Light and heat control over secondary structure and amyloid-like fiber formation in an overcrowded-alkene-modified Trp zipper

The external photocontrol over peptide folding, by the incorporation of molecular photoswitches into their structure, provides a powerful tool to study biological processes. However, it is limited so far to switches that exhibit only a rather limited geometrical change upon photoisomerization and that show thermal instability of the photoisomer. Here we describe the use of an overcrowded alkene photoswitch to control a model β-hairpin peptide. This photoresponsive unit undergoes a large conformational change and has two thermally stable isomers which has major influence on the secondary structure and the aggregation of the peptide, permitting the phototriggered formation of amyloid-like fibrils.     

Heterogeneous photobleaching in confocal microscopy

Photobleaching was studied during recording of confocal scanning laser microscopy. Studies on fluorescent gels of FITC-labeled dextran were used to evaluate differential bleaching along thez-axis. Differential bleaching along the z-axis was observed and it was seen that this was related to the numerical aperture of the objective in use. This points to the conclusion that photon energy flux density is an important parameter in photobleaching. To check if photon energy flux density heterogeneity is affected by local variation in the refractive index of the sample, photobleaching rates were calculated for different fluorescent objects (sections of seeds, animal cells stained with nuclear stains, immunocytochemistry preparations) and a pronounced similarity was found between photobleaching rates and DIC images.     

Estimating a reduced rank regression model for non-normal variables

A method using third order moments for estimating the regression coefficients as well as the latent state scores of the reduced-rank regression model when the latent variable(s) are non-normally distributed is presented in this paper. It is shown that the factor analysis type indeterminacy of the regression coefficient matrices is eliminated. A real life example of the proposed method is presented. Differences of this solution with the reduced-rank regression eigen solution are discussed.     

Bulk and microprobe analysis for trace elements with synchrotron radiation

Blood serum samples were irradiated by monochromatic synchrotron radiation (15 keV) and by 2.4-MeV protons to compare their performance for the detection of trace elements by x-ray fluorescence (XRF) spectrometry. Absolute concentration assignment was based on the addition of an internal standard and on a method which uses the incoherent and coherent radiation as a flux monitor of the incoming beam. Preliminary experiments with a synchrotron XRF microprobe are reported.     

Mechanical mixtures of Me (Ni, Pd) Ce oxides and silica-supported heteropolyacids: Role and optimal concentration of each active species in n-hexane isomerization

Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me-Ce oxides (Me = Ni, Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better dispersion of the second metal species. Nickel and palladium, present as Ni⁰ and Pd⁰, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus, enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO-HPW/SiO₂ and 50–50 wt % Pd_0.1CeO-HPW/SiO₂. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively. Finally, the conversion of n-hexane is in the order HPW > HSiW > HBW, which seems to be consistent with the order of their acid strength as per the literature, but the isomerization selectivity appears to be slightly higher on HSiW. Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 1, pp. 24–28. The text was submitted by the authors in English.     

Metallo‐supramolecular block copolymer micelles: Improved preparation and characterization

Micelles prepared from amphiphilic block copolymers in which a poly(styrene) segment is connected to a poly(ethylene oxide) block via a bis‐(2,2′:6′,2″‐terpyridine‐ruthenium) complex have been intensely studied. In most cases, the micelle populations were found to be strongly heterogeneous in size because of massive micelle/micelle aggregation. In the study reported in this article we tried to improve the homogeneity of the micelle population. The variant preparation procedure developed, which is described here, was used to prepare two “protomer”‐type micelles: PS₂₀‐[Ru]‐PEO₇₀ and PS₂₀‐[Ru]‐PEO₃₇₅. The dropwise addition of water to a solution of the compounds in dimethylformamide was replaced by the controlled addition of water by a syringe pump. The resulting micelles were characterized by sedimentation velocity and sedimentation equilibrium analyses in an analytical ultracentrifuge and by transmission electron microscopy of negatively stained samples. Sedimentation analysis showed virtually unimodal size distributions, in contrast to the findings on micelles prepared previously. PS₂₀‐[Ru]‐PEO₇₀ micelles were found to have an average molar mass of 318,000 g/mol (corresponding to 53 protomers per micelle, which is distinctly less than after micelle preparation by the standard method) and an average hydrodynamic diameter (d_h) of 18 nm. For PS₂₀‐[Ru]‐PEO₃₇₅ micelles, the corresponding values were M = 603,000 g/mol (31 protomers per micelle) and d_h = 34 nm. The latter particles were found to be identical to the “equilibrium” micelles prepared in pure water. Both micelle types had a very narrow molar mass distribution but a much broader distribution of s values and thus of hydrodynamic diameters. This indicates a conformational heterogeneity that is stable on the time scale of sedimentation velocity analysis. The findings from electron microscopy were in disagreement with those from the sedimentation analysis both in average micelle diameter and in the width of the distributions, apparently because of imperfections in the staining procedure. The preparation procedure described also may be useful in micelle formation from other types of protomers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4458–4465, 2004     

Effect of the particle size distribution on the low shear viscosity of high‐solid‐content latexes

The production of high‐solid‐content, low‐viscosity latexes is an active field in both industry and academia. The viscosity of polymer dispersions has a clear dependence on the particle size distribution (PSD). An example is the rule of thumb that a bimodal PSD enables the reduction of the viscosity with respect to monomodal systems. Despite important progress in theoretical work, not much has been done to quantitatively predict the low shear viscosity of aqueous polymer dispersions as a function of the complex PSD. In this work, the capability of a low‐shear‐viscosity equation to quantitatively account for the influence of both the PSD and the physicochemical characteristics of the dispersions is experimentally assessed. An analysis, consistent with theoretical concepts, of the data with semiempirical correlations is proposed. Next, with values of the parameters of the viscosity equation obtained experimentally, the effect of a latex with a 70% solid content on the low shear viscosity is examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3936–3946, 2004