33,978 research outputs found
Glycerol Modulates Water Permeation through Escherichia coli Aquaglyceroporin GlpF
Among aquaglyceroporins that transport both water and glycerol across the
cell membrane, Escherichia coli glycerol uptake facilitator (GlpF) is the most
thoroughly studied. However, one question remains: Does glycerol modulate water
permeation? This study answers this fundamental question by determining the
chemical-potential profile of glycerol along the permeation path through GlpF's
conducting pore. There is a deep well near the Asn-Pro-Ala (NPA) motifs
(dissociation constant 14 microM) and a barrier near the selectivity filter
(10.1 kcal/mol above the well bottom). This profile owes its existence to
GlpF's perfect steric arrangement: The glycerol-protein van der Waals
interactions are attractive near the NPA but repulsive elsewhere in the
conducting pore. In light of the single-file nature of waters and glycerols
lining up in GlpF's amphipathic pore, it leads to the following conclusion:
Glycerol modulates water permeation in the microM range. At mM concentrations,
GlpF is glycerol-saturated and a glycerol dwelling in the well occludes the
conducting pore. Therefore, water permeation is fully correlated to glycerol
dissociation that has an Arrhenius activation barrier of 6.5 kcal/mol.
Validation of this theory is based on the existent in vitro data, some of which
have not been given the proper attention they deserved: The Arrhenius
activation barriers were found to be 7 kcal/mol for water permeation and 9.6
kcal/mol for glycerol permeation; The presence of up to 100 mM glycerol did not
affect the kinetics of water transport with very low permeability, in apparent
contradiction with the existent theories that predicted high permeability (0 M
glycerol)
Valid and efficient formula for free energy difference from nonequilibrium work
Atomic force microscopes and optical tweezers afford direct probe into the inner working of single biomolecules by mechanically unfolding them.^1-15^ Critical to the success of this type of probe is to correctly extract the free energy differences between the various conformations of a protein/nucleic acid along its forced unfolding pathways. Current studies rely on the Jarzynski equality^16^ (JE) or its undergirding Crooks fluctuation theorem^17^ (CFT), even though questions remain on its validity^17-19^ and on its accuracy.^13,20-21^ The validity of JE relies on the assumption of microscopic reversibility.^17,18^ The dynamics of biomolecules, however, is Langevin stochastic in nature. The frictional force in the Langevin equation breaks the time reversal symmetry and renders the dynamics microscopically irreversible even though detailed balance holds true. The inaccuracy of JE has largely been attributed to the fact that one cannot sample a large enough number of unfolding paths in a given study, experimental or computational.^13,15^ Here I show that both of these questions can be answered with a new equation relating the nonequilibrium work to the equilibrium free energy difference. The validity of this new equation requires detailed balance but not microscopic reversibility. Taking into the new equation equal number of unfolding and refolding paths, the accuracy is enhanced ten folds in comparison to a JE study based on a similar but larger number of unfolding paths
Some Issues in a Gauge Model of Unparticles
We address in a recent gauge model of unparticles the issues that are
important for consistency of a gauge theory, i.e., unitarity and Ward identity
of physical amplitudes. We find that non-integrable singularities arise in
physical quantities like cross section and decay rate from gauge interactions
of unparticles. We also show that Ward identity is violated due to the lack of
a dispersion relation for charged unparticles although the Ward-Takahashi
identity for general Green functions is incorporated in the model. A previous
observation that the unparticle's (with scaling dimension d) contribution to
the gauge boson self-energy is a factor (2-d) of the particle's has been
extended to the Green function of triple gauge bosons. This (2-d) rule may be
generally true for any point Green functions of gauge bosons. This implies that
the model would be trivial even as one that mimics certain dynamical effects on
gauge bosons in which unparticles serve as an interpolating field.Comment: v1:16 pages, 3 figures. v2: some clarifications made and presentation
improved, calculation and conclusion not modified; refs added and updated.
Version to appear in EPJ
Alignment and orientation of an adsorbed dipole molecule
Half-cycle laser pulse is applied on an absorbed molecule to investigate its
alignment and orientation behavior. Crossover from field-free to hindered
rotation motion is observed by varying the angel of hindrance of potential
well. At small hindered angle, both alignment and orientation show
sinusoidal-like behavior because of the suppression of higher excited states.
However, mean alignment decreases monotonically as the hindered angle is
increased, while mean orientation displays a minimum point at certain hindered
angle. The reason is attributed to the symmetry of wavefunction and can be
explained well by analyzing the coefficients of eigenstates.Comment: 4 pages, 4 figures, to appear in Phys. Rev. B (2004
Modulation efficiency of LiNbO<sub>3</sub> waveguide electro-optic intensity modulator operating at high microwave frequency
The modulation efficiency, at high-frequency microwave modulation, of a LiNbO3 waveguide electro-optic modulator is shown to be degraded severely, especially when it is used as a frequency translator in a Brillouin-distributed fiber-sensing system. We derive an analytical expression for this attenuation regarding the phase-velocity mismatch and the impedance mismatch during the modulation process. Theoretical results are confirmed by experimental results based on a 15 Gb/s LiNbO3 optical intensity modulator
DLC2 modulates angiogenic responses in vascular endothelial cells by regulating cell attachment and migration.
Deleted in liver cancer 1 (DLC1) is a RhoGTPase activation protein-containing tumor suppressor that associates with various types of cancer. Although DLC2 shares a similar domain structure with that of DLC1, the function of DLC2 is not well characterized. Here, we describe the expression and ablation of DLC2 in mice using a reporter-knockout approach. DLC2 is expressed in several tissues and in endothelial cells (ECs) of blood vessels. Although ECs and blood vessels show no histological abnormalities and mice appear overall healthy, DLC2-mutant mice display enhanced angiogenic responses induced by matrigel and by tumor cells. Silencing of DLC2 in human ECs has reduced cell attachment, increased migration, and tube formation. These changes are rescued by silencing of RhoA, suggesting that the process is RhoA pathway dependent. These results indicate that DLC2 is not required for mouse development and normal vessel formation, but may protect mouse from unwanted angiogenesis induced by, for example, tumor cells
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Provision of secondary frequency regulation by coordinated dispatch of industrial loads and thermal power plants
Demand responsive industrial loads with high thermal inertia have potential to provide ancillary service for frequency regulation in the power market. To capture the benefit, this study proposes a new hierarchical framework to coordinate the demand responsive industrial loads with thermal power plants in an industrial park for secondary frequency control. In the proposed framework, demand responsive loads and generating resources are coordinated for optimal dispatch in two-time scales: (1) the regulation reserve of the industrial park is optimally scheduled in a day-ahead manner. The stochastic regulation signal is replaced by the specific extremely trajectories. Furthermore, the extremely trajectories are achieved by the day-ahead predicted regulation mileage. The resulting benefit is to transform the stochastic reserve scheduling problem into a deterministic optimization; (2) a model predictive control strategy is proposed to dispatch the industry park in real time with an objective to maximize the revenue. The proposed technology is tested using a real-world industrial electrolysis power system based upon Pennsylvania, Jersey, and Maryland (PJM) power market. Various scenarios are simulated to study the performance of the proposed approach to enable industry parks to provide ancillary service into the power market. The simulation results indicate that an industrial park with a capacity of 500 MW can provide up to 40 MW ancillary service for participation in the secondary frequency regulation. The proposed strategy is demonstrated to be capable of maintaining the economic and secure operation of the industrial park while satisfying performance requirements from the real world regulation market
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