Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
to vesicles, which are homogenously filled membrane shells. The bending
rigidity κ of the membrane and the molecular interaction with external
substances have an important influence on the cell morphology and
the cell’s adhesion behavior. Rather few attention has been paid in the
past to the influence of thermal fluctuations on the adhesion behavior.
With the help of Monte Carlo simulations we study the behavior of a
one-component vesicle adhered to a substrate at finite temperature. The
adhesion behavior of vesicles was studied systematically as a function of
temperature, potential depth and potential width. For high enough adhesion
strength the contact area of the vesicle goes linear with T/κ. This
behavior can be justified by an analytic approach. Further, a master equation
has been derived which describes all our simulation results. Thus, by
measuring the adhesion area one can derive the adhesion strength from
κ or vice versa.
AKB 50.49 Fr 10:30 B
Morphogen transport by planar transcytosis — •Tobias
Bollenbach 1 , Karsten Kruse 1 , Periklis Pantazis 2 , Marcos
González-Gaitán 2 , and Frank Jülicher 1 — 1 MPI for Physics
of Complex Systems, Nöthnitzerstr. 38, 01187 Dresden — 2 MPI for
Molecular Cell Biology and Genetics, Pfotenhauerstr. 108, 01307
Dresden
Morphogens are signaling molecules that play a key role in development.
They spread from a restricted source into an adjacent target tissue
forming a concentration gradient therein. The fate of cells in the target
tissue is determined by the local concentration of such morphogens.
So far, the dominant mechanism by which morphogens are transported
through the tissue has not been clearly identifed. While diffusion through
the extracellular space is a possibility, recent in vivo experiments on the
morphogen Dpp in the fruit fly Drosophila provide evidence for an active
transport mechanism that was termed “planar transcytosis”. Here,
a theoretical description of this transport mechanism is presented. As a
consequence of nonlinearities in the current, this transport phenomenon
exhibits rich behavior. We compare our description to experimental observations
and aim at a quantitative description of morphogen transport.
AKB 50.50 Fr 10:30 B
Kinetics of salivary protein adsorption — •Anthony Quinn, Hubert
Mantz, and Karin Jacobs — Experimental Physics, Saarland
University, POB 151 150, 66041 Saarbrücken
Exposure to saliva in the oral cavity results in the formation of a layer
of proteins and related biopolymers on all solid surfaces, known as an
acquired salivary pellicle. Pellicle formation occurs immediately and is
considered to be complete within 1-2 hours, with a thickness of approximately
1 micron. Bacteria in the saliva then selectively attaches to the
pellicle (dependent on the surface characteristics of the bacteria and the
specific proteins), marking the onset of plaque formation. Observations
of plaque, which is known to play a crucial role in the development of
oral diseases, reveal an enhanced or diminished growth dependent on the
supporting substrate. This research therefore aims to identify the mechanisms
for salivary protein adsorption at solid-liquid interfaces, with the
intention of optimising the biocompatibility of dental replacement materials,
and reduce the incidence of disease inducing plaque formation.
In-situ ellipsometry is utilized to follow the adsorption kinetics of purified
protein solutions, and the competitive adsorption from mixed protein
solutions on a series of tailored substrates. The physicochemical composition
of these tailored surfaces is carefully controlled and fully characterised
via AFM and wettability analysis, enabling the structural and
interfacial tension components to be identified respectively.
AKB 50.51 Fr 10:30 B
Studying cell adhesion with holographic optical tweezers —
•Jennifer E. Curtis and Joachim P. Spatz — Biophysical Chemistry,
Institiute for Physical Chemistry, University of Heidelberg
Most cells must adhere to a surface if they are to survive. Cell adhesion
involves the orchestration of a stunning number of proteins to form focal
complexes, the physical link between the surface, plasma membrane, and
the stabilizing cytoskeleton. Despite this complexity, simple mechanical
force plays a unique role in the initiation of cell adhesion. The activities
of cells depend sensitively on the mechanical nature of their adhesion
substrate. When too soft, a cell cannot adhere and focal contacts fail to
form. When too hard, unphysiological actin stress fibers plague the cell.
Using holographic optical tweezers, we present dozens of microspheres
directly above the surface of a fibroblast to study the dynamics of focal
adhesion formation. We also study how the initial position and the force
exerted by the optical traps influences the final binding strength, the total
time for full binding, and the subsequent motion of the bead. Lastly,
we hope to elucidate the role of the hylauronic acid layer in focal adhesion
by studying binding dynamics with and without this sticky sugar
matrix.
