Friday, March 13, 2009

J Phys Chem B and C, vol 113, issues 10-11

Nothing in the two issues of J Phys Chem C this time around

Alamethicin Topology in Phospholipid Membranes by Oriented Solid-state NMR and EPR Spectroscopies: a Comparison

Evgeniy S. Salnikov, Marta De Zotti§, Fernando Formaggio§, Xing Li, Claudio Toniolo§, Joe D. J. ONeil, Jan Raap, Sergei A. Dzuba* and Burkhard Bechinger*

J. Phys. Chem. B, 2009, 113 (10), pp 3034–3042
Abstract: Alamethicin, a hydrophobic peptide that is considered a paradigm for membrane channel formation, was uniformly labeled with 15N, reconstituted into oriented phosphatidylcholine bilayers at concentrations of 1 or 5 mol %, and investigated by solid-state NMR spectroscopy as a function of temperature. Whereas the peptide adopts a transmembrane alignment in POPC bilayers at all temperatures investigated, it switches from a transmembrane to an in-plane orientation in DPPC membranes when passing the phase transition temperature. This behavior can be explained by an increase in membrane hydrophobic thickness and the resulting hydrophobic mismatch condition. Having established the membrane topology of alamethicin at temperatures above and below the phase transition, ESEEM EPR was used to investigate the water accessibility of alamethicin synthetic analogues carrying the electron spin label TOAC residue at one of positions 1, 8, or 16. Whereas in the transmembrane alignment the labels at positions 8 and 16 are screened from the water phase, this is only the case for the latter position when adopting an orientation parallel to the surface. By comparing the EPR and solid-state NMR data of membrane-associated alamethicin it becomes obvious that the TOAC spin labels and the cryo-temperatures required for EPR spectroscopy have less of an effect on the alamethicin−POPC interactions when compared to DPPC. Finally, at P/L ratios of 1/100, spectral line broadening due to spin−spin interactions and thereby peptide oligomerization within the membrane were detected for transmembrane alamethicin.


Impact of Reduction on the Properties of Metal Bisdithiolenes: Multinuclear Solid-State NMR and Structural Studies on Pt(tfd)2 and Its Reduced Forms

Joel A. Tang, Elzbieta Kogut, Danielle Norton, Alan J. Lough§, Bruce R. McGarvey, Ulrich Fekl* and Robert W. Schurko*
J. Phys. Chem. B, 2009, 113 (11), pp 3298–3313

Abstract: Transition-metal dithiolene complexes have interesting structures and fascinating redox properties, making them promising candidates for a number of applications, including superconductors, photonic devices, chemical sensors, and catalysts. However, not enough is known about the molecular electronic origins of these properties. Multinuclear solid-state NMR spectroscopy and first-principles calculations are used to examine the molecular and electronic structures of the redox series [Pt(tfd)2]z– (tfd = S2C2(CF3)2; z = 0, 1, 2; the anionic species have [NEt4]+ countercations). Single-crystal X-ray structures for the neutral (z = 0) and the fully reduced forms (z = 2) were obtained. The two species have very similar structures but differ slightly in their intraligand bond lengths. 19F−195Pt CP/CPMG and 195Pt magic-angle spinning (MAS) NMR experiments are used to probe the diamagnetic (z = 0, 2) species, revealing large platinum chemical shielding anisotropies (CSA) with distinct CS tensor properties, despite the very similar structural features of these species. Density functional theory (DFT) calculations are used to rationalize the large platinum CSAs and CS tensor orientations of the diamagnetic species using molecular orbital (MO) analysis, and are used to explain their distinct molecular electronic structures in the context of the NMR data. The paramagnetic species (z = 1) is examined using both EPR spectroscopy and 13C and 19F MAS NMR spectroscopy. Platinum g-tensor components were determined by using solid-state EPR experiments. The unpaired electron spin densities at 13C and 19F nuclei were measured by employing variable-temperature 13C and 19F NMR experiments. DFT and ab initio calculations are able to qualitatively reproduce the experimentally measured g-tensor components and spin densities. The combination of experimental and theoretical data confirm localization of unpaired electron density in the π-system of the dithiolene rings.

Location of a Metallic Cation Complexed in a Calixarene Cavity As Determined by Calixarene 13C Spin Relaxation. Application to Cesium and Thallium Complexed by p-Sulfonatocalix[4]arene in Water

Diana Cuc, Sabine Bouguet-Bonnet, Nicole Morel-Desrosiers, Jean-Pierre Morel, Pierre Mutzenhardt and Daniel Canet*

J. Phys. Chem. B, 2009, 113 (11), pp 3499–3503
Abstract: This study deals with the exact location of the monovalent metal cations Cs+ and Tl+ which are complexed by the p-sulfonatocalix[4]arene in water. This determination rests on the measurements of longitudinal relaxation times of carbon-13 not directly bonded to protons. The difference between the relaxation times of the free calixarene and of the complex definitely demonstrates that the monovalent metal cation is well inside the calixarene cavity. These features are in fact enhanced by the presence of paramagnetic species which act in a different way in the complexed form. Experimental results also show without any ambiguity that the calixarene cavity is essentially hydrophobic. Finally, it is observed that thallium is more mobile than cesium within the calixarene cavity.

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