Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
Animals like beetles, flies, spiders and geckos rely on hairy (fibrillar)
structures to adhere to natural surfaces enabling them to walk on vertical
surfaces or on the ceiling. Recently, experimental evidence has been
provided by Autumn et al. that the adhesion between single fibers and
the natural surfaces is caused by van der Waals interaction. Modeling
the adhesion of fibrillar structures by contact mechanics reveals that the
total adhesion of the system can be enhanced by contact splitting. In
effect the size of fibers in animals scales inversely with their body mass.
Derived from contact mechanics we present the effects of contact shape,
fiber material and fiber geometry on the total adhesion of the system.
Design rules are devised that can serve as guidelines for the fabrication
of artificial adhesion systems.
AKB 50.108 Fr 10:30 B
Geometrical parameters of periodical structures by 1 H NMR —
•Stefan Kirsch and Peter Bachert — Dept. of Medical Physics in
Radiology, Deutsches Krebsforschungszentrum, Heidelberg, Germany
Some biological tissue, e.g., muscle and trabecular bone, contains periodical
structures at different levels of organisation. Assessment of the
corresponding geometrical parameters is possible on the basis of intermolecular
dipole-dipole couplings. In liquid-state 1 H NMR the distant
dipolar field generated by the bulk of water protons can have a significant
effect on the evolution of the magnetization. Application of the
CRAZED sequence (two rf pulses, delay τ, gradient G) yields a train
of echo signals at times nτ (n = 1, 2, ...) which are explained by intermolecular
multiple-quantum coherences of the order n (Warren et al.)
and which are generated by dipolar-coupled spins of mutual distance
d = π/(γGτ). In periodical structures echo formation for n �= 2 is only
expected when d is a linear combination of the characteristic length scale
λs of the structure: d = Σi li · (λs/2); li = ±1, ±2,... . With the purpose
of possible application in vivo, we performed CRAZED experiments with
tightly packed microcapillaries immersed in water in a 1.5-T whole-body
MR tomograph (MAGNETOM Vision; Siemens). We demonstrate, that
for n = 1 and 3 information on λs of the structure can be obtained.
AKB 50.109 Fr 10:30 B
Asynchronous networks of switches and the reliability of gene
regulation — •Konstantin Klemm and Stefan Bornholdt — Interdiszplinäres
Zentrum für Bioinformatik, Universität Leipzig, Kreuzstr.
7b, 04103 Leipzig
How do networks of regulatory genes function in a near deterministic
way, although they consist of mostly autonomous non-synchronized
switches? This is studied using a network of asymmetrically coupled binary
threshold units with asynchronous serial update in random order.
Motivated by the biochemical processes during gene regulation involving
mRNA and protein, each unit updates its state according to an individual
clock that is subject to noise. In general, this asynchronous mode leads
to stochastic, non-deterministic system behavior. However, considering
a transmission delay between the nodes leads to synchronization and
ordering of the dynamics: On a coarse grained time scale, units switch
simultaneously. These results suggest that transmission delay may contribute
to the robustness of genetic regulation. Further, as some topological
features are more favorable than others for this effect, it hints at a
possible explanation of why some topological motifs, in particular loops,
are suppressed in the wiring of genetic networks.
[1] Konstantin Klemm and Stefan Bornholdt, preprint
http://www.arXiv.org/abs/q-bio/0309013
AKB 50.110 Fr 10:30 B
Spatially periodic patterns in nematic and biopolymer-motor
systems — •Falko Ziebert and Walter Zimmermann — FR Theoretische
Physik, Universität des Saarlandes, 66041 Saarbrücken
We present two possible mechanisms of pattern formation in rigid
biopolymer systems like actin filaments and microtubules. The first one
is the competition between diffusion and the finite lifetime of the filaments,
which are in vivo continuously assembled and disassembled. This
kinetics transforms the phase separation close to the isotropic-nematictransition
into the formation of spatially periodic patterns, as predicted
on the basis of a phenomenological model.
In the second scenario, motor proteins are added to a solution of filaments.
Because the motors walk on a filament only in one specified
direction defined by the structure of the filament and the motor species,
the �n ↔ −�n symmetry of the filament solution is broken which leads to
new patterns. This scenario can be described microscopically by means
of a Smoluchowski-equation.
