301 research outputs found
Generic phase diagram of active polar films
We study theoretically the phase diagram of compressible active polar gels
such as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations, we perform a linear stability analysis of the uniform states in the
case of an infinite bidimensional active gel to obtain the dynamic phase
diagram of active polar films. We predict in particular modulated flowing
phases, and a macroscopic phase separation at high activity. This qualitatively
accounts for experimental observations of various active systems, such as
acto-myosin gels, microtubules and kinesins in vitro solutions, or swimming
bacterial colonies.Comment: 4 pages, 1 figur
Nonequilibrium Fluctuations, Travelling Waves, and Instabilities in Active Membranes
The stability of a flexible fluid membrane containing a distribution of
mobile, active proteins (e.g. proton pumps) is shown to depend on the structure
and functional asymmetry of the proteins. A stable active membrane is in a
nonequilibrium steady state with height fluctuations whose statistical
properties are governed by the protein activity. Disturbances are predicted to
travel as waves at sufficiently long wavelength, with speed set by the normal
velocity of the pumps. The unstable case involves a spontaneous, pump-driven
undulation of the membrane, with clumping of the proteins in regions of high
activity.Comment: 4 two-column pages, two .eps figures included, revtex, uses eps
Mechanochemical action of the dynamin protein
Dynamin is a ubiquitous GTPase that tubulates lipid bilayers and is
implicated in many membrane severing processes in eukaryotic cells. Setting the
grounds for a better understanding of this biological function, we develop a
generalized hydrodynamics description of the conformational change of large
dynamin-membrane tubes taking into account GTP consumption as a free energy
source. On observable time scales, dissipation is dominated by an effective
dynamin/membrane friction and the deformation field of the tube has a simple
diffusive behavior, which could be tested experimentally. A more involved,
semi-microscopic model yields complete predictions for the dynamics of the tube
and possibly accounts for contradictory experimental results concerning its
change of conformation as well as for plectonemic supercoiling.Comment: 17 pages, 4 figures; typos corrected, reference adde
Energy Transduction of Isothermal Ratchets: Generic Aspects and Specific Examples Close to and Far from Equilibrium
We study the energetics of isothermal ratchets which are driven by a chemical
reaction between two states and operate in contact with a single heat bath of
constant temperature. We discuss generic aspects of energy transduction such as
Onsager relations in the linear response regime as well as the efficiency and
dissipation close to and far from equilibrium. In the linear response regime
where the system operates reversibly the efficiency is in general nonzero.
Studying the properties for specific examples of energy landscapes and
transitions, we observe in the linear response regime that the efficiency can
have a maximum as a function of temperature. Far from equilibrium in the fully
irreversible regime, we find a maximum of the efficiency with values larger
than in the linear regime for an optimal choice of the chemical driving force.
We show that corresponding efficiencies can be of the order of 50%. A simple
analytic argument allows us to estimate the efficiency in this irreversible
regime for small external forces.Comment: 16 pages, 10 figure
International Consensus Conference on Atopic Dermatitis II (ICCAD II * ): clinical update and current treatment strategies
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74915/1/j.1365-2133.148.s63.1.x.pd
Animated molecular dynamics simulations of hydrated caesium-smectite interlayers
Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs(+ )formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs(+ )within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs(+ )for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs(+ )and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output
Polydispersity and ordered phases in solutions of rodlike macromolecules
We apply density functional theory to study the influence of polydispersity
on the stability of columnar, smectic and solid ordering in the solutions of
rodlike macromolecules. For sufficiently large length polydispersity (standard
deviation ) a direct first-order nematic-columnar transition is
found, while for smaller there is a continuous nematic-smectic and
first-order smectic-columnar transition. For increasing polydispersity the
columnar structure is stabilized with respect to solid perturbations. The
length distribution of macromolecules changes neither at the nematic-smectic
nor at the nematic-columnar transition, but it does change at the
smectic-columnar phase transition. We also study the phase behaviour of binary
mixtures, in which the nematic-smectic transition is again found to be
