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NMR Shows (nmr + shows)

Distribution by Scientific Domains

Chemistry	83%

Selected Abstracts

Synergistic Ordering of Side-Group Liquid Crystal Polymer and Small Molecule Liquid Crystal: Order and Phase Behavior of Nematic Polymer Solutions,

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 19-20 2007
Neal R. Scruggs
Abstract Addition of a small-molecule liquid crystal (5CB) to a cyanobiphenyl-based side-group liquid crystal polymer (SGLCP) stabilizes nematic order, increasing the isotropization temperature (TNI) more than 15,°C. Despite synergistic ordering at high concentration, small amounts of polymer destabilize nematic order. Even though TNI(SGLCP) is 27,°C greater than TNI(5CB), 2H NMR shows that the order parameter of the SGLCP is less than that of 5CB at concentrations for which monodomains were accessible (,10 wt.-%). The results imply that nematic order is frustrated in the bulk polymer and addition of small molecule LC relaxes this frustration by allowing greater configurational freedom. Conversely, adding small amounts of polymer to the bulk 5CB introduces frustration, resulting in the strong asymmetry of the phase diagram. [source]

Cooperative interaction of n -butylammonium ion with 1,3-alternate tetrapropoxycalix [4]arene: NMR and theoretical study

MAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2008
Jaroslav K
Abstract The interaction of 1,3-alternate tetrapropoxycalix[4]arene (1) with n -butylammonium ion (2) in CD2Cl2 was examined using 1H, 13C and 14N NMR spectroscopy and DFT (density functional theory) calculations. NMR shows that 1 forms with 2 an equimolecular hydrogen-bonded complex with the equilibrium constant 5.91 × 103 l/mol at 296 K. The structure of the complex can be shown to be asymmetric at 203 K, with 2 interacting by hydrogen bonds with the two ethereal oxygen atoms of one half of 1 and with the , system of the other half, but is rapidly averaged to an apparent C4h symmetry by chemical exchange at higher temperatures. Using two related but independent techniques based on transverse and rotating-frame proton relaxation, it is shown that only an intermolecular exchange of 2 between the bound and free states takes place, in contrast to previously studied interaction of 1 with H3O+. Its correlation time is 0.169 ms. It is shown by DFT calculations that such swift exchange is not possible without a cooperative interaction of both 2 and 1 with several molecules of water present. Similarities and contrasts between the exchange processes of 2 and H3O+ bound to 1 are discussed, in particular with respect to the apparent quantum tunneling of the latter inside the molecule of the complex. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Determination of the human type I interferon receptor binding site on human interferon-,2 by cross saturation and an NMR-based model of the complex

PROTEIN SCIENCE, Issue 11 2006
Sabine R. Quadt-Akabayov
Abstract Type I interferons (IFNs) are a family of homologous helical cytokines that exhibit pleiotropic effects on a wide variety of cell types, including antiviral activity and antibacterial, antiprozoal, immunomodulatory, and cell growth regulatory functions. Consequently, IFNs are the human proteins most widely used in the treatment of several kinds of cancer, hepatitis C, and multiple sclerosis. All type I IFNs bind to a cell surface receptor consisting of two subunits, IFNAR1 and IFNAR2, associating upon binding of interferon. The structure of the extracellular domain of IFNAR2 (R2-EC) was solved recently. Here we study the complex and the binding interface of IFN,2 with R2-EC using multidimensional NMR techniques. NMR shows that IFN,2 does not undergo significant structural changes upon binding to its receptor, suggesting a lock-and-key mechanism for binding. Cross saturation experiments were used to determine the receptor binding site upon IFN,2. The NMR data and previously published mutagenesis data were used to derive a docking model of the complex with an RMSD of 1 Å, and its well-defined orientation between IFN,2 and R2-EC and the structural quality greatly improve upon previously suggested models. The relative ligand,receptor orientation is believed to be important for interferon signaling and possibly one of the parameters that distinguish the different IFN I subtypes. This structural information provides important insight into interferon signaling processes and may allow improvement in the development of therapeutically used IFNs and IFN-like molecules. [source]

Removal of the N-terminal hexapeptide from human ,2-microglobulin facilitates protein aggregation and fibril formation

PROTEIN SCIENCE, Issue 5 2000
G. Esposito
Abstract The solution structure and stability of N-terminally truncated ,2-microglobulin (,N6,2-m), the major modification in ex vivo fibrils, have been investigated by a variety of biophysical techniques. The results show that ,N6,2-m has a free energy of stabilization that is reduced by 2.5 kcal/mol compared to the intact protein. Hydrogen exchange of a mixture of the truncated and full-length proteins at ,M concentrations at pH 6.5 monitored by electrospray mass spectrometry reveals that ,N6,2-m is significantly less protected than its wild-type counterpart. Analysis of ,N6,2-m by NMR shows that this loss of protection occurs in , strands I, III, and part of II. At mM concentration gel filtration analysis shows that ,N6,2-m forms a series of oligomers, including trimers and tetramers, and NMR analysis indicates that strand V is involved in intermolecular interactions that stabilize this association. The truncated species of ,2-microglobulin was found to have a higher tendency to self-associate than the intact molecule, and unlike wild-type protein, is able to form amyloid fibrils at physiological pH. Limited proteolysis experiments and analysis by mass spectrometry support the conformational modifications identified by NMR and suggest that ,N6,2-m could be a key intermediate of a proteolytic pathway of ,2-microglobulin. Overall, the data suggest that removal of the six residues from the N-terminus of ,2-microglobulin has a major effect on the stability of the overall fold. Part of the tertiary structure is preserved substantially by the disulfide bridge between Cys25 and Cys80, but the pairing between ,-strands far removed from this constrain is greatly perturbed. [source]

Crystal structures and cation ordering in Cs2MgSi5O12, Rb2MgSi5O12 and Cs2ZnSi5O12 leucites

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 4 2009
A. M. T. Bell
The crystal structures of the leucite analogues Cs2MgSi5O12, Cs2ZnSi5O12 and Rb2MgSi5O12 have been determined by synchrotron X-ray powder diffraction using Rietveld refinement in conjunction with 29Si MAS NMR spectroscopy. These leucites are framework structures with distinct tetrahedral sites (T sites) occupied by Si and a divalent cation (either Mg or Zn in these samples); there is also a monovalent extra-framework cation (either Cs or Rb in these samples). The refined crystal structures were based on the Pbca leucite structure of Cs2CdSi5O12, thus a framework with five ordered Si T sites and one ordered Cd T site was used as the starting model for refinement. 29Si MAS NMR shows five distinct Si T sites for Cs2MgSi5O12 and Rb2MgSi5O12, but six Si T sites for Cs2ZnSi5O12. The refined structures for Cs2MgSi5O12 and Rb2MgSi5O12 were determined with complete T -site ordering, but the refined structure for Cs2ZnSi5O12 was determined with partial disorder of Mg and Si over two of the T sites. [source]

Alkyl exchange reaction between dialkylzinc compounds and methylaluminoxane and the effect on propylene polymerization

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 9 2010
Chuanhui Zhang
Abstract Alkyl exchange reaction between dialkylzinc compounds and vacuum-dried methylaluminoxane (MAO) was investigated. 1H NMR shows a clear and direct proof of alkyl exchange reaction between ZnEt2 and trimethylaluminum associated with MAO. Detailed analysis of polymers produced in the presence of dialkylzinc compounds gave other indirect but equally strong evidence of alkyl exchange. Therefore, care must be taken to investigate dialkylzinc-based chain transfer reaction in combination with a precatalyst and MAO. Copyright © 2010 John Wiley & Sons, Ltd. [source]