52 research outputs found
Why Microtubules run in Circles - Mechanical Hysteresis of the Tubulin Lattice
The fate of every eukaryotic cell subtly relies on the exceptional mechanical
properties of microtubules. Despite significant efforts, understanding their
unusual mechanics remains elusive. One persistent, unresolved mystery is the
formation of long-lived arcs and rings, e.g. in kinesin-driven gliding assays.
To elucidate their physical origin we develop a model of the inner workings of
the microtubule's lattice, based on recent experimental evidence for a
conformational switch of the tubulin dimer. We show that the microtubule
lattice itself coexists in discrete polymorphic states. Curved states can be
induced via a mechanical hysteresis involving torques and forces typical of few
molecular motors acting in unison. This lattice switch renders microtubules not
only virtually unbreakable under typical cellular forces, but moreover provides
them with a tunable response integrating mechanical and chemical stimuli.Comment: 5 pages, 4 Movies in the Supplemen
A Solvable Model for Polymorphic Dynamics of Biofilaments
We investigate an analytically tractable toy model for thermally induced
polymorphic dynamics of cooperatively rearranging biofilaments - like
microtubules. The proposed 4 -block model, which can be seen as a
coarse-grained approximation of the full polymorphic tube model, permits a
complete analytical treatment of all thermodynamic properties including
correlation functions and angular fourier mode distributions. Due to its
mathematical tractability the model straightforwardly leads to some physical
insights in recently discussed phenomena like the "length dependent persistence
length". We show that a polymorphic filament can disguise itself as a classical
worm like chain on small and on large scales and yet display distinct anomalous
tell-tale features indicating an inner switching dynamics on intermediate
length scales
Kinetics of non-ionic surfactant adsorption at a fluid-fluid interface from a micellar solution
The kinetics of non-ionic surfactant adsorption at a fluid-fluid interface
from a micellar solution is considered theoretically. Our model takes into
account the effect of micelle relaxation on the diffusion of the free
surfactant molecules. It is shown that non-ionic surfactants undergo either a
diffusion or a kinetically limited adsorption according to the characteristic
relaxation time of the micelles. This gives a new interpretation for the
observed dynamical surface tension of micellar solutions.Comment: 4 page
Renormalization Group in Quantum Mechanics
We establish the renormalization group equation for the running action in the
context of a one quantum particle system. This equation is deduced by
integrating each fourier mode after the other in the path integral formalism.
It is free of the well known pathologies which appear in quantum field theory
due to the sharp cutoff. We show that for an arbitrary background path the
usual local form of the action is not preserved by the flow. To cure this
problem we consider a more general action than usual which is stable by the
renormalization group flow. It allows us to obtain a new consistent
renormalization group equation for the action.Comment: 20 page
Crunching Biofilament Rings
We discuss a curious example for the collective mechanical behavior of
coupled non-linear monomer units entrapped in a circular filament. Within a
simple model we elucidate how multistability of monomer units and exponentially
large degeneracy of the filament's ground state emerge as a collective feature
of the closed filament. Surprisingly, increasing the monomer frustration, i.e.,
the bending prestrain within the circular filament, leads to a conformational
softening of the system. The phenomenon, that we term polymorphic crunching, is
discussed and applied to a possible scenario for membrane tube deformation by
switchable dynamin or FtsZ filaments. We find an important role of cooperative
inter-unit interaction for efficient ring induced membrane fission
Spin Hall effect of Photons in a Static Gravitational Field
Starting from a Hamiltonian description of the photon within the set of
Bargmann-Wigner equations we derive new semiclassical equations of motion for
the photon propagating in static gravitational field. These equations which are
obtained in the representation diagonalizing the Hamiltonian at the order
, present the first order corrections to the geometrical optics. The
photon Hamiltonian shows a new kind of helicity-magnetotorsion coupling.
However, even for a torsionless space-time, photons do not follow the usual
null geodesic as a consequence of an anomalous velocity term. This term is
responsible for the gravitational birefringence phenomenon: photons with
distinct helicity follow different geodesics in a static gravitational field.Comment: 6 page
Equation of State of Looped DNA
We derive the equation of state of DNA under tension that features a loop. Such loops occur transiently during DNA condensation in the presence of multivalent ions or permanently through sliding protein linkers such as condensin. The force-extension relation of such looped-DNA modeled as a wormlike chain is calculated via path integration in the semiclassical limit. This allows us to rigorously determine the high stretching asymptotics. Notably the functional form of the force-extension curve resembles that of straight DNA, yet with a strongly renormalized apparent persistence length. We also present analogous results for DNA under tension with several protein-induced kinks and/or loops. That means that the experimentally extracted single-molecule elasticity does not necessarily only reflect the bare DNA stiffness, but can also contain additional contributions that depend on the overall chain conformation and length
Helices at Interfaces
Helically coiled filaments are a frequent motif in nature. In situations
commonly encountered in experiments coiled helices are squeezed flat onto two
dimensional surfaces. Under such 2-D confinement helices form "squeelices" -
peculiar squeezed conformations often resembling looped waves, spirals or
circles. Using theory and Monte-Carlo simulations we illuminate here the
mechanics and the unusual statistical mechanics of confined helices and show
that their fluctuations can be understood in terms of moving and interacting
discrete particle-like entities - the "twist-kinks". We show that confined
filaments can thermally switch between discrete topological twist quantized
states, with some of the states exhibiting dramatically enhanced
circularization probability while others displaying surprising
hyperflexibility
- …