continuous. Demixing according to rod length in the smectic phase is always
preempted by transitions to solid or columnar ordering.Comment: 13 pages (TeX), 2 Postscript figures uuencode
Feasibility of Onchocerciasis Elimination with Ivermectin Treatment in Endemic Foci in Africa: First Evidence from Studies in Mali and Senegal
The control of onchocerciasis, or river blindness, is based on annual or six-monthly ivermectin treatment of populations at risk. This has been effective in controlling the disease as a public health problem, but it is not known whether it can also eliminate infection and transmission to the extent that treatment can be safely stopped. Many doubt that this is feasible in Africa. A study was undertaken in three hyperendemic onchocerciasis foci in Mali and Senegal where treatment has been given for 15 to 17 years. The results showed that only few infections remained in the human population and that transmission levels were everywhere below postulated thresholds for elimination. Treatment was subsequently stopped in test areas in each focus, and follow-up evaluations did not detect any recrudescence of infection or transmission. Hence, the study has provided the first evidence that onchocerciasis elimination is feasible with ivermectin treatment in some endemic foci in Africa. Although further studies are needed to determine to what extent these findings can be extrapolated to other areas in Africa, the principle of onchocerciasis elimination with ivermectin treatment has been established
Topical immunotherapy of severe alopecia areata with diphenylcyclopropenone (DPCP): experience in an Iranian population
BACKGROUND: Highly variable results of topical diphenylcyclopropenone (DPCP) in the treatment of alopecia areata have been reported so far. The purposes of the present study were to evaluate the efficacy and tolerability of DPCP treatment in severe alopecia areata. METHODS: Twenty-eight patients (16 female and 12 male, 10–35 years old, mean age 25 years) with extensive alopecia areata were enrolled in an open-label clinical trial. After sensitization with 2% DPCP, progressively higher concentrations beginning at 0.001% were applied weekly for 6 months to one side of the scalp, after which, if terminal hair growth was noted, the entire scalp was then treated under the same weekly protocol. The maximum concentration of DPCP in acetone was 2%. RESULTS: Twenty-seven of 28 patients completed therapy. The overall response rate was 81.5% (22/27), complete remission (90%-100% terminal hair re-growth) was obtained 22.2% (6/27) and partial remission (10%-90% terminal hair re-growth) in 59.3% (16/27). In all patients an eczematous reaction consisting of erythema, itching, and scaling at the site of application were observed. During therapy, other side effects including, occipital lymphadenopathy 40.7% (11/27), severe eczema/blister formation 40.7% (11/27), hyperpigmentation 18.5% (5/27) were observed, but no hypopigmentation, vitiligo, contact urticaria, and erythema multiforme-like reaction were seen in the patients. Nineteen of 27 (70.4%) patients had at least one side effect, other than eczematous reaction. Notably, partial recurrence was observed in 50.9% (13/22) of these patients after 6 to 12 months of follow-up. During the follow-up time the maintenance DPCP immunotherapy was continued. CONCLUSION: Topical DPCP treatment for alopecia areata is an effective therapy with a slightly high relapse rate during bilateral maintenance treatment. According to the author's knowledge this is the first experience with DPCP in Iran
Quantum dynamics in strong fluctuating fields
A large number of multifaceted quantum transport processes in molecular
systems and physical nanosystems can be treated in terms of quantum relaxation
processes which couple to one or several fluctuating environments. A thermal
equilibrium environment can conveniently be modelled by a thermal bath of
harmonic oscillators. An archetype situation provides a two-state dissipative
quantum dynamics, commonly known under the label of a spin-boson dynamics. An
interesting and nontrivial physical situation emerges, however, when the
quantum dynamics evolves far away from thermal equilibrium. This occurs, for
example, when a charge transferring medium possesses nonequilibrium degrees of
freedom, or when a strong time-dependent control field is applied externally.
Accordingly, certain parameters of underlying quantum subsystem acquire
stochastic character. Herein, we review the general theoretical framework which
is based on the method of projector operators, yielding the quantum master
equations for systems that are exposed to strong external fields. This allows
one to investigate on a common basis the influence of nonequilibrium
fluctuations and periodic electrical fields on quantum transport processes.
Most importantly, such strong fluctuating fields induce a whole variety of
nonlinear and nonequilibrium phenomena. A characteristic feature of such
dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres
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