718 research outputs found
Mechanistic investigations into the cyclopropanation of electron deficient alkenes with ethyl diazoacetate using [Co(MeTAA)]
A detailed mechanistic study of the cyclopropanation of electron-deficient alkenes, namely methyl acrylate with ethyl diazoacetate (EDA), was carried out, aiming at understanding both the superior activity and the higher sensitivity of the cobalt(II) tetramethyldibenzotetraaza[14]annulene [Co(MeTAA)] catalyst as compared with cobalt(II) tetraphenylporphyrin [Co(TPP)]. Cobalt(III)-carbene radicals were demonstrated to be present as key intermediates in the reaction, using a combination of kinetic studies, experimental EPR spin-trapping experiments, and supporting DFT studies. Reaction progress was monitored in real time by observing N2 formation and measuring its partial pressure under isothermal conditions. Reaction progress kinetic analysis (RPKA) was used to analyze the experimental data. Results showed that the reaction is first-order in both [catalyst] and [EDA] and zero-order in [methyl acrylate], in agreement with the DFT-calculated mechanism. The calculated activation parameters corresponding to the rate-determining step of the reaction are in agreement with the experimental values, thus providing strong support for the proposed metalloradical mechanism
Energetics and mechanism of drug transport mediated by the lactococcal multidrug transporter LmrP
The gene encoding the secondary multidrug transporter LmrP of Lactococcus lactis was heterologously expressed in Escherichia coli. The energetics and mechanism of drug extrusion mediated by LmrP were studied in membrane vesicles of E. coli, LmrP-mediated extrusion of tetraphenyl phosphonium (TPP+) from right-side-out membrane vesicles and uptake of the fluorescent membrane probe 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) into inside-out membrane vesicles are driven by the membrane potential (Delta psi) and the transmembrane proton gradient (Delta pH), pointing to an electrogenic drug/proton antiport mechanism, Ethidium bromide, a substrate for LmrP, inhibited the LmrP-mediated TPP+ extrusion from right-side-out membrane vesicles, showing that LmrP is capable of transporting structurally unrelated drugs. Kinetic analysis of LmrP-mediated TMA-DPH transport revealed a direct relation between the transport rate and the amount of TMA-DPH associated with the cytoplasmic leaflet of the lipid bilayer. This observation indicates that drugs are extruded from the inner leaflet of the cytoplasmic membrane into the external medium. This is the first report that shows that drug extrusion by a secondary multidrug resistance (MDR) transporter occurs by a ''hydrophobic vacuum cleaner'' mechanism in a similar way as was proposed for the primary lactococcal MDR transporter, LmrA
Generalized thermodynamics and Fokker-Planck equations. Applications to stellar dynamics, two-dimensional turbulence and Jupiter's great red spot
We introduce a new set of generalized Fokker-Planck equations that conserve
energy and mass and increase a generalized entropy until a maximum entropy
state is reached. The concept of generalized entropies is rigorously justified
for continuous Hamiltonian systems undergoing violent relaxation. Tsallis
entropies are just a special case of this generalized thermodynamics.
Application of these results to stellar dynamics, vortex dynamics and Jupiter's
great red spot are proposed. Our prime result is a novel relaxation equation
that should offer an easily implementable parametrization of geophysical
turbulence. This relaxation equation depends on a single key parameter related
to the skewness of the fine-grained vorticity distribution. Usual
parametrizations (including a single turbulent viscosity) correspond to the
infinite temperature limit of our model. They forget a fundamental systematic
drift that acts against diffusion as in Brownian theory. Our generalized
Fokker-Planck equations may have applications in other fields of physics such
as chemotaxis for bacterial populations. We propose the idea of a
classification of generalized entropies in classes of equivalence and provide
an aesthetic connexion between topics (vortices, stars, bacteries,...) which
were previously disconnected.Comment: Submitted to Phys. Rev.
Scaling laws and vortex profiles in 2D decaying turbulence
We use high resolution numerical simulations over several hundred of turnover
times to study the influence of small scale dissipation onto vortex statistics
in 2D decaying turbulence. A self-similar scaling regime is detected when the
scaling laws are expressed in units of mean vorticity and integral scale, as
predicted by Carnevale et al., and it is observed that viscous effects spoil
this scaling regime. This scaling regime shows some trends toward that of the
Kirchhoff model, for which a recent theory predicts a decay exponent .