AKB 50.52 Fr 10:30 B
Electronic Transport in DNA — •Daphne Klotsa, Rudolf A.
Römer, and Matthew S. Turner — Department of Physics, University
of Warwick, CV4 7AL, Coventry, United Kingdom
Based on a theoretical model proposed by Cuniberti et al. [1] we are focusing
on electron localisation along the Deoxyribose Nucleic Acid (DNA)
double helix, using a tight-binding Hamiltonian. The possibility that this
organic super molecule might facilitate electron transfer, along the overlapping
π-orbitals [2], as a means of signalling other biomolecules, has
led us to perform calculations varying the DNA sequences. We have performed
simulations on random sequence, λ- and telomeric DNA. For the
first two the resulting localisation lengths as functions of energy and
disorder show similar behaviour whereas the latter — specific sequence
DNA — gives significantly larger localisation lengths. In all cases an energy
bandgap, indicating semiconducting behaviour, has been observed.
Counter intuitively, for random absorption of Sodium (Na) atoms onto
the backbone, preliminary results have shown localisation lengths to increase
with increasing disorder. A “moving window”technique will be
used in order to assess whether particular shorter fragments of a sequence
behave differently, whose contribution would inevitably be smoothed out
when the whole sequence is considered. Finally, we are looking at another
theoretical way of modelling DNA’s complex structure and properties,
the “ladder model”— an extension of the aforementioned model.
[1] G. Cuniberti, et al., Phys. Rev. B 65, 241314 (2002)
[2] P. J. de Pablo, et al. Phys. Rev. Letters 85, 4992 (2000)
AKB 50.53 Fr 10:30 B
Interlamellar variation of the 3D bone nanostructure
— •Wolfgang Wagermaier 1 , Himadri S. Gupta 1 , Paul
Roschger 2 , Manfred Burghammer 3 , Klaus Klaushofer 2 , and
Peter Fratzl 1 — 1 MPI-KGF, Biomaterials, Potsdam, Germany —
2 LBIO, 4th Med. Dept., Hanusch Hospital and UKH Meidling, Vienna,
Austria — 3 ESRF, Grenoble, France
Bone is a biomineralised tissue structurally optimised for its biological
function. Diverse fibrillar array types - at the 0.1 - 10 micrometer range,
adapted to their local load bearing requirements - bring about this optimisation.
We used a novel combination of synchrotron scanning small
angle X-ray scattering combined with sample rotation to see how the 3D
mineralised nanostructure varied in bone with inter-lamellar resolution.
Several osteons around blood vessels in compact bone were scanned over
a total area of typically (100 x 100 micrometer) and a simple physical
model was developed to reconstruct the 3D orientation. Our results show
mineral crystallite orientation has a fiber texture within single lamellae,
but shows a smooth spatial variation around the cylindrical core of the
osteon. Biomechanically the variation may be explained in terms of the
in-vivo stresses developed around osteonal channels, requiring a composite
structure able to resist large deformation and bending stresses. Our
results provide insights into how bone is designed at the lamellar level
for its biophysical function.
AKB 50.54 Fr 10:30 B
Fusion of giant vesicles observed with time resolution below
a millisecond — •Rumiana Dimova 1 , Christopher Haluska 1 ,
Karin A. Riske 1 , Valerie Marchi-Arztner 1 , and Reinhard
Lipowsky 2 — 1 Max Planck Institute of Colloids and Interfaces, Am
Muehlenberg 1, 14476 Golm, Germany — 2 Universite de Rennes, Rennes
France
Using optical microscopy and a high speed camera, we are able to
record the fusion process with a high temporal resolution of 100us. We
consider two model membrane systems: functionalized giant unilamellar
vesicles (GUVs) in the presence of multivalent ions, and lipid or polymer
GUVs subjected to electrical pulses.
In the first case, we functionalize lecithin membranes with synthetic
molecules bearing a ligand, which form complexes with Eu3+ (2:1). Micropipettes
are used to manipulate GUV’s and inject Eu3+ solution in
the contact zone between two vesicles. The injections induce adhesion
and formation of intermembrane complexes, followed by fusion. This system
has the potential to mimic the action of fusogenic SNARE proteins