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Abstract: Anionic surfactant-templated mesoporous silicas (AMS) are synthesized with a co-structure directing agent (CSDA) that interacts with both the organic and inorganic components of the system. The AMS materials structure is controlled by pH. We investigated the formation of AMS cubic and hexagonal phases, prepared under the same conditions, except pH, by EPR spectroscopic measurements. We used silica-like and surfactant-like spin probes added to the reaction mixtures in minute amounts. Through the spin probes we resolved the specific interactions of the CSDA (N-trimethoxylsilylpropyl-N,N,N-trimethyl ammonium chloride (TMAPS)) with the surfactant (sodium myristate (C14AS)) and the polymerizing silica. We observed for both phases a fast formation of a mesophase upon addition of the silica precursor (TEOS, tetraethoxysilane) and the CSDA into the surfactant solution, attributed to the strong attraction between the CSDA and the anionic surfactant. This is then followed by a slow condensation of the silica. Electron spin echo envelope modulation (ESEEM) spectra of both spin probes in the as-synthesized materials indicated the presence of two types of CSDA molecules; one interacting with the surfactant and the other with the silica wall. Continuous wave EPR spectra showed different spin probe motilities in the two as-synthesized materials that indicated that the relative populations of the two CSDA types are different in the two phases. We attribute this difference to the pH differences in the reaction mixtures. A soft extraction of the surfactant from the pores did not alter the structure of the final materials, but it abolished the observed molecular level differences between them. The extraction allowed the pending ammonium groups to acquire a high degree of freedom and accessibility to water molecules. [Copyright &y& Elsevier]

Calmodulin (CaM) is a calcium-binding protein that regulates the function of many proteins by indirectly conferring Ca2+ sensitivity, and it undergoes a large conformational change on partners' binding. We compared the solution binding mode of the target peptides MARCKS and IQ by double electron-electron resonance (DEER) distance measurements and paramagnetic NMR. We combined nitroxide and Gd(III) spin labels, including specific substitution of one of the Ca2+ ions in the CaM mutant N60D by a Gd(III) ion. The binding of MARCKS to holo -CaM resulted neither in a closed conformation nor in a unique relative orientation between the two CaM domains, in contrast with the crystal structure. Binding of IQ to holo -CaM did generate a closed conformation. Using elastic network modeling and 12 distance restraints obtained from multiple holo -CaM/IQ DEER data, we derived a model of the solution structure, which is in reasonable agreement with the crystal structure. [Display omitted] • The target peptides MARCKS and IQ bind CaM both in its apo - and holo -states • In solution, the binding of MARCKS to holo -CAM does not generate a closed conformation • In solution, holo -CaM with bound IQ has a closed conformation, albeit heterogeneous • Gd(III)/nitroxide DEER distance restraints give a solution holo -CaM/IQ structural model Jash et al. track the conformational changes calmodulin undergoes on binding MARCKS and IQ peptides in solution and report significant differences in their binding modes. This is achieved by various Gd(III) and nitroxide spin labeling schemes for CaM and the peptides combined with EPR inter-spin distance and paramagnetic NMR measurements. [ABSTRACT FROM AUTHOR]

Nitroxide spin-labelled lipid analogues are often used to study model membrane properties using EPR spectroscopy. Whereas in liquid phase membranes the spin label assumes, on average, its putative location, in gel phases and frozen membrane, depending on its position along the acyl chain, it may exhibit a different average location. Here we used2H three-pulse Electron Spin Echo Envelope Modulation (ESEEM) of phospholipid spin probes, combined with various deuteration schemes to detect the effect of the model membrane curvature and cholesterol on vertical migrations of the spin label. We compared large and small unilamellar 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles with and without cholesterol (10%). The vertical displacement of the spin label was manifested as an apparently flat trans-membrane profile of water concentration and of label proximity to the head group choline. The spin-label propensity to migrate was found to increase with vesicle curvature and decrease in the presence of cholesterol. This in turn reflects the effect of packing and ordering of the membrane lipids. The results show that in curved vesicles lacking cholesterol, the label attached to carbon 16 may travel as far high along the membrane normal as the location of the label on carbon 5, due to the presence of U-shaped lipid conformations. This phenomenon must be taken into account when using spin-labelled lipids as membrane depth markers or to trace trans-membrane profiles. [ABSTRACT FROM PUBLISHER]

Abstract: Double electron–electron resonance (DEER) at W-band (95GHz) was applied to measure the distance between a pair of nitroxide and Gd3+ chelate spin labels, about 6nm apart, in a homodimer of the protein ERp29. While high-field DEER measurements on systems with such mixed labels can be highly attractive in terms of sensitivity and the potential to access long distances, a major difficulty arises from the large frequency spacing (about 700MHz) between the narrow, intense signal of the Gd3+ central transition and the nitroxide signal. This is particularly problematic when using standard single-mode cavities. Here we show that a novel dual-mode cavity that matches this large frequency separation dramatically increases the sensitivity of DEER measurements, allowing evolution times as long as 12μs in a protein. This opens the possibility of accessing distances of 8nm and longer. In addition, orientation selection can be resolved and analyzed, thus providing additional structural information. In the case of W-band DEER on a Gd3+–nitroxide pair, only two angles and their distributions have to be determined, which is a much simpler problem to solve than the five angles and their distributions associated with two nitroxide spin labels. [Copyright &y& Elsevier]

