Monday, July 27, 2009

Journal of Physical Chemistry B, Vol. 113, Issues 25 to 30

Glassy Dynamics in Nanoconfinement as Revealed by 31P NMR
S. Gradmann, P. Medick and E. A. Rssler
Experimentalphysik II, Universitt Bayreuth, 95440 Bayreuth, Germany
J. Phys. Chem. B, 2009, 113 (25), pp 8443–8445

Abstract: We investigated the glass former m-tricresyl-phosphate confined in different nanoporous silica matrices with defined pore radii from 2−150 nm. While applying different 31P NMR techniques, we were able to detect the extremely stretched correlation functions extending over 7−8 decades in time and reflecting strong dynamic heterogeneities. The experimental results were explained by a topological model for which the broad distribution of correlation times G(ln τ) becomes inhomogeneous in space; that is, the “local” dynamics given by a correlation time τ(r) depend on the distance from the pore center. As τ(r) changes with temperature, we were able to reintroduce the idea of a dynamic correlation length.

Hierarchical Dynamics of As2P2S8 Quasi-Molecular Units in a Supercooled Liquid in the As−P−S System: A 31P NMR Spectroscopic Study

E. L. Gjersing and S. Sen*
J. Phys. Chem. B, 2009, 113 (25), pp 8514–8519
DOI: 10.1021/jp901388j
Abstract: The dynamics of As2P2S8 quasi-molecular units caged in an As−S network in the supercooled chalcogenide liquid of composition (As2S3)90(P2S5)10 have been studied near the glass transition region (Tg = 468 ≤ T ≤ 628 K) using 31P NMR line shape analysis and spin−lattice relaxation techniques. 31P NMR line shape analysis indicates the presence of isotropic rotational reorientation of As2P2S8 quasi-molecular units at frequencies on the order of tens of kilohertz at T <>

Proton Assisted Recoupling at High Spinning Frequencies
Jzef R. Lewandowski
§, Gal De Pape, Matthew T. Eddy, Jochem Struppe, Werner Maas and Robert G. Griffin*
J. Phys. Chem. B, 2009, 113 (27), pp 9062–9069
DOI: 10.1021/jp810280t

2008 marked the Centennial of the American Chemical Society’s Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem. Senior Editors.,

Abstract: We demonstrate the successful application of 13C−13C proton assisted recoupling (PAR) on [U−13C,15N] N-f-MLF-OH and [U−13C,15N] protein GB1 at high magic angle spinning (MAS) frequencies (ωr/2π = 65 kHz). Specifically, by combining PAR mixing with low power heteronuclear decoupling (ω1H/2π 16 kHz) and high spinning frequencies, we obtain high resolution 2D spectra displaying long-range 13C−13C contacts from which distance estimates can be extracted. These experiments therefore demonstrate the possibility of performing high resolution structural studies in the limit of high spinning frequency and low power 1H decoupling, a regime which optimizes the resolution of protein samples and preserves their integrity.

Self-Diffusion and Mutual Diffusion of Small Molecules in High-Set Curdlan Hydrogels Studied by 31P NMR
Marc-Andr Gagnon and Michel Lafleur
J. Phys. Chem. B, 2009, 113 (27), pp 9084–9091
DOI: 10.1021/jp811105p

Abstract: Self-diffusion and mutual diffusion are two different transport mechanisms experimentally characterized on different length and time scales. NMR spectroscopy is a highly suitable technique to characterize these two phenomena as both mechanisms can be studied on the same system and in the same experimental conditions. Pulsed field gradient (PFG) NMR was used to measure the self-diffusion whereas 31P NMR profiling provided an approach to determine the mutual diffusion coefficients. We have characterized the diffusion of phosphate, trimetaphosphate, alendronate, and d-glucose-6-phosphate in hydrogels prepared with 10% (w/v) curdlan, a bacterial polysaccharide built of linear (1→3)-β-d-glucose repeating units. These solutes are small compared to the average pore size of the hydrogel, as inferred from environmental scanning electron microscopy (eSEM). Our results show that the self- and mutual-diffusion coefficients of small molecules in curdlan hydrogels are similar and are reduced by 30% compared to those measured in aqueous solutions. These observations are validated for the complete series of investigated analytes. It is therefore concluded that, for this system, the analyte diffusion in the gel is essentially reduced because of interactions at the molecular level and that the open structure of this gel has a very limited influence at the mesoscopic length scale. A literature survey indicates that these conditions prevail for the large majority of the systems that have been investigated up to now.

