Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
AKB 50.101 Fr 10:30 B
Biomechanical investigations of collagen fibrils — •Stefan
Strasser, Wolfgang M. Heckl, and Stefan Thalhammer —
Department for Geo und Environmental Sciences, GeoBioCenter and
Center for Nanoscience, Ludwig Maximilians University, Theresienstr.
41, 80333 Munich, Germany
Collagen fibrils, type I and II with various periodicities were investigated,
using the Atomic Force Microscope both as tool for imaging and
nanomanipulation. Native and fibrous long spacing (FLS) collagen fibrils
were formed by self assembling in vitro, using a special setup for
dialysing. Depending on the conditions of assembly, collagen may form a
variety of different structures. The received collagen fibrils of type I had
a bending pattern of 67nm for the native fibrils, and 200nm to 300nm
for the FLS fibrils. Collagen is a system which shows a large degree of
polymorphism. All fibrils with a periodicity greater than 67nm can be
considered as FLS collagen. The collagen fibrils were imaged in air and
in liquids. To determine the elastic properties of collagen fibrils the tip of
the AFM was used as a nano-indentor. Force displacement curves were
recorded in liquids at different hights of collagen clusters as well as on
single fibrils and the youngs modulus has been calculated utilizing the
Hertzian theory. Results will be shown and discussed on single fibrils and
on multilayer stacks.
AKB 50.102 Fr 10:30 B
Contact Resistance of Cell Silicon Junctions Measured with
Voltage-Sensitive Dye — •Raimund Gleixner and Peter
Fromherz — MPI für Biochemie, Abt. Membran und Neurophysik,
Martinsried
The electrical sheet resistance between a cell and a solid substrate is
a crucial parameter for bioelectronic coupling. It can be determined by
applying transient voltages to a silicon chip, observing the response of
the membrane with a voltage-sensitive dye (Braun:PRL86,2905,2002).
We used the novel dye ANNINE-5 (Hübener:JPC,107,7896,2003) to
study the contact between HEK293 cells and fibronectin-coated oxidized
silicon. We determined the sheet resistance as a function of salt concentration
in the bath. A train of voltage transients was applied to the
chip and the fluorescence in the attached membrane was averaged in a
confocal microscope over several periods. For sinusoidal stimulation, the
frequency dependent spatially resolved transfer function was fitted with
a core-coat conductor model of the junction.
We found that the sheet resistance was proportional to the bulk specific
resistance. This indicated a cleft of 80nm filled with bulk electrolyte.
¿From fluorescence interference contrast (FLIC)microscopy we obtained
a distance of 70nm between membrane and chip, independent of salt
concentration. We conclude: (i)The narrow extracellular space of cell
adhesion is filled with bulk electrolyte. (ii)The sheet resistance can be
enhanced by enhancing the resistance of the bath.
AKB 50.103 Fr 10:30 B
Correlation functions and Boltzmann Langevin approach for
driven one dimensional lattices gases — •Paolo Pierobon 1,2 , Felix
Von Oppen 2 , Andrea Parmeggiani 1 , and Erwin Frey 1,2 for the
collaboration — 1 Hahn-Meitner Institut, Abteilung Theorie, Glienicker
Str.100, D-14109 Berlin, Germany — 2 Fachbereich Physik, Freie Universitaet
Berlin, Arnimallee 14, D-14195 Berlin, Germany
The Totally Asymmetric Simple Exclusion Process (TASEP) is a simple
one dimensional driven lattice gas model. It may serve as a simplified
model highlighting some aspects of traffic flow and biological transport
phenomena, e.g. motion of molecular motors on microtubules. At the
same time it has become a paradigm of non-equilibrium systems which
exhibit dynamic phase transitions driven by the boundaries. Although
the stationary density profile has been understood for long time, the time
dependent properties are still under investigation. We study the dynamic
correlation function of the particle density in such a system using real
time Monte Carlo simulations. In certain regimes the results can be rationalized
using a Boltzmann-Langevin approach i.e. adding a fluctuating
current to the mean field equation in a phenomenological way. We also
analyze our results within the ”domain wall”picture, where a “shock”
separates two phases and fluctuates as a Brownian walker with reflective
boundary conditions. Critical exponents are measured with great accuracy
from the properties of autocorrelation function at the critical point
(where three phases coexist), and in the maximal current phase. The
results corroborate that the system, in such conditions, belongs to the
KPZ universality class.
