Controlled/living radical polymerization of isoprene and butadiene in emulsion

A process for RAFT-controlled radical polymerization in emulsion [36] has been applied to the polymerizations of isoprene and of butadiene in emulsion systems, with the goal of producing latex particles containing block copolymers of acrylic acid (stabilizer and starting polymer), styrene (second polymer) and isoprene or butadiene (third polymer). The microstructure of the polymer chains was examined using dual-detection size-exclusion chromatography, and the nanostructure of the materials was investigated by differential scanning calorimetry and solid-state nuclear magnetic resonance. Reactions were always slow (although faster than the corresponding processes in solution), and exhibited limited reinitiation by isoprene when in emulsion. The materials containing isoprene exhibit a nanostructure with a phase separation into high-T_g polystyrene-rich domains and low-T_g polyisoprene-rich domains, revealed by DSC and NMR. This has the potential to lead to barrier materials with novel physical properties.     

Investigating the structural properties of amyloid-like fibrils formed in vitro from beta2-microglobulin using limited proteolysis and electrospray ionisation mass spectrometry

The protein beta(2)-microglobulin (beta(2)m) aggregates to form classical amyloid fibrils in patients undergoing long-term haemodialysis. Amyloid-like fibrils with a cross-beta fold can also be formed from wild-type beta(2)m under acidic conditions in vitro. The morphology of such fibrils depends critically on the conditions used: incubation of beta(2)m in low ionic strength buffers at pH 2.5 results in the formation of long (microm), straight fibrils while, at pH 3.6, short (<500 nm) fibrils form. At higher ionic strengths (0.2-0.4 M) at pH 1.5-3.6, the fibrils have a distinct curved and nodular morphology. To determine the conformational properties of beta(2)m within in vitro fibrils of different morphologies, limited proteolysis of each fibril type using pepsin was performed and the resulting peptide fragments identified by tandem mass spectrometry. For comparison, the proteolytic degradation patterns of monomeric beta(2)m and seven synthetic peptides spanning the entire sequence of the intact protein were similarly analysed. The results show that fibrils with different morphologies result in distinct digestion patterns. While the curved, worm-like fibrils are relatively weakly protected from proteolysis, the long, straight fibrils formed at pH 2.5 at low ionic strength show only a single cut-site at Val9, demonstrating that substantial refolding of the initially acid-denatured and unprotected state of beta(2)m occurs during assembly. The data demonstrate that the organisation of the polypeptide chain in fibrils with different morphological features differs considerably, despite the fact that the fibrils possess a common cross-beta architecture.     

Electrochemical reactivity at redox-molecule-based nanoelectrode ensembles

A model describing electrochemical reactivity at nanoelectrode ensembles consisting of redox-molecule-based active sites immobilized on otherwise passivated electrode surfaces is presented. A mathematical treatment in terms of hemispherical diffusion of redox-active solutes to a layer of independent molecule-based nanoelectrode sites is shown to be equivalent to one in terms of a bimolecular diffusion-limited reaction between a layer of immobilized redox molecules and a reservoir of redox-active solutes. This equivalence derives from the fact that in both cases the mass-transfer problem is essentially that of hemispherical diffusion. The model is further developed to consider rate limitation by both the bimolecular redox reaction between the active-site molecule and redox molecules in solution and the heterogeneous redox reaction between the electrode and the active-site molecule. Analytical expressions are derived for the current–voltage relation corresponding to catalyzed electron transfer at an ensemble of redox-molecule-based nanoelectrode sites, and the expressions are used to interpret preliminary data for ultrasensitive electrochemical detection in flow streams via an electrochemical amplification process that is thought to involve redox mediation by individual analyte molecules adsorbed onto monolayer-coated electrodes.     

Gestures, Speech, and the Sprouting of Signs: A Semiotic-Cultural Approach to Students' Types of Generalization

To improve our understanding of novice students' production of symbolic algebraic expressions, this article contrasts students' presymbolic and symbolic procedures in generalizing activities. Although a significant amount of previous research on the learning of algebra has dealt with students' errors in the mastering of the algebraic syntax, the semiotic cultural theoretical approach presented here focuses on the role that body, discourse, and signs play when students' refer to mathematical objects. Three types of generalizations are identified: factual, contextual, and symbolic. The results suggest that the passage from presymbolic to symbolic generalizations requires a specific kind of rupture with the ostensive gestures and contextually based key linguistic terms underpinning presymbolic generalizations. This rupture means a disembodiment of the students' previous spatial temporal embodied mathematical experience.     

