropy tensors measured from nPBBC by fitting the 13C powder patterns are now extensively used on these molecules to interpret their observed 13C chemical shifts in the nematic phase. The bend angles are estimated as a function of temperature in two members of the nPBBC series. Conformation twists in the bent-core region inferred from the local molecular biaxial paramters are discussed.

Revisiting Magnesium Chelation by Teichoic Acid with Phosphorus Solid-State NMR and Theoretical Calculations
Jason R. Wickham, Jeffrey L. Halye, Stepan Kashtanov, Jana Khandogin and Charles V. Rice

J. Phys. Chem. B, 2009, 113 (7), pp 2177–2183
Abstract:Teichoic acids are essential components of the Gram-positive bacterial cell wall. One of their many functions is metal binding, a vital process for bacterial growth. With the combination of phosphorus-31 solid-state NMR spectroscopy and theoretical calculations using density functional theory (DFT), we have determined that the binding mode between teichoic acids and magnesium involves bidentate coordination by the phosphate groups of teichoic acid. Measurement of chemical shift anisotropy tensors gave a reduced anisotropy (δ) of 49.25 ppm and an asymmetry (η) of 0.7. DFT calculations with diglycerol phosphate and triglycerol diphosphate model compounds were completed with Mg2+ in anhydrous as well as partially hydrated bidentate and fully hydrated monodentate, bidentate, and bridging binding modes. 31P CSA tensors were calculated from the energy-minimized model compounds using the combined DFT and GIAO methods, resulting in dramatically different tensor values for each binding mode. The anhydrous bidentate chelation mode was found to be a good approximation of the experimental data, an observation that alters the current monodentate paradigm for metal chelation by teichoic acids.

Thermal Denaturation of Hydrated Wool Keratin by 1H Solid-State NMR
Maria Baias
*, Dan E. Demco*§, Crisan Popescu*, Radu Fechete§, Claudiu Melian, Bernhard Blmich and Martin Mller

J. Phys. Chem. B, 2009, 113 (7), pp 2184–2192
Abstract:Thermal denaturation of hydrated keratin in wool was investigated by NMR using 1H wide-line spectra to obtain the phase composition and 1H spin-diffusion experiments using a double-quantum filter to obtain the domain sizes for the wool fibers. The denaturation process detected by DSC takes place for wool fibers in deuterated water in the temperature range 140−144 °C. The phase composition measured by 1H wide line NMR spectra reveals a rigid, semirigid and an amorphous phase for temperatures in the range 25−160 °C. A dramatic change in the phase composition was detected around 142 °C, corresponding to the denaturation temperature. The morphological domain sizes measured by 1H spin-diffusion NMR experiments were obtain from the solutions of the spin-diffusion equations for two-dimensional rectangular and cylindrical morphologies. The keratin mobility gradient in the interfacial region at different denaturation temperatures was measured from the 1H spin-diffusion data. A qualitative model describing the denaturation process of hydrated keratin protein was developed that explains the changes in domain thickness, spin diffusivities, phase composition, and thermodynamic parameters.

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