180 research outputs found
Pattern formation in 2-frequency forced parametric waves
We present an experimental investigation of superlattice patterns generated
on the surface of a fluid via parametric forcing with 2 commensurate
frequencies. The spatio-temporal behavior of 4 qualitatively different types of
superlattice patterns is described in detail. These states are generated via a
number of different 3--wave resonant interactions. They occur either as
symmetry--breaking bifurcations of hexagonal patterns composed of a single
unstable mode or via nonlinear interactions between the two primary unstable
modes generated by the two forcing frequencies. A coherent picture of these
states together with the phase space in which they appear is presented. In
addition, we describe a number of new superlattice states generated by 4--wave
interactions that arise when symmetry constraints rule out 3--wave resonances.Comment: The paper contains 34 pages and 53 figures and provides an extensive
review of both the theoretical and experimental work peformed in this syste
Anthropogenic Space Weather
Anthropogenic effects on the space environment started in the late 19th
century and reached their peak in the 1960s when high-altitude nuclear
explosions were carried out by the USA and the Soviet Union. These explosions
created artificial radiation belts near Earth that resulted in major damages to
several satellites. Another, unexpected impact of the high-altitude nuclear
tests was the electromagnetic pulse (EMP) that can have devastating effects
over a large geographic area (as large as the continental United States). Other
anthropogenic impacts on the space environment include chemical release ex-
periments, high-frequency wave heating of the ionosphere and the interaction of
VLF waves with the radiation belts. This paper reviews the fundamental physical
process behind these phenomena and discusses the observations of their impacts.Comment: 71 pages, 35 figure
Mapping the internal recognition surface of an octanuclear coordination cage using guest libraries
Size and shape criteria for guest binding inside the cavity of an octanuclear cubic coordination cage in water have been established using a new fluorescence displacement assay to quantify guest binding. For aliphatic cyclic ketones of increasing size (from C5 to C11), there is a linear relationship between ΔG for guest binding and the guest’s surface area: the change in ΔG for binding is 0.3 kJ mol–1 Å–2, corresponding to 5 kJ mol–1 for each additional CH2 group in the guest, in good agreement with expectations based on hydrophobic desolvation. The highest association constant is K = 1.2 × 106 M–1 for cycloundecanone, whose volume is approximately 50% of the cavity volume; for larger C12 and C13 cyclic ketones, the association constant progressively decreases as the guests become too large. For a series of C10 aliphatic ketones differing in shape but not size, ΔG for guest binding showed no correlation with surface area. These guests are close to the volume limit of the cavity (cf. Rebek’s 55% rule), so the association constant is sensitive to shape complementarity, with small changes in guest structure resulting in large changes in binding affinity. The most flexible members of this series (linear aliphatic ketones) did not bind, whereas the more preorganized cyclic ketones all have association constants of 104–105 M–1. A crystal structure of the cage·cycloundecanone complex shows that the guest carbonyl oxygen is directed into a binding pocket defined by a convergent set of CH groups, which act as weak hydrogen-bond donors, and also shows close contacts between the exterior surface of the disc-shaped guest and the interior surface of the pseudospherical cage cavity despite the slight mismatch in shape
An Interconverting Family of Coordination Cages and a meso-Helicate; Effects of Temperature, Concentration, and Solvent on the Product Distribution of a Self-Assembly Process
The
self-assembly between a water-soluble bis-bidentate ligand
L<sup>18w</sup> and Co(II) salts in water affords three high-spin
Co(II) products: a dinuclear <i>meso</i>-helicate [Co<sub>2</sub>(L<sup>18w</sup>)<sub>3</sub>]X<sub>4</sub>; a tetrahedral
cage [Co<sub>4</sub>(L<sup>18w</sup>)<sub>6</sub>]X<sub>8</sub>; and
a dodecanuclear truncated-tetrahedral cage [Co<sub>12</sub>(L<sup>18w</sup>)<sub>18</sub>]X<sub>24</sub> (X = BF<sub>4</sub> or ClO<sub>4</sub>). All three products were crystallized under different conditions
and structurally characterized. In [Co<sub>2</sub>(L<sup>18w</sup>)<sub>3</sub>]X<sub>4</sub> all three bridging ligands span a pair
of metal ions; in the two larger products, there is a metal ion at
each vertex of the Co<sub>4</sub> or Co<sub>12</sub> polyhedral cage
array with a bridging ligand spanning a pair of metal ions along every
edge. All three structural types are known: what is unusual here is
the presence of all three from the same reaction. The assemblies <b>Co</b><sub><b>2</b></sub>, <b>Co</b><sub><b>4</b></sub>, and <b>Co</b><sub><b>12</b></sub> are in slow
equilibrium (hours/days) in aqueous solution, and this can be conveniently
monitored by <sup>1</sup>H NMR spectroscopy because (i) the paramagnetism
of Co(II) disperses the signals over a range of ca. 200 ppm and (ii)
the different symmetries of the three species give characteristically
different numbers of independent <sup>1</sup>H NMR signals, which
makes identification easy. From temperature- and concentration-dependent <sup>1</sup>H NMR studies it is clear that increasing temperature and
increasing dilution favors fragmentation to give a larger proportion
of the smaller assemblies for entropic reasons. High concentrations
and low temperature favor the larger assembly despite the unfavorable
entropic and electrostatic factors associated with its formation.