AKB 50.104 Fr 10:30 B
An off-lattice model for multicellular tumor spheroid growth —
•Gernot Schaller and Michael Meyer-Hermann — Institut für
Theoretische Physik, Zellescher Weg 17, 01062 Dresden
We model three-dimensional multicellular tumor spheroids using an
agent-based Delaunay/Voronoi-model which relies on experimentally accesible
cellular properties.
For the detection of natural neighbors we use three-dimensional
weighted dynamic and kinetic Delaunay/Voronoi tessellations. The
resulting bidirectional graph can be dynamically updated with in
average linear complexity versus the number of cells. Cells are assumed
to be elastically deformable spheres with varying radii. The growth of
cells is modeled by coupling the cells to a reaction-diffusion-equation for
nutrients. The dynamics is then calculated using the Langevin equation
in the overdamped limit.
We study the emergence of typical avascular tumor morphologies starting
with a single tumor cell. The influence of several mechanisms such
as nutrient limitation, external tissue pressure and intercellular adhesion
on growth and morphology is studied.
This work has been financially supported by the SMWK.
AKB 50.105 Fr 10:30 B
Molecular dynamics of intramyocellular metabolites from highresolution
in vivo 1 H NMR spectroscopy — •Leif Schröder 1 ,
Christian Schmitz 2 , and Peter Bachert 1 — 1 Dept. of Medical
Physics in Radiology, Deutsches Krebsforschungszentrum, Heidelberg,
Germany — 2 Biophysikalische Chemie, Physikalisch-Chemisches Institut,
Universität Heidelberg, Germany
Residual dipolar couplings affecting resonances in 1 H NMR spectra of
living tissue have been discovered 10 years ago (Kreis et. al.). Our purpose
was to explore molecular dynamics of creatine (Cr), taurine (Tau),
and carnosine (Cs) in human calf muscle (m. gastrocnemius) in vivo by
analyzing dipolar-coupled multiplets in localized high-resolution 1 H NMR
spectra (STEAM and PRESS technique). Measurements were performed
in healthy volunteers on a whole-body MR tomograph at B0 = 1.5T. The
residual coupling strength SD0 derived from the spectra was compared
to the coupling constant D0 calculated using the internuclear distance
under the assumption of completely frozen molecular libration. The order
parameter S is a measure of molecular mobility. We found a large
S (1.2 × 10 −2 ) for the detectable Cs spin system, i.e. the imidazole ring
protons, which indicates restricted reorientational motion of this compound
in contrast to high mobilities of the CH2 and CH3 groups of Tau
and Cr (S = 1.4 − 3 × 10 −4 ). The observed motional restriction of Cs is
explained by interaction of the metabolite with phospholipids in muscle
cell membranes. These results demonstrate that 1 H NMR spectroscopy
permits noninvasive studies of intermolecular processes in vivo.
AKB 50.106 Fr 10:30 B
Are there lipid rafts? — •Heiko Heerklotz — Biozentrum Basel,
Klingelbergstr. 70, CH-4056 Basel
Functional domains in biological membranes and the role of lipids in
the formation of such structures are still not unequivocally clarified. A
classical definition of the term lipid rafts refers to large, long-lived domains
in native cellular membranes that are formed by a spontaneous demixing
of a liquid ordered phase rich in sphingomyelin (SM) and cholesterol
(Cho) from the fluid, phosphatidylcholine (PC)-rich matrix. Rafts
have been believed to accumulate large numbers of proteins and it has
been assumed that rafts can be isolated from membranes as so-called detergent
resistant membrane fragments (DRMs). Our investigations have
revealed however that these common assumptions are inconsistent. Putative
lipid rafts having stability properties similar to liquid ordered domains
in a PC-SM-Cho mixture cannot be isolated by detergent without
changing their properties substantially. We came to this conclusion on
the basis of studies of the stability of domains and intermolecular interactions
between lipids (POPC-SM-Cho) and detergents (Triton X-100
and others) by various microcalorimetric techniques (isothermal titration
calorimetry, differential scanning calorimetry, pressure perturbation
calorimetry) and solid-state NMR. A simple thermodynamic model based
on the enthalpy and entropy of the interactions of different lipids with
the detergent explains the phenomena.
AKB 50.107 Fr 10:30 B
Contact mechanics of bioinspired fibrillar structures — •Ralph
Spolenak, Stanislav Gorb, and Eduard Arzt — Max-Planck-
Institut für Metallforschung, Heisenbergstr. 3, D- 70569 Stuttgart