Electroactive Polymers: Developments of and Perspectives for Dielectric Elastomers

We present the development and applications of dielectric elastomers. For the last 10 years the significance of this class of polymers has risen as more applications seem possible and first products have been commercialized.     

Thermodynamics of the formaldehyde-water and formaldehyde-ice systems for atmospheric applications

Formaldehyde (HCHO) is a species involved in numerous key atmospheric chemistry processes that can significantly impact the oxidative capacity of the atmosphere. Since gaseous HCHO is soluble in water, the water droplets of clouds and the ice crystals of snow exchange HCHO with the gas phase and the partitioning of HCHO between the air, water, and ice phases must be known to understand its chemistry. This study proposes thermodynamic formulations for the partitioning of HCHO between the gas phase and the ice and liquid water phases. A reanalysis of existing data on the vapor-liquid equilibrium has shown the inadequacy of the Henry's law formulation, and we instead propose the following equation to predict the mole fraction of HCHO in liquid water at equilibrium, X(HCHO,liq), as a function of the partial pressure P(HCHO) (Pa) and temperature T (K): X(HCHO,liq) = 1.700 × 10(-15)?e((8014/T))(P(HCHO))(1.105). Given the paucity of data on the gas-ice equilibrium, the solubility of HCHO and the diffusion coefficient (D(HCHO)) in ice were measured by exposing large single ice crystals to low P(HCHO). Our recommended value for D(HCHO) over the temperature range 243-266 K is D(HCHO) = 6 × 10(-12) cm(2) s(-1). The solubility of HCHO in ice follows the relationship X(HCHO,ice) = 9.898 × 10(-13)?e((4072/T))(P(HCHO))(0.803). Extrapolation of these data yields the P(HCHO) versus 1/T phase diagram for the H(2)O-HCHO system. The comparison of our results to existing data on the partitioning of HCHO between the snow and the atmosphere in the high arctic highlights the interplay between thermodynamic equilibrium and kinetics processes in natural systems.     

Ultrafast hybridization screening in Fe3+ aqueous solution

We report here on the electron binding energies and ultrafast electronic relaxation of the Fe(3+)(aq) complex in FeCl(3) aqueous solution as measured by soft X-ray photoelectron (PE) spectroscopy from a vacuum liquid microjet. Covalent mixing between the 3d valence orbitals of the iron cation and the molecular orbitals of water in the ground-state solution is directly revealed by spectroscopy of the highest partially occupied molecular orbitals. Valence PE spectra, obtained for photon energies near the iron 2p absorption edge, exhibit large resonant enhancements. These resonant PE features identify 3d-O2p transient hybridization between iron and water-derived orbitals and are an indication of charge transfer within the electronically excited Fe(3+)(aq)* complex. Charge transfer from water to iron is also revealed by the 2p core-level PE spectrum, and the asymmetric peak shape additionally identifies the characteristic multiplet interactions in the 2p core-hole state. The electronic structure of water molecules in the first hydration shell is selectively probed by Auger decay from water molecules, at excitation energies well below the O1s absorption edge of neat water. These experiments lay the groundwork for establishing resonant PE spectroscopy for the study of electronic-structure dynamics in the large family of transition metal (aqueous) solutions.     

Electronic structure of sub-10 nm colloidal silica nanoparticles measured by in situ photoelectron spectroscopy at the aqueous-solid interface

X-Ray photoelectron spectroscopy has been extended to colloidal nanoparticles in aqueous solution using a liquid microjet in combination with synchrotron radiation, which allowed for depth-dependent measurements. Two distinct electronic structures are evident in the Si 2p photoelectron spectrum of 7 nm SiO(2)-nanoparticles at pH 10. A core-shell model is proposed where only the outermost layer of SiO(2) nanoparticles, which is mainly composed of deprotonated silanol groups, >Si-O(-), interacts with the solution. The core of the nanoparticles is not affected by the solvation process and retains the same electronic structure as measured in vacuum. Future opportunities of this new experiment are also highlighted.     

Silylated Cyclohexadienes in Radical Chain Hydrosilylations

A new method for the mild radical hydrosilylation of alkenes and alkynes is described. Silylated cyclohexadienes that can be readily prepared on large scale are used as radical hydrosilylating reagents. Non‐activated alkenes and alkynes are hydrosilylated in high yields. The reaction can be combined with C C bond formation, as demonstrated for the preparation of silylated cycloalkanes from the corresponding dienes. Furthermore, radical hydrosilylations in combination with β‐fragmentation reactions for the synthesis of allylsilanes and hydrosilylations of aldehydes and ketones providing protected alcohols can be readily performed by this strategy.     

