Descripción: Energetics, structural features, polarity, and melting transitions in water clusters containing up to eight molecules were studied using ab initio methods and empirical force field models. Our quantum approach was based on density functional theory performed at the generalized gradient approximation level. For the specific case of (H2O)6, we selected five conformers of similar energy with different geometries and dipolar moments. For these cases, the cyclic arrangement was found to be the only nonpolar aggregate. For (H2O)8, the most stable structures corresponded to nonpolar, cubic-like, D2d and S4 conformers. Higher energy aggregates exhibit a large spectrum in their polarities. The static polarizability was found to be proportional to the size of the aggregates and presents a weak dependence with the number of hydrogen bonds. In order to examine the influence of thermal fluctuations on the aggregates, we have performed a series of classical molecular dynamics experiments from low temperature up to the melting transition using two different effective pseudopotentials: the TIP4P and MCY models. Minimum energy structures for both classical potentials were found to reproduce reasonably well the results obtained using ab initio methods. Isomerization and phase transitions were monitored by following changes in dipole moments, number of hydrogen bonds and Lindemann's parameter. For (H2O)6 and (H2O)8, the melting transitions were found at Tm≈50 and 160 K, respectively; for both aggregates, we observed premelting transitions between well differentiated conformers as well. © 1999 American Institute of Physics.

Descripción: Structural and dynamical properties of liquid-state, binary, water-acetonitrile nanoclusters are studied. When acetonitrile is the solute species, it exhibits a propensity to reside on the cluster surface, although one may identify regions interior to the cluster that are favorable for solvation. The dynamics of the interchange of acetonitrile between surface and interior solvation regions and structural aspects of surface solvation states are studied. When water is the solute it tends to be solvated in the interior of the cluster and form aggregates. The nature and dynamics of contact and solvent separated water pairs in these clusters is investigated, and the recombination dynamics of larger water aggregates in concentrated water-acetonitrile clusters is examined. © 1998 American Institute of Physics.

Descripción: We assessed the intracellular association states of the Parkinson's disease related protein α-synuclein (AS) in living cells by transfection with a functional recombinant mutant protein (AS-C4) bearing a tetracysteine tag binding the fluorogenic biarsenical ligands FlAsH and ReAsH, The aggregation states of AS-C4 were assessed by in situ microscopy of molecular translational mobility with FRAP (fluorescence recovery after photobleaching) and of local molecular density with confocal fluorescence anisotropy (CFA). FRAP recovery was quantitative and rapid in regions of free protein, whereas AS in larger aggregates was>80% immobile. A small 16% recovery characterized by an apparent diffusion constant of 0.03-0.04 μm 2/s was attributed to the dynamics of smaller, associated forms of AS-C4 and the exchange of mobile species with the larger immobile aggregates. By CFA, the larger aggregates exhibited high brightness and very low anisotropy, consistent with homoFRET between closely packed AS, for which a Förster distance (R o) of 5.3 nm was calculated. Other bright regions had high anisotropy values, close to that of monomeric AS, and indicative of membrane-associated protein with both low mobility and low degree of association. The anisotropy-fluorescence intensity correlations also revealed regions of free protein or of small aggregates, undetectable by conventional fluorescence imaging alone. The combined strategy (FRAP+CFA) provides a highly sensitive means for elucidating both the dynamics and structural features of protein aggregates and other intracellular complexes in living cells, and can be extended to other amyloid systems and to drug screening protocols. © 2011 Roberti et al.

Descripción: The kinetics of the reaction of 2,4-dinitrochlorobenzene (DNClB) with aniline and substituted anilines such as p-anisidine, p-toluidine and N-methylaniline have been studied in toluene. Except for N-methylaniline the reactions have shown a third order in amine rate dependence which is consistent with aggregates of the amine acting as the nucleophile. On the other hand, the reaction of DNClB with N-methylaniline under the same conditions shows a linear dependence of the second order rate coefficient, kA, vs [amine], which is consistent with the previous mechanism.

Descripción: The competition between chemical equilibrium, for example protonation, and physical interactions determines the molecular organization and functionality of biological and synthetic systems. Charge regulation by displacement of acid-base equilibrium induced by changes in the local environment provides a feedback mechanism that controls the balance between electrostatic, van der Waals, steric interactions and molecular organization. Which strategies do responsive systems follow to globally optimize chemical equilibrium and physical interactions? We address this question by theoretically studying model layers of end-grafted polyacids. These layers spontaneously form self-assembled aggregates, presenting domains of controlled local pH and whose morphologies can be manipulated by the composition of the solution in contact with the film. Charge regulation stabilizes micellar domains over a wide range of pH by reducing the local charge in the aggregate at the cost of chemical free energy and gaining in hydrophobic interactions. This balance determines the boundaries between different aggregate morphologies. We show that a qualitatively new form of organization arises from the coupling between physical interactions and protonation equilibrium. This optimization strategy presents itself with polyelectrolytes coexisting in two different and well-defined protonation states. Our results underline the need of considering the coupling between chemical equilibrium and physical interactions due to their highly nonadditive behavior. The predictions provide guidelines for the creation of responsive polymer layers presenting self-organized patterns with functional properties and they give insights for the understanding of competing interactions in highly inhomogeneous and constrained environments such as those relevant in nanotechnology and those responsible for biological cells function.

