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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]

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]

Summary We have applied high-field (W-band) pulse electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) to characterize the coordination sphere of the Mn 2+ co-factor in the nucleotide binding sites (NBSs) of ABC transporters. MsbA and BmrCD are two efflux transporters hypothesized to represent divergent catalytic mechanisms. Our results reveal distinct coordination of Mn 2+ to ATP and transporter residues in the consensus and degenerate NBSs of BmrCD. In contrast, the coordination of Mn 2+ at the two NBSs of MsbA is similar, which provides a mechanistic rationale for its higher rate constant of ATP hydrolysis relative to BmrCD. Direct detection of vanadate ion, trapped in a high-energy post-hydrolysis intermediate, further supports the notion of asymmetric hydrolysis by the two NBSs of BmrCD. The integrated spectroscopic approach presented here, which link energy input to conformational dynamics, can be applied to a variety of systems powered by ATP turnover. [ABSTRACT FROM AUTHOR]