Chemoenzymatic approach to enantiopure streptogramin B variants: characterization of stereoselective pristinamycin I cyclase from Streptomyces pristinaespiralis

Streptogramin B antibiotics are cyclic peptide natural products produced by Streptomyces species.In combination with the synergistic group A component, they are "last line of defense" antimicrobial agents against multiresistant cocci. The racemization sensitivity of the phenylglycine (Phg(7)) ester is a complex challenge in total chemical synthesis of streptogramin B molecules. To provide fast and easy access to novel streptogramin antibiotics, we introduce a novel chemoenzymatic strategy in which diversity is generated by standard solid phase protocols and stereoselectivity by subsequent enzymatic cyclization. For this approach, we cloned, overproduced, and biochemically characterized the recombinant thioesterase domain SnbDE TE of the pristinamycin I nonribosomal peptide synthetase from Streptomyces pristinaespiralis. SnbDE TE catalyzes regioselective ring closure of linear peptide thioester analogues of pristinamycin I as well as stereoselective cyclization out of complex in situ racemizing substrate mixtures, enabling synthesis of Streptogramin B variants via a dynamic kinetic resolution assay. A remarkable substrate tolerance was detected for the enzymatic cyclization including all the seven positions of the peptide backbone. Interestingly, SnbDE TE was observed to be the first cyclase from a macrolactone forming NRPS which is additionally able to catalyze macrolactamization of peptide thioester substrates. An N-methylated peptide bond between positions 4 and 5 is mandatory for a high substrate turnover. The presented strategy is potent to screen for analogues with improved activity and guides our understanding of structure--activity relationships in the important class of streptogramin antibiotics.     

Probing the water coordination of protein-targeted MRI contrast agents by pulsed ENDOR spectroscopy.

A novel methodology based on electron-nuclear double resonance (ENDOR) spectroscopy is used for the direct determination of the water coordination number (q) of gadolinium-based magnetic resonance imaging (MRI) contrast agents. Proton ENDOR spectra can be obtained at approximately physiological concentrations for metal complexes in frozen aqueous solutions either in the presence or absence of protein targets. It is shown that, depending on the structure of the co-ligand, the water hydration number of a complex in aqueous solution can be significantly different to when the complex is noncovalently bound to a protein. From the ENDOR spectra of the exchangeable protons, precise information on the metal-proton distance can be derived as well. These essential parameters directly correlate with the efficacy of MRI contrast agents and should therefore aid the development of novel, highly efficient compounds targeted to various proteins.     

Ultra-trace analysis of platinum in human tissue samples

Background levels of platinum were determined in human autopsy tissues taken from five individuals. The investigated specimens were lung, liver and kidney. Sample preparation involved microwave digestion followed by an open vessel treatment. Inductively-coupled plasma sector field mass spectrometry (ICP-SFMS) was applied in combination with an ultrasonic nebulization/membrane desolvation system for sample introduction. Isotope dilution analysis was employed for accurate quantification of platinum. Excellent procedural detection limits (3 s validation) of 20, 20 and 34 pg g^–1 dry weight were obtained for lung, liver and kidney tissue, respectively. Due to the lack of appropriate biological reference material, road dust (BCR-723) was used for method validation. Platinum levels ranging between 0.03 and 1.42 ng g^–1 were determined in the investigated samples. The platinum concentrations observed in human lung tissue may reflect the increasing atmospheric background levels of platinum originating from car catalysts. The presence of platinum in kidney and liver tissue samples clearly indicates the bioavailability of the element.     

Rationalisation of unusual changes in efficiency and retention with temperature shown for bases in reversed-phase high-performance liquid chromatography at intermediate pH

Despite the possibility of poorer peak shapes, analysis of pharmaceuticals and other bases using reversed-phase high-performance liquid chromatography (RP-HPLC) at intermediate pH gives useful increases in retention, selectivity and column loading capacity compared with low pH. Retention times of some bases showed anomalous increases with column temperature. Peak shapes for bases improved significantly at elevated temperature (up to 70 degrees C), with the weakest bases studied (pKa approximately 8) giving greater improvement than the strongest bases (pKa approximately 10). In contrast, quaternary ammonium compounds showed reduced retention with increasing column temperature (normal behaviour) and only modest improvements in peak shape. Considering these results, and pKa measurements of the bases made using capillary electrophoresis, it appears that increases in retention and improvement in efficiency may be influenced significantly by reduction in the pKa of bases with temperature, leading to reduction in protonation. It is less likely that efficiency improvements are due to the speeding up of the kinetics of silanol ion exchange, at least in the temperature range studied here.