Descripción: 2-Cys peroxiredoxins (2-Cys Prxs) are ubiquitous peroxidases with important roles in cellular antioxidant defense and hydrogen peroxide-mediated signaling. Post-translational modifications of conserved cysteines cause the transition from low to high molecular weight oligomers, triggering the functional change from peroxidase to molecular chaperone. However, it remains unclear how non-covalent interactions of 2-Cys Prx with metabolites modulate the quaternary structure. Here, we disclose that ATP and Mg2+ (ATP/Mg) promote the self-polymerization of chloroplast 2-Cys Prx (polypeptide 23.5 kDa) into soluble higher order assemblies (>2 MDa) that proceed to insoluble aggregates beyond 5mMATP. Remarkably, the withdrawal of ATP or Mg2+ brings soluble oligomers and insoluble aggregates back to the native conformation without compromising the associated functions. As confirmed by transmission electron microscopy, ATP/Mg drive the toroid-like decamers (diameter 13 nm) to the formation of large sphere-like particles (diameter ∼30 nm). Circular dichroism studies on ATP-labeled 2-Cys Prx reveal that ATP/Mg enhance the proportion of β-sheets with the concurrent decrease in the content of α-helices. In line with this observation, the formation of insoluble aggregates is strongly prevented by 2,2,2-trifluoroethanol, a cosolvent employed to induce α-helical conformations. We further find that the response of self-polymerization to ATP/Mg departs abruptly from that of the associated peroxidase and chaperone activities when two highly conserved residues, Arg129 and Arg152, are mutated. Collectively, our data uncover that non-covalent interactions of ATP/Mg with 2-Cys Prx modulate dynamically the quaternary structure, thereby coupling the non-redox chemistry of cell energy with redox transformations at cysteine residues. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc.

Descripción: Photoactive materials based on dye molecules incorporated into thin films or bulk solids are useful for applications as photosensitization, photocatalysis, solar cell sensitization and fluorescent labeling, among others. In most cases, high concentrations of dyes are desirable to maximize light absorption. Under these circumstances, the proximity of dye molecules leads to the formation of aggregates and statistical traps, which dissipate the excitation energy and lower the population of excited states. The search for enhancement of light collection, avoiding energy wasting requires accounting the photophysical parameters quantitatively, including the determination of quantum yields, complicated by the presence of light scattering when particulate materials are considered. In this work we summarize recent advances on the photophysics of dyes in light-scattering materials, with particular focus on the effect of dye concentration. We show how experimental reflectance, fluorescence and laser-induced optoacoustic spectroscopy data can be used together with theoretical models for the quantitative evaluation of inner filter effects, fluorescence and triplet formation quantum yields and energy transfer efficiencies. © 2013 The American Society of Photobiology.

Descripción: The spontaneous and reversible formation of foci and filaments that contain proteins involved in different metabolic processes is common in both the nucleus and the cytoplasm. Stress granules (SGs) and processing bodies (PBs) belong to a novel family of cellular structures collectively known as mRNA silencing foci that harbour repressed mRNAs and their associated proteins. SGs and PBs are highly dynamic and they form upon stress and dissolve thus releasing the repressed mRNAs according to changes in cell physiology. In addition, aggregates containing abnormal proteins are frequent in neurodegenerative disorders. In spite of the growing relevance of these supramolecular aggregates to diverse cellular functions a reliable automated tool for their systematic analysis is lacking. Here we report a MATLAB Script termed BUHO for the high-throughput image analysis of cellular foci. We used BUHO to assess the number, size and distribution of distinct objects with minimal deviation from manually obtained parameters. BUHO successfully addressed the induction of both SGs and PBs in mammalian and insect cells exposed to different stress stimuli. We also used BUHO to assess the dynamics of specific mRNA-silencing foci termed Smaug 1 foci (S-foci) in primary neurons upon synaptic stimulation. Finally, we used BUHO to analyze the role of candidate genes on SG formation in an RNAi-based experiment. We found that FAK56D, GCN2 and PP1 govern SG formation. The role of PP1 is conserved in mammalian cells as judged by the effect of the PP1 inhibitor salubrinal, and involves dephosphorylation of the translation factor eIF2α. All these experiments were analyzed manually and by BUHO and the results differed in less than 5% of the average value. The automated analysis by this user-friendly method will allow high-throughput image processing in short times by providing a robust, flexible and reliable alternative to the laborious and sometimes unfeasible visual scrutiny. © 2012 Perez-Pepe et al.