AKB 50.111 Fr 10:30 B
Entropic Forces between Biopolymers — •Azam Gholami 1 and
Erwin Frey 1,2 — 1 Hahn-Meitner-Institut, Abteilung Theoretische
Physik, Glienicker Strasse 100, D-14109 Berlin, Germany — 2 Fachbereich
Physik, Freie Universitat Berlin, Arnimallee 4, D-14195 Berlin, Germany
The interaction between biopolymers in the cytoskeleton is characterized
by the interplay of energy and entropy. There are electrostatic forces,
interactions mediated by crosslinking proteins, and steric interactions induced
by thermal fluctuations. We study these steric interactions in various
geometries using Monte Carlo methods. The Monte Carlo data give
clear evidence for the existence of repulsive interaction between biopolymers
at short distances. The form of the repulsive interaction is in good
agreement with the analytical results obtained from scaling arguments.
AKB 50.112 Fr 10:30 B
Electrophoretic Accumulation of Membrane Bound Proteins on
Polymer Supports — •Joachim Hermann 1 , Markus Fischer 2 ,
Steven Boxer 3 , and Motomu Tanaka 1 — 1 Physik Department,
Lehrstuhl f”ur Biophysik E22, TU M”unchen, James-Franck-Str., D-
85748 Garching — 2 Fakult”at f”ur Chemie, TU M”unchen, James-
Franck-Str., D-85748 Garching — 3 Department of Chemistry, Stanford
University, CA 94305-5080, USA
The presented work deals with the local biofunctionalization of solid
surfaces by means of electrophoretic accumulation of genetically engineered
recombinant proteins docking onto membranes deposited on polymer
supports. His-tagged fluorescent proteins (GFP and DsRed) were
firstly coupled homogeneously to lipid monolayers doped with chelator
(NTA) lipids, then a DC electric field was applied for accumulation of
the proteins. The resulting concentration gradient of proteins at equilibrium
could empirically be analyzed by a simple differential equation.
As a result, the mean velocities and mobilties of the two proteins were
obtained.
The proteins on the membrane surface experienced not only electrophoretic
forces but also electroosmosis in the opposite direction. The
counter balance between these two opposing forces was further studied
by shifting the surface charge densities, namely, the difference in zeta
potentials between the membrane and the proteins. In fact, incorporation
of oppositely charged lipids into the membrane led to larger mean
velocities for both proteins, which supports the proposed scenario semiquantitatively.
AKB 50.113 Fr 10:30 B
Vesicles in the Optical Stretcher - Shape changes induced
by stress-dependent flip-flop processes — •Frank Sauer, Stefan
Schinkinger, Falk Wottawah, Bryan Lincoln, and Jochen
Guck — Fakultät für Physik und Geowissenschaften, Univeristät Leipzig
We have investigated giant unilamellar vesicles (DLPC and C12E4)
with an optical stretcher. This laser-based micromanipulation tool induces
optical surface stresses that can trap vesicles and deform them
from spherical into a prolate shape. The observed deformation increased
linearly with applied stresses ranging from 1-20 Pa. This deformation
was reversible upon immediate removal of the stress. Surprisingly, under
continued stress for more than 1s, the vesicle returned to its spherical
shape with the return rate increasing with stress. When the stress was
then removed, the vesicle became oblate and recovered its equilibrium
spherical shape only after a very long time (10-20 min). This effect could
be repeated several times with the same vesicle, excluding damage to
the vesicle as possible cause. The most likely explanation of this phenomenon
is a stress dependent flip-flop rate between outer and inner
leaflets. This leads to a stress dependent spontaneous curvature, which
changes the equilibrium shape of the vesicle. Further experiments to test
this hypothesis are underway.
AKB 50.114 Fr 10:30 B
Pattern formation in systems with conservation laws — •Ronny
Peter and Walter Zimmermann — Theoretische Physik, Universität
des Saarlandes, 66041 Saarbrücken
Pattern formation in dissipative systems, especially in systems with
unconserved order parameters, were intensively explored during the recent
decade. Pattern formation in systems with conservation laws, that
recently emerged in models for biological systems, however exhibit new
aspects. For both, stationary and spatially periodic pattern as well as for
traveling and standing waves, in systems with conservation laws largescale
neutral modes must be included in the asymptotic analysis for pattern
formation near onset. Various consequences are related to these slow