In terms of scaled variables, the vortices have a similar profile close to a
Fermi-Dirac distribution.Comment: 4 Latex pages and 4 figures. Submitted to Phys. Rev. Let
Relaxation equations for two-dimensional turbulent flows with a prior vorticity distribution
Using a Maximum Entropy Production Principle (MEPP), we derive a new type of
relaxation equations for two-dimensional turbulent flows in the case where a
prior vorticity distribution is prescribed instead of the Casimir constraints
[Ellis, Haven, Turkington, Nonlin., 15, 239 (2002)]. The particular case of a
Gaussian prior is specifically treated in connection to minimum enstrophy
states and Fofonoff flows. These relaxation equations are compared with other
relaxation equations proposed by Robert and Sommeria [Phys. Rev. Lett. 69, 2776
(1992)] and Chavanis [Physica D, 237, 1998 (2008)]. They can provide a
small-scale parametrization of 2D turbulence or serve as numerical algorithms
to compute maximum entropy states with appropriate constraints. We perform
numerical simulations of these relaxation equations in order to illustrate
geometry induced phase transitions in geophysical flows.Comment: 21 pages, 9 figure
Slow relaxation in the two dimensional electron plasma under the strong magnetic field
We study slow relaxation processes in the point vortex model for the
two-dimensional pure electron plasma under the strong magnetic field. By
numerical simulations, it is shown that, from an initial state, the system
undergoes the fast relaxation to a quasi-stationary state, and then goes
through the slow relaxation to reach a final state. From analysis of simulation
data, we find (i) the time scale of the slow relaxation increases linearly to
the number of electrons if it is measured by the unit of the bulk rotation
time, (ii) during the slow relaxation process, each electron undergoes an
superdiffusive motion, and (iii) the superdiffusive motion can be regarded as
the Levy flight, whose step size distribution is of the power law. The time
scale that each electron diffuses over the system size turns out to be much
shorter than that of the slow relaxation, which suggests that the correlation
among the superdiffusive trajectories is important in the slow relaxation
process.Comment: 11pages, 19 figures. Submitted to J. Phys. Soc. Jp
Decreased parenchymal arteriolar tone uncouples vessel-to-neuronal communication in a mouse model of vascular cognitive impairment
Chronic hypoperfusion is a key contributor to cognitive decline and neurodegenerative conditions, but the cellular mechanisms remain ill-defined. Using a multidisciplinary approach, we sought to elucidate chronic hypoperfusion-evoked functional changes at the neurovascular unit. We used bilateral common carotid artery stenosis (BCAS), a well-established model of vascular cognitive impairment, combined with an ex vivo preparation that allows pressurization of parenchymal arterioles in a brain slice. Our results demonstrate that mild (~ 30%), chronic hypoperfusion significantly altered the functional integrity of the cortical neurovascular unit. Although pial cerebral perfusion recovered over time, parenchymal arterioles progressively lost tone, exhibiting significant reductions by day 28 post-surgery. We provide supportive evidence for reduced adenosine 1 receptor-mediated vasoconstriction as a potential mechanism in the adaptive response underlying the reduced baseline tone in parenchymal arterioles. In addition, we show that in response to the neuromodulator adenosine, the action potential frequency of cortical pyramidal neurons was significantly reduced in all groups. However, a significant decrease in adenosine-induced hyperpolarization was observed in BCAS 14Â days. At the microvascular level, constriction-induced inhibition of pyramidal neurons was significantly compromised in BCAS mice. Collectively, these results suggest that BCAS uncouples vessel-to-neuron communicationâvasculo-neuronal couplingâa potential early event in cognitive decline.Fil: Kim, Ki Jung. Augusta University. Departament of Physiology; Estados UnidosFil: Diaz, Juan Ramiro. Augusta University. Departament of Physiology; Estados UnidosFil: Presa, Jessica Lorena. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Instituto de BiologĂa y Medicina Experimental. FundaciĂłn de Instituto de BiologĂa y Medicina Experimental. Instituto de BiologĂa y Medicina Experimental; Argentina. Augusta University. Departament of Physiology; Estados UnidosFil: Muller, P. Robinson. Augusta University. Departament of Physiology; Estados UnidosFil: Brands, Michael W.. Augusta University. Departament of Physiology; Estados UnidosFil: Khan, Mohammad B.. Augusta University. Medical College of Georgia; Estados UnidosFil: Hess, David C.. Augusta University. Medical College of Georgia; Estados UnidosFil: Althammer, Ferdinand. Georgia State University; Estados UnidosFil: Stern, Javier E.. Georgia State University; Estados UnidosFil: Filosa, Jessica A.. Augusta University. Departament of Physiology; Estados Unido
Simplified instantaneous non-local quantum computation with applications to position-based cryptography
Instantaneous measurements of non-local observables between space-like
separated regions can be performed without violating causality. This feat
relies on the use of entanglement. Here we propose novel protocols for this
task and the related problem of multipartite quantum computation with local
operations and a single round of classical communication. Compared to
previously known techniques, our protocols reduce the entanglement consumption
by an exponential amount. We also prove a linear lower bound on the amount of
entanglement required for the implementation of a certain non-local
measurement.
These results relate to position-based cryptography: an amount of
entanglement scaling exponentially in the number of communicated qubits is
sufficient to render any such scheme insecure. Furthermore, we show that
certain schemes are secure under the assumption that the adversary has less
entanglement than a given linear bound and is restricted to classical
communication.Comment: 18 pages, 2 figures. V2: clarified results of the crypto section,
improved readabilit
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