318 research outputs found
Molecular Chemical Engines: Pseudo-Static Processes and the Mechanism of Energy Transduction
We propose a simple theoretical model for a molecular chemical engine that
catalyzes a chemical reaction and converts the free energy released by the
reaction into mechanical work. Binding and unbinding processes of reactant and
product molecules to and from the engine are explicitly taken into account. The
work delivered by the engine is calculated analytically for infinitely slow
(``pseudo-static'') processes, which can be reversible (quasi-static) or
irreversible, controlled by an external agent. It is shown that the work larger
than the maximum value limited by the second law of thermodynamics can be
obtained in a single cycle of operation by chance, although the statistical
average of the work never exceeds this limit and the maximum work is delivered
if the process is reversible. The mechanism of the energy transductionis also
discussed.Comment: 8 pages, 3 figues, submitted to J. Phys. Soc. Jp
Efficiency of Energy Transduction in a Molecular Chemical Engine
A simple model of the two-state ratchet type is proposed for molecular
chemical engines that convert chemical free energy into mechanical work and
vice versa. The engine works by catalyzing a chemical reaction and turning a
rotor. Analytical expressions are obtained for the dependences of rotation and
reaction rates on the concentrations of reactant and product molecules, from
which the performance of the engine is analyzed. In particular, the efficiency
of energy transduction is discussed in some detail.Comment: 4 pages, 4 fugures; title modified, figures 2 and 3 modified, content
changed (pages 1 and 4, mainly), references adde
Direct Observation of the Myosin Va Recovery Stroke That Contributes to Unidirectional Stepping along Actin
Myosins are ATP-driven linear molecular motors that work as cellular force
generators, transporters, and force sensors. These functions are driven by
large-scale nucleotide-dependent conformational changes, termed
“strokes”; the “power stroke” is the force-generating
swinging of the myosin light chain–binding “neck” domain
relative to the motor domain “head” while bound to actin; the
“recovery stroke” is the necessary initial motion that primes, or
“cocks,” myosin while detached from actin. Myosin Va is a processive
dimer that steps unidirectionally along actin following a “hand over
hand” mechanism in which the trailing head detaches and steps forward
∼72 nm. Despite large rotational Brownian motion of the detached head about
a free joint adjoining the two necks, unidirectional stepping is achieved, in
part by the power stroke of the attached head that moves the joint forward.
However, the power stroke alone cannot fully account for preferential forward
site binding since the orientation and angle stability of the detached head,
which is determined by the properties of the recovery stroke, dictate actin
binding site accessibility. Here, we directly observe the recovery stroke
dynamics and fluctuations of myosin Va using a novel, transient caged
ATP-controlling system that maintains constant ATP levels through stepwise
UV-pulse sequences of varying intensity. We immobilized the neck of monomeric
myosin Va on a surface and observed real time motions of bead(s) attached
site-specifically to the head. ATP induces a transient swing of the neck to the
post-recovery stroke conformation, where it remains for ∼40 s, until ATP
hydrolysis products are released. Angle distributions indicate that the
post-recovery stroke conformation is stabilized by ≥5
kBT of energy. The high kinetic
and energetic stability of the post-recovery stroke conformation favors
preferential binding of the detached head to a forward site 72 nm away. Thus,
the recovery stroke contributes to unidirectional stepping of myosin Va
One-loop QCD Corrections to Top Quark Decay into a Neutralino and Light Stop
We calculate the one-loop QCD corrections to using dimensional reduction scheme, including QCD and supersymmetric
QCD corrections. The analytic expressions for the corrections to the decay
width are given, which can easily be extended to . The numerical results show that the correction amounts to more
than a 10\% reduction in the partial width relative to the tree level result.
