472 research outputs found
Effect of Viscosity on Mechanics of Single, Skinned Fibers from Rabbit Psoas Muscle
AbstractMuscle contraction is highly dynamic and thus may be influenced by viscosity of the medium surrounding the myofilaments. Single, skinned fibers from rabbit psoas muscle were used to test this hypothesis. Viscosity within the myofilament lattice was increased by adding to solutions low molecular weight sugars (disaccharides sucrose or maltose or monosaccharides glucose or fructose). At maximal Ca2+ activation, isometric force (Fi) was inhibited at the highest solute concentrations studied, but this inhibition was not directly related to viscosity. Solutes readily permeated the filament lattice, as fiber diameter was unaffected by added solutes (except for an increased diameter with Fi<30% of control). In contrast, there was a linear dependence upon 1/viscosity for both unloaded shortening velocity and also the kinetics of isometric tension redevelopment; these effects were unrelated to either variation in solution osmolarity or inhibition of force. All effects of added solute were reversible. Inhibition of both isometric as well as isotonic kinetics demonstrates that viscous resistance to filament sliding was not the predominant factor affected by viscosity. This was corroborated by measurements in relaxed fibers, which showed no significant change in the strain-rate dependence of elastic modulus when viscosity was increased more than twofold. Our results implicate cross-bridge diffusion as a significant limiting factor in cross-bridge kinetics and, more generally, demonstrate that viscosity is a useful probe of actomyosin dynamics
Electronic and optical properties of electromigrated molecular junctions
Electromigrated nanoscale junctions have proven very useful for studying
electronic transport at the single-molecule scale. However, confirming that
conduction is through precisely the molecule of interest and not some
contaminant or metal nanoparticle has remained a persistent challenge,
typically requiring a statistical analysis of many devices. We review how
transport mechanisms in both purely electronic and optical measurements can be
used to infer information about the nanoscale junction configuration. The
electronic response to optical excitation is particularly revealing. We briefly
discuss surface-enhanced Raman spectroscopy on such junctions, and present new
results showing that currents due to optical rectification can provide a means
of estimating the local electric field at the junction due to illumination.Comment: 19 pages, 8 figures, invited paper for forthcoming special issue of
Journal of Physics: Condensed Matter. For other related papers, see
http://www.ruf.rice.edu/~natelson/publications.htm
Quantum transport in a resonant tunnel junction coupled to a nanomechanical oscillator
We discuss the quantum transport of electrons through a resonant tunnel
junction coupled to a nanomechanical oscillator at zero temperature. By using
the Green's function technique we calculate the transport properties of
electrons through a single dot strongly coupled to a single oscillator. We
consider a finite chemical potential difference between the right and left
leads. In addition to the main resonant peak of electrons on the dot, we find
satellite peaks due to the creation of phonons. These satellite peaks become
sharper and more significant with increasing coupling strength between the
electrons and the oscillator. We also consider the energy transferred from the
electrons to the oscillator.Comment: Updated in response to referees' comments. Section IV amended
including figure
Decoherence in elastic and polaronic transport via discrete quantum states
Here we study the effect of decoherence on elastic and polaronic transport
via discrete quantum states. The calculations are performed with the help of
nonperturbative computational scheme, based on the Green's function theory
within the framework of polaron transformation (GFT-PT), where the many-body
electron-phonon interaction problem is mapped exactly into a single-electron
multi-channel scattering problem. In particular, the influence of dephasing and
relaxation processes on the shape of the electrical current and shot noise
curves is discussed in detail under the linear and nonlinear transport
conditions.Comment: 11 pages, 3 figure
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
Thermoelectric effect in molecular electronics
We provide a theoretical estimate of the thermoelectric current and voltage
over a Phenyldithiol molecule. We also show that the thermoelectric voltage is
(1) easy to analyze, (2) insensitive to the detailed coupling to the contacts,
(3) large enough to be measured and (4) give valuable information, which is not
readily accessible through other experiments, on the location of the Fermi
energy relative to the molecular levels. The location of the Fermi-energy is
poorly understood and controversial even though it is a central factor in
determining the nature of conduction (n- or p-type). We also note that the
thermoelectric voltage measured over Guanine molecules with an STM by Poler et
al., indicate conduction through the HOMO level, i.e., p-type conduction.Comment: 4 pages, 3 figure
Large-Scale Atomistic Simulations of Environmental Effects on the Formation and Properties of Molecular Junctions
Using an updated simulation tool, we examine molecular junctions comprised of
benzene-1,4-dithiolate bonded between gold nanotips, focusing on the importance
of environmental factors and inter-electrode distance on the formation and
structure of bridged molecules. We investigate the complex relationship between
monolayer density and tip separation, finding that the formation of
multi-molecule junctions is favored at low monolayer density, while
single-molecule junctions are favored at high density. We demonstrate that tip
geometry and monolayer interactions, two factors that are often neglected in
simulation, affect the bonding geometry and tilt angle of bridged molecules. We
further show that the structures of bridged molecules at 298 and 77 K are
similar.Comment: To appear in ACS Nano, 30 pages, 5 figure
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Roles of the creatine kinase system and myoglobin in maintaining energetic state in the working heart
<p>Abstract</p> <p>Background</p> <p>The heart is capable of maintaining contractile function despite a transient decrease in blood flow and increase in cardiac ATP demand during systole. This study analyzes a previously developed model of cardiac energetics and oxygen transport to understand the roles of the creatine kinase system and myoglobin in maintaining the ATP hydrolysis potential during beat-to-beat transient changes in blood flow and ATP hydrolysis rate.</p> <p>Results</p> <p>The theoretical investigation demonstrates that elimination of myoglobin only slightly increases the predicted range of oscillation of cardiac oxygenation level during beat-to-beat transients in blood flow and ATP utilization. In silico elimination of myoglobin has almost no impact on the cytoplasmic ATP hydrolysis potential (Δ<it>G</it><sub>ATPase</sub>). In contrast, disabling the creatine kinase system results in considerable oscillations of cytoplasmic ADP and ATP levels and seriously deteriorates the stability of Δ<it>G</it><sub>ATPase </sub>in the beating heart.</p> <p>Conclusion</p> <p>The CK system stabilizes Δ<it>G</it><sub>ATPase </sub>by both buffering ATP and ADP concentrations and enhancing the feedback signal of inorganic phosphate in regulating mitochondrial oxidative phosphorylation.</p
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