We suggest that this arises from the hydrophobic effect: reorganization
of several smaller complexes into one larger one results in a smaller
proportion of the hydrophobic ligand surface being exposed to water,
with a larger proportion of the ligand surface protected in the interior
of the assembly. In agreement with this, <sup>1</sup>H NMR spectra
in a nonaqueous solvent (MeNO<sub>2</sub>) show formation of only
[Co<sub>2</sub>(L<sup>18w</sup>)<sub>3</sub>]X<sub>4</sub> because
the driving force for reorganization into larger assemblies is now
absent. Thus, we can identify the contributions of temperature, concentration,
and solvent on the result of the metal/ligand self-assembly process
and have determined the speciation behavior of the <b>Co</b><sub><b>2</b></sub>/<b>Co</b><sub><b>4</b></sub>/<b>Co</b><sub><b>12</b></sub> system in aqueous solution
Identification of a structural motif crucial for infectivity of hepatitis B viruses
Infectious entry of hepatitis B viruses (HBV) has nonconventional facets. Here we analyzed whether a cell-permeable peptide [translocation motif (TLM)] identified within the surface protein of human HBV is a general feature of all hepadnaviruses and plays a role in the viral life cycle. Surface proteins of all hepadnaviruses contain conserved functional TLMs. Genetic inactivation of the duck HBV TLMs does not interfere with viral morphogenesis; however, these mutants are noninfectious. TLM mutant viruses bind to cells and are taken up into the endosomal compartment, but they cannot escape from endosomes. Processing of surface protein by endosomal proteases induces their exposure on the virus surface. This unmasking of TLMs mediates translocation of viral particles across the endosomal membrane into the cytosol, a prerequisite for productive infection. The ability of unmasked TLMs to translocate processed HBV particles across cellular membranes was shown by confocal immunofluorescence microscopy and by infection of nonpermissive cell lines with HBV processed in vitro with endosomal lysate. Based on these data, we propose an infectious entry mechanism unique for hepadnaviruses that involves virus internalization by receptor-mediated endocytosis followed by processing of surface protein in endosomes. This processing activates the function of TLMs that are essential for viral particle translocation through the endosomal membrane into the cytosol and productive infection
Optimized production strategy of the major capsid protein HPV 16L1 non-assembly variant in E. coli
The capsid of human papillomavirus (HPV) consists of two capsid proteins - the major capsid protein L1 and the minor capsid protein L2. Assembled virus-like particles, which only consist of L1 proteins, are successfully applied as prophylactic vaccines against HPV infections. The capsid subunits are L1-pentamers, which are also reported to protect efficiently against HPV infections in animals. The recombinant production of L1 has been previously shown in E. coli, yeast, insect cells, plants and mammalian cell culture. Principally, in E. coli-based expression system L1 shows high expression yields but the protein is largely insoluble. In order to overcome this problem reported strategies address fusion proteins and overexpression of bacterial chaperones. However, an insufficient cleavage of the fusion proteins and removal of co-purified chaperones can hamper subsequent down streaming. We report a significant improvement in the production of soluble L1-pentamers by combining (I) a fusion of a N-terminal SUMO-tag to L1, (II) the heterologous co-expression of the chaperon system GroEL/ES and (III) low expression temperature. The fusion construct was purified in a 2-step protein purification including efficient removal of GroEL/ES and complete removal of the N-terminal SUMO-tag. The expression strategy was transferred to process-controlled high-cell-density fermentation with defined media according to the guidelines of good manufacturing practice. The produced L1 protein is highly pure (>95%), free of DNA (260:280 = 0.5) and pentameric. The production strategy yielded 5.73 mg of purified L1-pentamers per gram dry biomass. The optimized strategy is a suitable alternative for high yield L1-pentamer production and purification as a cheaper process for vaccine production
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