Structure of Polyelectrolytes in Poor Solvent

We present simulations on charged polymers in poor solvent. First we investigate in detail the dilute concentration range with and without imposed extension constraints. The resulting necklace polymer conformations are analyzed in detail. We find strong fluctuations in the number of pearls and their sizes leading only to small signatures in the form factor and the force-extension relation. The scaling of the peak in the structure factor with the monomer density shows a pertinent different behavior from good solvent chains.Comment: 7 pages, 5 figures. submitted to EP

Physico-Chemical Differences Between Particle- and Molecule-Derived Toxicity: Can We Make Inherently Safe Nanoparticles?

The rapidly growing applications of nanotechnology require a detailed understanding of benefits and risks, particularly in toxicology. The present study reviews the physical and chemical differences between particles and molecules when interacting with living organisms. In contrast to classical chemicals, the mobility of nanoparticles is governed by agglomeration, a clustering process that changes the characteristic size of the nanomaterials during exposure, toxicity tests or in the environment. The current status of nanotoxicology highlights non-classical toxic interactions through catalytic processes inside living cells and the enhanced heavy metal transport into the cytosol through the 'Trojan horse mechanism'. The safety of nanoparticles in consumer goods is proposed to be rendered inherently safer by substituting the currently used persistent oxides through biodegradable materials

Serrated flow of CuZr-based bulk metallic glasses probed by nanoindentation: Role of the activation barrier, size and distribution of shear transformation zones

We report on the effect of Al and Co alloying in vitreous Cu50Zr50 on local deformation and serrated flow as a model for relating the size and localization of shear transformation zones (STZ) to Poisson ratio and strain-rate sensitivity of metallic glasses. Alloying with Al results in significant variations in mechanical performance, in particular, in Young's modulus, hardness and strain-rate sensitivity. Increasing strain-rate sensitivity with increasing degree of alloying indicates a reduced tendency for shear localization. In parallel, a gradual transition from inhomogeneous to homogeneous plastic flow is observed. Using a statistical analysis of the shear stress associated with the initiation of the first pop-in in the load-displacement curve during spherical indentation, the activation volume for plastic flow at the onset of yielding is reported. This analysis is employed for experimental evaluation of the compositional dependence of activation barrier, size and distribution of STZs. It is demonstrated that the STZ size does not change significantly upon Al alloying and encompasses a local volume of around 22–24 atoms. However, the barrier energy density for the initiation of a single STZ progressively increases. The broader distribution of STZs impedes their accumulation into larger-size flow units, leading to a lower number and reduced size of serrations in the load-displacement curve. On the contrary, lower barrier energy densities enable a larger quantity of STZs to be activated simultaneously. These STZs can easily percolate into large flow units, promoting plastic flow through their interaction. We employ Poisson's ratio as an indicator for plasticity to shown that this interpretation can be transferred to other types of metallic glasses. That is, larger flow units were found for metallic glasses with higher Poisson ratio and more pronounced plasticity, while the flow units in alloys with very low Poisson ratio and high brittleness are significantly reduced in size and more homogeneously distributed throughout the material

Identification of oxidized amino acid residues in the vicinity of the Mn \u3csub\u3e4\u3c/sub\u3eCaO \u3csub\u3e5\u3c/sub\u3e cluster of photosystem II: Implications for the identification of oxygen channels within the photosystem

As a light-driven water-plastoquinone oxidoreductase, Photosystem II produces molecular oxygen as an enzymatic product. Additionally, under a variety of stress conditions, reactive oxygen species are produced at or near the active site for oxygen evolution. In this study, Fourier-transform ion cyclotron resonance mass spectrometry was used to identify oxidized amino acid residues located in several core Photosystem II proteins (D1, D2, CP43, and CP47) isolated from spinach Photosystem II membranes. While the majority of these oxidized residues (81%) are located on the oxygenated solvent-exposed surface of the complex, several residues on the CP43 protein ( 354E, 355T, 356M, and 357R) which are in close proximity (\u3c15 \u3eÅ) to the Mn 4CaO 5 active site are also modified. These residues appear to be associated with putative oxygen/reactive oxygen species exit channel(s) in the photosystem. These results are discussed within the context of a number of computational studies which have identified putative oxygen channels within the photosystem. © 2012 American Chemical Society

Association of the 17-kDa extrinsic protein with photosystem II in higher plants

The structural association of the spinach 17-kDa extrinsic protein of photosystem II with other extrinsic and membrane-bound components of the photosystem was investigated by labeling the 17-kDa extrinsic protein with the amino-group-specific reagent N-hydroxysuccinimidobiotin both on intact photosystem II membranes or as a free protein in solution. After isolation of the biotinylated molecules, the modified 17-kDa proteins were allowed to rebind to photosystem II membranes which were depleted of the 17-kDa component. Differential binding of the protein biotinylated in solution compared to unmodified 17-kDa protein or 17-kDa protein modified on PSII membranes was observed. This indicated possible steric or ionic interference because of biotinylated lysyl residues present on the protein modified in solution. Biotinylated sites on the different modified 17-kDa proteins were identified by trypsin and Staphylococcus V8 protease digestion, followed by affinity chromatography enrichment of the biotinylated peptides and analysis of the peptide fragment mixture by nanospray liquid chromatography-tandem mass spectrometry. Four lysyl residues that were modified when the protein was biotinylated in solution were not biotinylated when the protein was modified on the PS II membrane (90K, 96K, 101K, and 102K). These residues appear to identify a protein domain involved in the interaction of the 17-kDa protein with the other components of the photosystem. © 2005 American Chemical Society

Radiolytic mapping of solvent-contact surfaces in photosystem II of higher plants: Experimental identification of putative water channels within the photosystem

Background: Substrate water must reach the buried Mn4O 5Ca cluster in Photosystem II. Results: OH produced by radiolysis modified buried amino acid residues. These were mapped onto the PS II crystal structure. Conclusion: Two groups of oxidized residues were identified which form putative pathways to the Mn4O5Ca cluster. Significance: Identification of water and oxygen channels is crucial for our understanding of Photosystem II function. © 2013 by The American Society for Biochemistry and Molecular Biology, Inc