The formation mechanism of the cubic mesoporous carbon, FDU-16, synthesized by evaporation-induced self-assembly (EISA) was investigated at the molecular level by electron paramagnetic resonance (EPR) spectroscopic techniques. This material is synthesized using F127 pluronic block copolymer [poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO106-PPO70-PEO106)] as a structure-directing agent (template) and phenolic resol as a carbon precursor. Using two spin probes derived from pluronics with PEO and PPO chains of different lengths that are designed to sense different regions of the system, we followed the evaporation and thermopolymerization stages of the synthesis in situ. To make such studies possible, we have used a polyurethane foam support, placed in the EPR tube, which allows for the efficient solvent evaporation as required for EISA. We focused on the evolution of the dynamics of the template and its interactions with the resol during the reaction. We observed that during the evaporation stage the resol is distributed throughout the entire PEO blocks, all the way to the PPO–PEO interface, interacting with them via H-bonds, thus hindering the local motion of the PEO chains. At the end of this stage there is no polarity gradient along the PEO blocks, as found for traditional F127 micelles in water or during the synthesis of silica materials, and the mesostructure is not well-defined. A polarity and a resol gradient developed during the thermopolymerization stage where the polymerizing resol is driven out to the outer region of the PEO corona. This produces a corona of resin-pluronic composite and a resol-free PPO core with high mobility of the PEO segments close to the PPO–PEO interface and restricted mobility in the composite corona. During this stage the final structure sets in. During the synthesis of ordered mesoporous carbon by evaporation-induced self-assembly on a polyurethane foam support, the carbon source, resol, interacts with the poly(ethylene oxide) (PEO) block of the pluronic template all the way to the poly(propylene oxide) (PPO) region. Segregation of PEO and PPO regions occur during the thermopolymerization stage, leading to the final structure. [ABSTRACT FROM AUTHOR]

Abstract: Nitroxide spin probe electron paramagnetic resonance (EPR) has proven to be a very successful method to probe local polarity and solvent hydrogen bonding properties at the molecular level. The gxx and the 14N hyperfine Azz principal values are the EPR parameters of the nitroxide spin probe that are sensitive to these properties and are therefore monitored experimentally. Recently, the 14N quadrupole interaction of nitroxides has been shown to be also highly sensitive to polarity and H-bonding (A. Savitsky et al., J. Phys. Chem. B 112 (2008) 9079). High-field electron spin echo envelope modulation (ESEEM) was used successfully to determine the Pxx and Pyy principal components of the 14N quadrupole tensor. The Pzz value was calculated from the traceless character of the quadrupole tensor. We introduce here high-field (W-band, 95GHz, 3.5T) electron–electron double resonance (ELDOR)-detected NMR as a method to obtain the 14N Pzz value directly, together with Azz . This is complemented by W-band hyperfine sublevel correlation (HYSCORE) measurements carried out along the gxx direction to determine the principal Pxx and Pyy components. Through measurements of TEMPOL dissolved in solvents of different polarities, we show that Azz increases, while |Pzz | decreases with polarity, as predicted by Savitsky et al. [Copyright &y& Elsevier]

Abstract: An EPR spectrum of as synthesized [G.A. Tsigdinos, C.J. Hallada, Inorg. Chem. 7 (1968) 437–441], orange colored, H5PV2Mo10O40 polyoxometalate showed the presence of a reduced vanadium(IV) addenda atom. Surprisingly, further 31P ENDOR (electron-nuclear double resonance) measurements indicated the absence of a phosphorous heteroatom leading to the suggestion that H5VVVIVMo11O40 exists as a previously unrecognized impurity in the typically synthesized H5PV2Mo10O40 compound. H5/4PVVO4VIV/VMo11O36 was then synthesized in low yield (0.8mol%) by omitting the addition of phosphate in a typical H5PV2Mo10O40 preparation. The molecular formulation and structure was supported by X-ray crystallography, infrared and mass spectrometry. Further use of EPR/ENDOR/ESEEM (electron-spin echo envelope modulation) allowed the formulation of [VVVIVMo11O40]5− as [VVO4VIVMo11O36]5−. Accordingly, the polyoxometalate has a heteroatom core with 11 molybdenum addenda and one VO2+ moiety at the polyoxometalate surface. The redox potential and the catalytic activity of the new vanadomolybdate polyoxometalate compound were essentially identical to the often-studied H5PV2Mo10O40 polyoxometalate isomeric mixture. [Copyright &y& Elsevier]