Validating a Strategy for Molecular Dynamics Simulations of Cyclodextrin Inclusion Complexes through Single-Crystal X-ray and NMR Experimental Data: A Case Study
Giuseppina Raffaini
*, Fabio Ganazzoli, Luciana Malpezzi, Claudio Fuganti, Giovanni Fronza, Walter Panzeri and Andrea Mele*
J. Phys. Chem. B, 2009, 113 (27), pp 9110–9122
DOI: 10.1021/jp901581e

Abstract: A theoretical and experimental study about the formation and structure of the inclusion complex (−)-menthyl-O-β-D-glucopyranoside 1 with β-cyclodextrin (β-CD) 2 is presented as paradigmatic case study to test the results of molecular dynamics (MD) simulations. The customary methodological approach—the use of experimental geometrical parameters as restraints for MD runs—is logically reversed and the calculated structures are a posteriori compared with those obtained from NMR spectroscopy in D2O solution and single crystal X-ray diffraction so as to validate the simulation procedure. The guest molecule 1 allows for a broad repertoire of intermolecular interactions (dipolar, hydrophobic, hydrogen bonds) concurring to stabilize the host−guest complex, thus providing the general applicability of the simulation procedure to cyclodextrin physical chemistry. Many starting geometries of the host−guest association were chosen, not assuming any a priori inclusion. The simulation protocol, involving energy minimization and MD runs in explicit water, yielded four possible inclusion geometries, ruling out higher-energy outer adducts. By analysis of the average energy at room temperature, the most stable geometry in solution was eventually obtained, while the kinetics of formation showed that it is also kinetically favored. The reliability of such geometry was thoroughly checked against the NOE distances via the pair distribution functions, that is, the statistical distribution of intermolecular distances among selected diagnostic atoms calculated from the MD trajectories at room temperature. An analogous procedure was adopted both with implicit solvent and in vacuo. The most stable geometry matched that found with explicit solvent but major differences were observed in the relative stability of the metastable complexes as a consequence of the lack of hydration on the polar moiety of the guest. Finally, a control set of geometrical parameters of the thermodynamically favored complex matched the corresponding one obtained from the X-ray structure, while local conformational differences were indicative of packing effects.

1H Solid-State NMR Investigation of Structure and Dynamics of Anhydrous Proton Conducting Triazole-Functionalized Siloxane Polymers
mit Akbey
, Sergio Granados-Focil, E. Bryan Coughlin§, Robert Graf and Hans Wolfgang Spiess*
J. Phys. Chem. B, 2009, 113 (27), pp 9151–9160
DOI: 10.1021/jp9030909

Abstract:1H MAS solid-state NMR methods are applied to elucidate the conduction mechanism of an anhydrous proton conducting triazole-functionalized polysiloxane. At temperatures below T = 260 K, hydrogen bonding between neighboring heterocycles is observed and a dimer formation can be excluded. From the temperature dependence of 1H MAS NMR spectra, different dynamic processes of the triazole ring contributing to the proton conduction process are qualitatively and quantitatively analyzed and detailed insight into the conduction mechanism and temperature-dependent structural changes is obtained. Although the dynamics processes on the molecular level are qualitatively in good agreement with the findings from macroscopic conductivity measurements, temperature-dependent factors on mesoscopic scales beyond the local molecular mobility influence the macroscopic conductivity and hamper quantitative interpretation.

Intra- and Intermolecular Effects on 1H Chemical Shifts in a Silk Model Peptide Determined by High-Field Solid State 1H NMR and Empirical Calculations
Yu Suzuki
, Rui Takahashi, Tadashi Shimizu, Masataka Tansho, Kazuo Yamauchi, Mike P. Williamson§ and Tetsuo Asakura*
J. Phys. Chem. B, 2009, 113 (29), pp 9756–9761
DOI: 10.1021/jp903020p

Abstract: A combination of solid state 1H NMR chemical shift measurements and empirical chemical shift calculations has been used to interpret 1H solid state chemical shifts of a model peptide (Ala-Gly)15 for the crystalline domain of Bombyx mori silk fibroin in silk I and silk II structures, including a treatment of both intra- and intermolecular arrangements. Silk I and silk II are the structures of silk fibroin before and after spinning, respectively. Two peaks with equal intensity were observed for the amide protons of (AG)15 in silk I, whereas only one broad peak was observed for silk II, reflecting a difference of 1.1 ppm in Ala HN shift between silk I and silk II, but a difference of only 0.2 ppm in Gly HN shift. Chemical shift calculations predicted chemical shifts that are in good agreement with the experimental observations and showed that the origin of these chemical shift differences was predominantly the magnetic anisotropy effect from the C═O bond that hydrogen bonds with HN, which has a more favorable geometry for Ala HN in silk II than for the other HN. This result shows that we could distinguish between proton chemical shift effects arising from intermolecular interactions and those from intramolecular interactions by combining observation of the solid state 1H NMR chemical shift and empirical chemical shift calculation.

1H Photo-CIDNP Enhancements in Heteronuclear Correlation NMR Spectroscopy
Ashok Sekhar and Silvia Cavagnero
J. Phys. Chem. B, 2009, 113 (30), p 10548
DOI: 10.1021/jp905605u
Publication Date (Web): July 9, 2009
Copyright © 2009 American Chemical Society

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