Tuesday, April 29, 2008

Joel's Journal Updates

Effects of Guanidinium-Phosphate Hydrogen Bonding on the Membrane-Bound Structure and Activity of an Arginine-Rich Membrane Peptide from Solid-State NMR Spectroscopy
Ming Tang, Alan J. Waring, Robert I. Lehrer, Mei Hong
Angwandte (2008)47, 3202.
Barreling through: Guanidinium-phosphate hydrogen bonding significantly affects the structure and activity of the antimicrobial peptide PG-1. Solid-state NMR data show that a mutant of PG-1, having dimethylated Arg residues, adopts an in-plane orientation, interfacial location, and fast uniaxial motion around the membrane normal (see scheme). The less active mutant thus disrupts the membrane by in-plane diffusion, in contrast to the more active wild-type PG-1, which forms immobile transmembrane b-barrels to cause toroidal-pore membrane defects.

Application of Ultrahigh-Field 59Co Solid-State NMR Spectroscopy in the Investigation of the 1,2-Polybutadiene Catalyst [Co(C8H13)(C4H6)]
Patrick Crewdson, David L. Bryce, Frank Rominger, Peter Hofmann
Angwandte (2008)47, 3454.
Highly suitable: The 1,2-polybutadiene catalyst [Co(3:2-C8H13)(4-C4H6)] (1) was isolated and structurally characterized by ultrahigh-field 59Co solid-state NMR spectroscopy, demonstrating the utility of this technique. It can be applied to study the formation mechanism of syndiotactic 1,2-polybutadiene.

Determination of NMR Lineshape Anisotropy of Guest Molecules within Inclusion Complexes from Molecular Dynamics Simulations
Saman Alavi, Peter Dornan, Tom K. Woo
Nonspherical cages in inclusion compounds can result in non-uniform motion of guest species in these cages and anisotropic lineshapes in NMR spectra of the guest. Herein, we develop a methodology to calculate lineshape anisotropy of guest species in cages based on molecular dynamics simulations of the inclusion compound. The methodology is valid for guest atoms with spin 1/2 nuclei and does not depend on the temperature and type of inclusion compound or guest species studied. As an example, the nonspherical shape of the structure I (sI) clathrate hydrate large cages leads to preferential alignment of linear CO2 molecules in directions parallel to the two hexagonal faces of the cages. The angular distribution of the CO2 guests in terms of a polar angle and azimuth angle and small amplitude vibrational motions in the large cage are characterized by molecular dynamics simulations at different temperatures in the stability range of the CO2 sI clathrate. The experimental 13C NMR lineshapes of CO2 guests in the large cages show a reversal of the skew between the low temperature (77 K) and the high temperature (238 K) limits of the stability of the clathrate. We determine the angular distributions of the guests in the cages by classical MD simulations of the sI clathrate and calculate the 13C NMR lineshapes over a range of temperatures. Good agreement between experimental lineshapes and calculated lineshapes is obtained. No assumptions regarding the nature of the guest motions in the cages are required.

Proton Spin Diffusion in Polyethylene as a Function of Magic-Angle Spinning Rate. A Phenomenological Approach.
O. Dmitrenko, Shi Bai, Peter A. Beckmann, Scott van Bramer, Alexander J. Vega, and C. Dybowski
The analysis of heavy-metal solids with NMR spectroscopy provides a means of investigating the electronic environment through the dependence of the chemical shift on structure. We have investigated the relation of the 207Pb NMR isotropic chemical shift, span, and skew of a series of solid Pb(II) compounds to lattice parameters. Complementary relativistic spin-orbit density functional calculations on clusters such as PbI64- that model the local environment in the dihalides show a dependence of NMR properties on the local structure in good agreement with experimental results.

Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives
Juan Miguel Lopez del Amo, Gerd Buntkowsky, Hans-Heinrich Limbach,* Irene Resa, Rafael Fernndez, and Ernesto Carmona
Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(5-C5Me5)Zn(1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(5-C5Me5)Zn(1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMAS NMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as the presence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arising from a fast combined Zn tautomerism and 1/5 reorganization of the Cp* rings between two degenerate states A and B related by a molecular inversion. In the ordered phase, the degeneracy of A and B is lifted; that is, the two rings X and Y are inequivalent, where X exhibits a larger fraction of time in the 5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of H = 2.4 kJ mol-1 and a reaction entropy of S = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100 K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fast CD3 group rotations and a fast "5 rotation", corresponding to 72 rotational jumps of the 5 coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the 5 rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 group rotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchange regime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18 kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies 6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.

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