551 research outputs found
Comprehensive structural model of the mechanochemical cycle of a mitotic motor highlights molecular adaptations in the kinesin family
Kinesins are responsible for a wide variety of microtubule-based, ATP-dependent
functions. Their motor domain drives these activities but the molecular adaptations
that specify these diverse and essential cellular activities are poorly understood. It
has been assumed that the first identified kinesin - the transport motor kinesin-1 â is
the mechanistic paradigm for the entire superfamily, but accumulating evidence
suggests that this is not the case. To address the deficits in our understanding of the
molecular basis of functional divergence within the kinesin superfamily, we studied
kinesin-5s, which are essential mitotic motors whose inhibition blocks cell division.
Using cryo-electron microscopy and subnanometer resolution structure
determination, we have visualised conformations of microtubule-bound human
kinesin-5 motor domain at successive steps in its ATPase cycle. Following ATP
hydrolysis, nucleotide-dependent conformational changes in the active site are
allosterically propagated into rotations of the motor domain and uncurling of the drugbinding
loop L5. In addition, the mechanical neck-linker element that is crucial for
motor stepping undergoes discrete, ordered displacements. We also observed large
reorientations of the motor N-terminus that indicate its importance for kinesin-5
function through control of neck-linker conformation. A kinesin-5 mutant lacking this
N-terminus is enzymatically active, and ATP-dependent neck-linker movement and
motility is defective although not ablated. All these aspects of kinesin-5
mechanochemistry are distinct from kinesin-1. Our findings directly demonstrate the
regulatory role of the kinesin-5 N-terminus in collaboration with the motorâs structured
neck-linker, and highlight the multiple adaptations within kinesin motor domains that
tune their mechanochemistries according to distinct functional requirements
Imaginary Squashing Mode Spectroscopy of Helium Three B
We have made precision measurements of the frequency of a collective mode of
the superfluid 3He-B order parameter, the J=2- imaginary squashing mode.
Measurements were performed at multiple pressures using interference of
transverse sound in an acoustic cavity. Transverse waves propagate in the
vicinity of this order parameter mode owing to off-resonant coupling. At the
crossing of the sound mode and the order parameter mode, the sound wave is
strongly attenuated. We use both velocity and attenuation measurements to
determine precise values of the mode frequency with a resolution between 0.1%
and 0.25%.Comment: 6 pages, 4 figures, submitted to proceedings of Quantum Fluids and
Solids (QFS) Conference 2006; revised 9/26/0
S-, P- and D-wave resonances in positronium-sodium and positronium-potassium scattering
Scattering of positronium (Ps) by sodium and potassium atoms has been
investigated employing a three-Ps-state coupled-channel model with Ps(1s,2s,2p)
states using a time-reversal-symmetric regularized electron-exchange model
potential fitted to reproduce accurate theoretical results for PsNa and PsK
binding energies. We find a narrow S-wave singlet resonance at 4.58 eV of width
0.002 eV in the Ps-Na system and at 4.77 eV of width 0.003 eV in the Ps-K
system. Singlet P-wave resonances in both systems are found at 5.07 eV of width
0.3 eV. Singlet D-wave structures are found at 5.3 eV in both systems. We also
report results for elastic and Ps-excitation cross sections for Ps scattering
by Na and K.Comment: 9 pages, 5 figures, Accepted in Journal of Physics
Discovery of the Acoustic Faraday Effect in Superfluid 3He-B
We report the discovery of the acoustic Faraday effect in superfluid 3He-B.
The observation of this effect provides the first direct evidence for
propagating transverse acoustic waves in liquid 3He, a mode first predicted by
Landau in 1957. The Faraday rotation is large and observable because of
spontaneously broken spin-orbit symmetry in 3He-B. We compare the experimental
observations with a simulation of the transverse acoustic impedance that
includes the field-induced circular birefringence of transverse waves.Comment: 4 pages in RevTex plus 3 postscript figures; new version includes:
minor corrections to the text and an updated of list of reference
Solitary waves of nonlinear nonintegrable equations
Our goal is to find closed form analytic expressions for the solitary waves
of nonlinear nonintegrable partial differential equations. The suitable
methods, which can only be nonperturbative, are classified in two classes.
In the first class, which includes the well known so-called truncation
methods, one \textit{a priori} assumes a given class of expressions
(polynomials, etc) for the unknown solution; the involved work can easily be
done by hand but all solutions outside the given class are surely missed.
In the second class, instead of searching an expression for the solution, one
builds an intermediate, equivalent information, namely the \textit{first order}
autonomous ODE satisfied by the solitary wave; in principle, no solution can be
missed, but the involved work requires computer algebra.
We present the application to the cubic and quintic complex one-dimensional
Ginzburg-Landau equations, and to the Kuramoto-Sivashinsky equation.Comment: 28 pages, chapter in book "Dissipative solitons", ed. Akhmediev, to
appea
High frequency sound in superfluid 3He-B
We present measurements of the absolute phase velocity of transverse and
longitudinal sound in superfluid 3He-B at low temperature, extending from the
imaginary squashing mode to near pair-breaking. Changes in the transverse phase
velocity near pair-breaking have been explained in terms of an order parameter
collective mode that arises from f-wave pairing interactions, the so-called
J=4- mode. Using these measurements, we establish lower bounds on the energy
gap in the B-phase. Measurement of attenuation of longitudinal sound at low
temperature and energies far above the pair-breaking threshold, are in
agreement with the lower bounds set on pair-breaking. Finally, we discuss our
estimations for the strength of the f-wave pairing interactions and the Fermi
liquid parameter, F4s.Comment: 15 pages, 8 figures, accepted to J. Low Temp. Phy
Microfluidic and Nanofluidic Cavities for Quantum Fluids Experiments
The union of quantum fluids research with nanoscience is rich with
opportunities for new physics. The relevant length scales in quantum fluids,
3He in particular, are comparable to those possible using microfluidic and
nanofluidic devices. In this article, we will briefly review how the physics of
quantum fluids depends strongly on confinement on the microscale and nanoscale.
Then we present devices fabricated specifically for quantum fluids research,
with cavity sizes ranging from 30 nm to 11 microns deep, and the
characterization of these devices for low temperature quantum fluids
experiments.Comment: 12 pages, 3 figures, Accepted to Journal of Low Temperature Physic
One-Pot Silver Nanoring Synthesis
Silver colloidal nanorings have been synthesized by reducing silver ions with NaBH4 in trisodium citrate buffers. pH increase, by addition of NaOH, was used to speed up reduction reaction. The UVâvis absorption spectra of resulting silver nanorings showed two peaks accounting for transverse and longitudinal surface plasmon resonance, at â400 nm, and between 600 and 700 nm, respectively. The shapes of these silver nanoparticles (nanorings) depended on AgNO3/NaBH4 ratio, pH and reaction temperature. Particles were analysed by transmission electron microscopy, scanning electron microscopy and X-ray diffraction. A reaction pathway is proposed to explain silver nanoring formation
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