1,448 research outputs found
Unidirectional Reconstitution into Detergent-destabilized Liposomes of the Purified Lactose Transport System of Streptococcus thermophilus
The lactose transport protein (LacS) of Streptococcus thermophilus was amplified to levels as high as 8 and 30% of total membrane protein in Escherichia coli and S. thermophilus, respectively. In both organisms the protein was functional and the expression levels were highest with the streptococcal lacS promoter. Also a LacS deletion mutant, lacking the carboxyl-terminal regulatory domain, could be amplified to levels >20% of membrane protein. Membranes from S. thermophilus proved to be superior in terms of efficient solubilization and ease and extent of purification of LacS; >95% of LacS was solubilized with relatively low concentrations of Triton X-100, n-octyl-ÎČ-D-glucoside, n-dodecyl-ÎČ-D-maltoside, or C12E8. The LacS protein carrying a poly-histidine tag was purified in large quantities (~5 mg/liter of culture) and with a purity >98% in a two-step process involving nickel chelate affinity and anion exchange chromatography. The membrane reconstitution of LacS was studied systematically by stepwise solubilization of preformed liposomes, prepared from E. coli phospholipid and phosphatidylcholine, and protein incorporation at the different stages of liposome solubilization. The detergents were removed by adsorption onto polystyrene beads and H+-lactose symport and lactose counterflow were measured. Highest transport activities were obtained when Triton X-100 was used throughout the solubilization/purification procedure, whereas activity was lost irreversibly with n-octyl-ÎČ-D-glucoside. For reconstitutions mediated by n-dodecyl-ÎČ-D-maltoside, C12E8, and to a lesser extent Triton X-100, the highest transport activities were obtained when the liposomes were titrated with low amounts of detergent (onset of liposome solubilization). Importantly, under these conditions proteoliposomes were obtained in which LacS was reconstituted in an inside-out orientation, as suggested by the outside labeling of a single cysteine mutant with a membrane impermeable biotin-maleimide. The results are consistent with a mechanism of reconstitution in which the hydrophilic regions of LacS prevent a random insertion of the protein into the membrane. Consistent with the in vivo lactose/galactose exchange catalyzed by the LacS protein, the maximal rate of lactose counterflow was almost 2 orders of magnitude higher than that of H+-lactose symport.
The isolation and purification of the elvapeptins A family of peptide inhibitors of mitochondrial ATPase activity
Node-weighted measures for complex networks with spatially embedded, sampled, or differently sized nodes
When network and graph theory are used in the study of complex systems, a
typically finite set of nodes of the network under consideration is frequently
either explicitly or implicitly considered representative of a much larger
finite or infinite region or set of objects of interest. The selection
procedure, e.g., formation of a subset or some kind of discretization or
aggregation, typically results in individual nodes of the studied network
representing quite differently sized parts of the domain of interest. This
heterogeneity may induce substantial bias and artifacts in derived network
statistics. To avoid this bias, we propose an axiomatic scheme based on the
idea of node splitting invariance to derive consistently weighted variants of
various commonly used statistical network measures. The practical relevance and
applicability of our approach is demonstrated for a number of example networks
from different fields of research, and is shown to be of fundamental importance
in particular in the study of spatially embedded functional networks derived
from time series as studied in, e.g., neuroscience and climatology.Comment: 21 pages, 13 figure
On the validity of the Boltzmann equation to describe low density granular systems
The departure of a granular gas in the instable region of parameters from the
initial homogeneous cooling state is studied. Results from Molecular Dynamics
and from Direct Monte Carlo simulation of the Boltzmann equation are compared.