We also compare the corrections in the no-mixing stop case with those in the
mixing stop case.Comment: 13 pages, 5 figures, revised version(using dimensional reduction
technique, the corrections have been recalculated
The dynamic stator stalk of rotary ATPases
Rotary ATPases couple ATP hydrolysis/synthesis with proton translocation across biological membranes and so are central components of the biological energy conversion machinery. Their peripheral stalks are essential components that counteract torque generated by rotation of the central stalk during ATP synthesis or hydrolysis. Here we present a 2.25-Å resolution crystal structure of the peripheral stalk from Thermus thermophilus A-type ATPase/synthase. We identify bending and twisting motions inherent within the structure that accommodate and complement a radial wobbling of the ATPase headgroup as it progresses through its catalytic cycles, while still retaining azimuthal stiffness necessary to counteract rotation of the central stalk. The conformational freedom of the peripheral stalk is dictated by its unusual right-handed coiled-coil architecture, which is in principle conserved across all rotary ATPases. In context of the intact enzyme, the dynamics of the peripheral stalks provides a potential mechanism for cooperativity between distant parts of rotary ATPases
Diffusion-Weighted MRI for Verification of Electroporation-Based Treatments
Clinical electroporation (EP) is a rapidly advancing treatment modality that uses electric pulses to introduce drugs or genes into, e.g., cancer cells. The indication of successful EP is an instant plasma membrane permeabilization in the treated tissue. A noninvasive means of monitoring such a tissue reaction represents a great clinical benefit since, in case of target miss, retreatment can be performed immediately. We propose diffusion-weighted magnetic resonance imaging (DW-MRI) as a method to monitor EP tissue, using the concept of the apparent diffusion coefficient (ADC). We hypothesize that the plasma membrane permeabilization induced by EP changes the ADC, suggesting that DW-MRI constitutes a noninvasive and quick means of EP verification. In this study we performed in vivo EP in rat brains, followed by DW-MRI using a clinical MRI scanner. We found a pulse amplitude–dependent increase in the ADC following EP, indicating that (1) DW-MRI is sensitive to the EP-induced changes and (2) the observed changes in ADC are indeed due to the applied electric field
Exploring metabolic responses of potato tissue induced by electric pulses
In this study, we investigated the metabolic
responses of potato tissue induced by pulsed electric field
(PEF). Potato tissue was subjected to field strengths ranging
from 30 to 500 V/cm, with a single rectangular pulse of 10 μs,
100 μs, or 1 ms. Metabolic responses were monitored using
isothermal calorimetry, changes on electrical resistance during
the delivery of the pulse, as well as impedance measurements.
Our results show that the metabolic response involves oxygen
consuming pathways as well as other unidentified events that
are shown to be insensitive to metabolic inhibitors such as
KCN and sodium azide. The metabolic response is strongly
dependent on pulsing conditions and is independent of the
total permeabilization achieved by the pulse. Evidence shows
that calorimetry is a simple and powerful method for
exploring conditions for metabolic stimulation, providing
information on metabolic responses that can not be obtained
from electrical measurements. This study set the basis for
further investigations on defense-related consequences of
PEF-induced stress.Sparbanksstiftelsen Färs & Frosta (Sweden).Fundação para a Ciência e a Tecnologia (FCT).Lund University (Sweden).Department of Cell and Organism Biology; Department of Plant Biochemistry
Haloarchaea swim slowly for optimal chemotactic efficiency in low nutrient environments
Archaea have evolved to survive in some of the most extreme environments on earth. Life in extreme, nutrient-poor conditions gives the opportunity to probe fundamental energy limitations on movement and response to stimuli, two essential markers of living systems. Here we use three-dimensional holographic microscopy and computer simulations to reveal that halophilic archaea achieve chemotaxis with power requirements one hundred-fold lower than common eubacterial model systems. Their swimming direction is stabilised by their flagella (archaella), enhancing directional persistence in a manner similar to that displayed by eubacteria, albeit with a different motility apparatus. Our experiments and simulations reveal that the cells are capable of slow but deterministic chemotaxis up a chemical gradient, in a biased random walk at the thermodynamic limit
Nanoelectropulse-driven membrane perturbation and small molecule permeabilization
BACKGROUND: Nanosecond, megavolt-per-meter pulsed electric fields scramble membrane phospholipids, release intracellular calcium, and induce apoptosis. Flow cytometric and fluorescence microscopy evidence has associated phospholipid rearrangement directly with nanoelectropulse exposure and supports the hypothesis that the potential that develops across the lipid bilayer during an electric pulse drives phosphatidylserine (PS) externalization. RESULTS: In this work we extend observations of cells exposed to electric pulses with 30 ns and 7 ns durations to still narrower pulse widths, and we find that even 3 ns pulses are sufficient to produce responses similar to those reported previously. We show here that in contrast to unipolar pulses, which perturb membrane phospholipid order, tracked with FM1-43 fluorescence, only at the anode side of the cell, bipolar pulses redistribute phospholipids at both the anode and cathode poles, consistent with migration of the anionic PS head group in the transmembrane field. In addition, we demonstrate that, as predicted by the membrane charging hypothesis, a train of shorter pulses requires higher fields to produce phospholipid scrambling comparable to that produced by a time-equivalent train of longer pulses (for a given applied field, 30, 4 ns pulses produce a weaker response than 4, 30 ns pulses). Finally, we show that influx of YO-PRO-1, a fluorescent dye used to detect early apoptosis and activation of the purinergic P2X(7 )receptor channels, is observed after exposure of Jurkat T lymphoblasts to sufficiently large numbers of pulses, suggesting that membrane poration occurs even with nanosecond pulses when the electric field is high enough. Propidium iodide entry, a traditional indicator of electroporation, occurs with even higher pulse counts. CONCLUSION: Megavolt-per-meter electric pulses as short as 3 ns alter the structure of the plasma membrane and permeabilize the cell to small molecules. The dose responses of cells to unipolar and bipolar pulses ranging from 3 ns to 30 ns duration support the hypothesis that a field-driven charging of the membrane dielectric causes the formation of pores on a nanosecond time scale, and that the anionic phospholipid PS migrates electrophoretically along the wall of these pores to the external face of the membrane
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