First Investigation of 115In in the High Spin Regime

The ¹¹⁵In nucleus was studied by means of γ-ray spectroscopic measurements. This nucleus was populated with the ¹⁰⁰Mo(¹⁸O, p2n)¹¹⁵In reaction and a charged-particle detector array was used to identify the proton evaporation channel. A level scheme with 17 new transitions is presented and compared with theoretical predictions based on the particle + core model.     

Far infrared laser frequencies of CH3OD and N2H4

The frequencies of 26 laser lines with wavelengths between 57 and 534 m have been measured in the optically pumped laser gases CH₃OD and N₂H₄. A pair of stabilized cw 12_{CO 2} lasers was used as a frequency standard for the heterodyne frequency measurements. Seven of the 26 lines are new.     

Stability and rheology of emulsions containing sodium caseinate: combined effects of ionic calcium and alcohol

We have investigated the combined effect of ionic calcium and ethanol on the visual creaming behavior and rheology of sodium caseinate-stabilized emulsions (4 wt% protein, 30 vol% oil, pH 6.8, mean droplet diameter 0.4 microm). A range of ionic calcium concentrations, expressed as a calcium/caseinate molar ratio R, was adjusted prior to homogenization and varying concentrations of ethanol were added shortly after homogenization. A stability map was produced on the basis of visual creaming behavior over a minimum period of 8 h for different calcium/caseinate/ethanol emulsion compositions. A single narrow stable (noncreaming) region was identified, indicating limited cooperation between calcium ions and ethanol. The shear-thinning behavior of the caseinate-stabilized emulsions is typical of systems undergoing depletion flocculation. Addition of calcium ions and/or ethanol was found to lead to a pronounced reduction in viscosity and the onset of Newtonian flow. The state of aggregation was correlated with emulsion microstructure from confocal laser scanning microscopy. Time-dependent rheology (18 h) with a density-matched oil phase (1-bromohexadecane) revealed that the visually stable emulsions were time-independent low-viscosity fluids. Surface coverage data showed that increasing amounts of caseinate were associated with the oil-water interface with increasing R and ethanol content. A decrease in free calcium ions in the aqueous phase with moderate increases in R and ethanol content was observed, which is consistent with greater calcium-caseinate binding (aggregation). Ostwald ripening occurred at the high-ethanol emulsion compositions that were stable to depletion flocculation. While the coarsening rate was low, this can account for the cream plug formation observed during gravity creaming experiments. The caseinate emulsion with no ionic calcium or ethanol exhibits depletion flocculation from excess nonadsorbed caseinate submicelles. Addition of calcium ions reduces the submicelle number density via specific calcium-binding in the aqueous phase (fewer, larger calcium-caseinate aggregates) and at the droplet surface (increased surface coverage). Nonspecific ethanol-induced (calcium-dependent) caseinate submicelle aggregation in the bulk phase and on the droplet surface (increased surface coverage) culminates in a reduction in the number density of caseinate submicelles. A narrow window of inhibition of depletion flocculation occurs in systems containing both calcium ions and ethanol, both species combining to aggregate the protein and so reduce the density of free submicelles.     

Force mode atomic force microscopy as a tool for protein folding studies

The advent of a new class of force microscopes designed specifically to “pull” biomolecules has allowed non-specialists to use force microscopy as a tool to study single-molecule protein unfolding. This powerful new technique has the potential to explore regions of the protein energy landscape that are not accessible in conventional bulk studies. It has the added advantage of allowing direct comparison with single-molecule simulation experiments. However, as with any new technique, there is currently no well described consensus for carrying out these experiments. Adoption of standard schemes of data selection and analysis will facilitate comparison of data from different laboratories and on different proteins. In this review, some guidelines and principles, which have been adopted by our laboratories, are suggested. The issues associated with collecting sufficient high quality data and the analysis of those data are discussed. In single-molecule studies, there is an added complication since an element of judgement has to be applied in selecting data to analyse; we propose criteria to make this process more objective. The principal sources of error are identified and standardised methods of selecting and analysing the data are proposed. The errors associated with the kinetic parameters obtained from such experiments are evaluated. The information that can be obtained from dynamic force experiments is compared, both quantitatively and qualitatively to that derived from conventional protein folding studies.