Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Montag
Fachsitzungen
– Hauptvorträge und Posterbeiträge –
AKB 10 Cytoskeleton and Polymer Networks
Zeit: Montag 09:30–11:30 Raum: H40
Hauptvortrag AKB 10.1 Mo 09:30 H40
Micromechanics of Biopolymer Networks and Shells —
•Christoph F. Schmidt 1 , Maryam Atakhorrami 1 , Gijsberta
H. Koenderink 1 , Pedro J. de Pablo 2 , Iwan A.T. Schaap 1 ,
Irena L. Ivanovska 1 , Gijs J.L. Wuite 1 , and Frederick C.
MacKintosh 1 — 1 Vrije Universiteit Amsterdam, Dept. Physics, De
Boelelaan 1081, — 2 Universidad Autonoma de Madrid, Departamento
de Fisica de la
Polymeric macromolecular assemblies play crucial roles in biology, from
DNA to the cytoskeleton or the cell membrane. We are generally interested
in the mechanical physical properties of such structures and in
their interactions with force generating motor proteins. I will report here
on measurements of the viscoelastic properties of model 3D actin networks
by microrheology and on measurements of the elastic properties
of two types of 2D-crystalline protein shells, microtubules and bacteriophage
capsids, probed by indentation with a scanning force microscope.
Actin networks are determining cell elastic response and we have probed
their complex viscoelastic response over a large range of length and time
scales, under a variety of conditions approaching those found in cells.
Both, microtubules and viral capsids are 2D protein shells and we found
linear elastic regimes that can be described by thin-shell theory and finite
element methods. We also found non-linear regimes and catastrophic collapse
under large loads. The mechanical response of these protein shells
at the nanometer scale shows simultaneously aspects of continuum elasticity,
as well as molecular graininess, particularly in their non-linear
behavior.
Hauptvortrag AKB 10.2 Mo 10:00 H40
Microrheology of Actin Networks — •Andreas Bausch — Lehrstuhl
fuer Biophysik E22, TU Muenchen
The actin cytoskeleton is of outstanding importance for the proper
functioning of cells. In order to study the underlying physical properties
of the actin polymer networks it is important to determine the mechanical
properties of these semiflexible polymer networks in vitro. Here, we measure
the viscoelasticity of entangled F-actin over length scales between
1 and 100 µm using one- and two-particle microrheology and directly
identify two distinct microscopic contributions to the elasticity. Filament
entanglements lead to a frequency-independent elastic modulus over an
extended frequency range of 0.01 to 30 rad/sec; this is probed only with
one-particle microrheology. Longitudinal fluctuations of the filaments increase
the elastic modulus between 0.1 and 30 rad/sec at length scales
larger than the filament persistence length; this is probed by two particle
microrheology.
AKB 11 Active Biomimetic Systems and Gels
Hauptvortrag AKB 10.3 Mo 10:30 H40
The Cytoskeleton: A Model System for Stiff Polymers — •Erwin
Frey — Abteilung Theorie, Hahn-Meitner-Institut, Glienicker Strasse
100, D-14109 Berlin
The structure responsible for the extraordinary mechanical properties
of cells is the cytoskeleton, a rigid yet flexible and dynamic network
of protein fibers, combined with a variety of associated regulatory proteins,
whose tasks range from passive cross-linking and active transport
to the regulation of biochemical processes. Due to their enormous stiffness,
the physics of cytoskeletal filaments is fundamentally different from
the physics of synthetic polymers like polyethylene. It is determined by
a subtle interplay between entropic and energetic contributions. Over
the past years novel interesting concepts of polymer physics have been
established. We will discuss the statistical mechanics and dynamics of
single filaments and show how the theoretical predictions can be confirmed
by fluorescence microscopy on single F-actin filaments. Building
on the physics of single filaments we will give a detailed theoretical model
for the viscoelasticity of solutions and the complex micro-mechanics of
crosslinked networks.
Hauptvortrag AKB 10.4 Mo 11:00 H40
Models of Biological Fiber Systems — •Camilla Mohrdieck
and Eduard Arzt — Max-Planck-Institut für Metallforschung, Heisenbergstr.
3, 70569 Stuttgart
Biological fiber systems have to be able to sustain large loads but they
must also be capable of undergoing large deformations, depending on
which environmental conditions they are subject to. Many fiber systems
meet these needs by responding in a nonlinear way to external loads.
Key to this nonlinearity is the crosslinking of fibers into a meshwork
and in some cases also a prestress in the fibers. To relate the influence
of topology and prestress to the mechanical stability and flexibility of
a fiber system, we perform computer simulations in which fiber systems
are iteratively deformed by applying forces to their nodal points. To this
end, we construct fiber sytems that reflect characteristic features of their
biological counterparts as for example the icosahedral capsids of some
viruses or the spherical actin cortex as it can be observed in quiescent
cells. In our study we combine large deflection analysis with a determination
of the topological properties such as the connectivity of the
meshwork, states of self stress and inextensional modes of deformation.
This allows to evaluate the mechanical properties of the fiber system as
well as to follow the evolution of its intrinsic structural properties as the
whole system deforms under a given load.
Zeit: Montag 12:00–13:00 Raum: H40
Hauptvortrag AKB 11.1 Mo 12:00 H40
Active Biomimetic Systems — •Reinhard Lipowsky — MPI für
Kolloid und Grenzflächenforschung, 14424 Potsdam
Biological systems have the ability to reorganize and to reconstruct
their spatial structure on the nano- and microscale. This capability is
based on molecular assemblies which can transform chemical (free) energy
into mechanical work. The simplest nanomachines of this type are filaments,
which grow by self-assembly, and molecular motors, which move
along such filaments. These active nanostructures generate force, provide
transport of cargo, and form biomimetic scaffolds. The talk will discuss
the basic phasics of these processes and their potential applications.
Hauptvortrag AKB 11.2 Mo 12:30 H40
Membranes and Active Gels — •Hans-Günther Döbereiner —
Departments of Biology & Physics, Columbia University, New York, NY
10027, USA
During the last decade, Biological Physics has become a major research
field. Especially, the fascinating physics of membranes continues to drive
forward our quantitative understanding of cellular behavior. Giant vesicles
[1] are an important tool to monitor the properties of biomembranes
[2,3] and soft matter in general [4]. Recently, active gels have drawn a lot
of attention. Combining the physics of polymers and molecular motors
to model biological phenotype [5] will require new concepts. We propose
to view cell spreading and migration as a sequence of dynamic phase
transitions of active gels and their regulatory networks.
[1] H.-G. Döbereiner, in Giant Vesicles, Perspectives in Supramolecular
Chemistry 6, 150, Eds. P.L. Luisi and P. Walde, Wiley & Sons (2000).