It is shown that the Boltzmann equation accurately predicts the low density
limit of the system. The relevant role played by the parallelization of the
velocities as time proceeds and the dependence of this effect on the density is
analyzed in detail
A Hedged Monte Carlo Approach to Real Option Pricing
In this work we are concerned with valuing optionalities associated to invest
or to delay investment in a project when the available information provided to
the manager comes from simulated data of cash flows under historical (or
subjective) measure in a possibly incomplete market. Our approach is suitable
also to incorporating subjective views from management or market experts and to
stochastic investment costs. It is based on the Hedged Monte Carlo strategy
proposed by Potters et al (2001) where options are priced simultaneously with
the determination of the corresponding hedging. The approach is particularly
well-suited to the evaluation of commodity related projects whereby the
availability of pricing formulae is very rare, the scenario simulations are
usually available only in the historical measure, and the cash flows can be
highly nonlinear functions of the prices.Comment: 25 pages, 14 figure
Renormalized Path Integral for the Two-Dimensional Delta-Function Interaction
A path-integral approach for delta-function potentials is presented.
Particular attention is paid to the two-dimensional case, which illustrates the
realization of a quantum anomaly for a scale invariant problem in quantum
mechanics. Our treatment is based on an infinite summation of perturbation
theory that captures the nonperturbative nature of the delta-function bound
state. The well-known singular character of the two-dimensional delta-function
potential is dealt with by considering the renormalized path integral resulting
from a variety of schemes: dimensional, momentum-cutoff, and real-space
regularization. Moreover, compatibility of the bound-state and scattering
sectors is shown.Comment: 26 pages. The paper was significantly expanded and numerous equations
were added for the sake of clarity; the main results and conclusions are
unchange
RX J1856.5-3754 as a possible Strange Star candidate
RX J1856.5-3754 has been proposed as a strange star candidate due to its very
small apparent radius measured from its X-ray thermal spectrum. However, its
optical emission requires a much larger radius and thus most of the stellar
surface must be cold and undetectable in X-rays. In the case the star is a
neutron star such a surface temperature distribution can be explained by the
presence of a strong toroidal field in the crust (Perez-Azorin et al. 2006,
Geppert et al. 2006). We consider a similar scenario for a strange star with a
thin baryonic crust to determine if such a magnetic field induced effect is
still possible.Comment: 4 pages, 4 figures; to appear in proceedings of the conference
"Isolated Neutron Stars: From the Interior to the Surface", eds. S. Zane, R.
Turolla, D. Page; Astrophysics & Space Science in pres
Fusion evaporation-residue cross sections for Si28+40Ca at E(28Si)=309, 397, and 452 MeV
Velocity distributions of mass-identified evaporation residues produced in the Si28+40Ca reaction have been measured at bombarding energies of 309, 397, and 452 MeV using time-of-flight techniques. These distributions were used to identify evaporation residues and to separate the complete-fusion and incomplete-fusion components. Angular distributions and upper limits for the total evaporation-residue and complete-fusion evaporation-residue cross sections were extracted at all three bombarding energies. The complete-fusion evaporation-residue cross sections and the deduced critical angular momenta are compared with earlier measurements and the predictions of existing models. The ratios of the complete-fusion evaporation-residue cross section to the total evaporation-residue cross section, along with those measured for the Si28+12C and Si28+28Si systems at the same energies, support the entrance-channel mass-asymmetry dependence of the incomplete-fusion evaporation-residue process reported earlier
C10 and 11B+ 12C reactions from 4 to 9 MeV/nucleon
Reaction products arising from the interaction of 11B+ 12C and 10B+ 13C have been studied in the energy range 4<Elab(B)<9 MeV/nucleon. From the total fusion cross sections for the two entrance channels, the critical angular momenta have been extracted and then compared as a function of compound nucleus excitation energy. Even though a limitation in the fusion cross section was observed, no common limitation was found in the critical angular momenta for these two systems up to at least a Na23 excitation energy of 60 MeV. Above this excitation energy, the experimental uncertainties make this point less clear. Up to an excitation energy of 60 MeV in Na23, a fusion limitation based on reaching a critical density of compound nucleus states like the yrast or ''statistical'' yrast line cannot be responsible for the fusion cross section limitations observed for these entrance channels. The present data suggest that competing entrance channel processes are responsible for the observed fusion cross section limitations
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