Descripción: The distribution and abundance of gymnosome gastropods in the Argentine Sea and Brazil--Malvinas Confluence during 1978-1979 and 1988 were studied. The collections analyzed included 768 quantitative samples obtained between 48°W and the coast, and from 35°S to 55°S. Two species were found. Spongiobranchaea australis was the most frequent and abundant (up to 730 per 1000 m3); its presence in the area was associated with the core of the Malvinas Current. Clione antarctica was less abundant (maximum abundance: 230 per 1000 m3) and was also associated with the Malvinas Current. The geographic ranges of both species in the area are wider than previously described. Since the range of S. australis in the area extends far from the range of its prey Clio, it is not clear whether S. australis can feed on the thecosomatous pteropod Limacina (and not only on Clio, as described in the bibliography) or it starves in that area. During the 1978-1979 annual cycle, the abundance of both species followed neither the abundance patterns of their prey nor of the total zooplankton, and differed from each other. The residence time of swarms of both gymnosomes were shorter than one month. As a general pattern, the aggregates are rapidly transported northward by the Malvinas Current and also penetrate the outer shelf water, but they remain there only during a short period and cannot preclude the final expatriation. So, the abundance of gymnosomes in the area depends on passive migration more than intrinsic population factors.

Descripción: Towards the northeast of the Sierra de San Luis several wolfram ore deposits occur. In San Martín district they are divided into three groups: Los Avestruces, La Esperanza and Cerrito Blanco. The geology of this area is formed of the Conlara metamorphic complex composed of schists (with a strike of 300° north), granitoids and pegmatites. In the same area subsaturated chalco-alkaline lamprophyres also appear as subvertical bodies with NW-NE strikes. Toward the boundaries of these rocks, the amount of biotite rises gradually from 80 to 90% developing a biotitite rock due to metasomatic processes and the superimposed deformation. This biotitite is rich in K, Rb and W among other elements. The scheelite mineralization (with quartz, biotite and tourmaline as non ore minerals) occurs in veins that are usually hosted between the lamprophyres and the wall rocks. Scheelite also appears between the schist planes of the biotitite rocks as well as disseminated crystals in quartz aggregates. This mineralization took place from fluids with XH2O: 0.87/0.82 - XCO2: 0.11/0.16 - XNaCl: 0.015/0.013 - (CH4) meanwhile the scheelite, according to the fluid inclusions studies, precipitated from unmixing processes below 320°C and 2.3 kb. Achaliana orogeny and the intrusion of Las Chacras-Piedras Coloradas batholiths (Devonian-Carboniferous) were the responsible for the slight rise of the fluids from the basement, the generation of fault structures through which they rose, the metasomatism and the mineralization.

Descripción: Stress granules (SGs) are ephemeral cytoplasmic aggregates containing stalled translation preinitiation complexes involved in mRNA storage and triage during the cellular stress response. SG formation is triggered by the phosphorylation of the alpha subunit of eIF2 (eIF2α), which provokes a dramatic blockage of protein translation. Our results demonstrate that acute infection of Vero cells with the arenavirus Juni{dotless} ́n (JUNV), aetiological agent of Argentine haemorrhagic fever, does not induce the formation of SGs. Moreover, JUNV negatively modulates SG formation in infected cells stressed with arsenite, and this inhibition correlates with low levels of eIF2α phosphorylation. Transient expression of JUNV nucleoprotein (N) or the glycoprotein precursor (GPC), but not of the matrix protein (Z), inhibits SG formation in a similar manner, comparable to infectious virus. Expression of N and GPC also impaired eIF2α phosphorylation triggered by arsenite. A moderate inhibition of SG formation was also observed when DTT and thapsigargin were employed as stress inducers. In contrast, no inhibition was observed when infected cells were treated with hippuristanol, a translational inhibitor and inducer of SGs that bypasses the requirement for eIF2α phosphorylation. Finally, we analysed SG formation in persistently JUNV-infected cells, where N and GPC are virtually absent and truncated N products are expressed abundantly.Wefound that persistently infected cells show a quite normal response to arsenite, with SG formation comparable to that of uninfected cells. This suggests that the presence of GPC and/or N is crucial to control the stress response upon JUNV infection of Vero cells © 2011 SGM.

Descripción: We carried out molecular dynamics simulation experiments to examine equilibrium and dynamical characteristics of the solvation of excess protons in mesoscopic, [m:n] binary polar clusters comprising m 50 water molecules and n 6, 25, and 100 acetone molecules. Contrasting from what is found in conventional macroscopic phases, the characteristics of the proton solvation are dictated, to a large extent, by the nature of the concentration fluctuations prevailing within the clusters. At low acetone contents, the overall cluster morphology corresponds to a segregated aqueous nucleus coated by an external aprotic phase. Under these circumstances, the proton remains localized at the surface of the water core, in a region locally deprived from acetone molecules. At higher acetone concentrations, we found clear evidence of the onset of the mixing process. The cluster structures present aqueous domains with irregular shape, fully embedded within the acetone phase. Still, the proton remains coordinated to the aqueous phase, with its closest solvation shell composed exclusively by three water molecules. As the relative concentration of acetone increases, the time scales characterizing proton transfer events between neighboring water molecules show considerable retardations, stretching into the nanosecond time domain already for n ∼ 25. In water-rich aggregates, and similarly to what is found in the bulk, proton transfers are controlled by acetone/water exchange processes taking place at the second solvation shell of the proton. As a distinctive feature of the transfer mechanism, translocation pathways also include diffusive motions of the proton from the surface down into inner regions of the underlying water domain. © 2012 American Institute of Physics.