Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
expresses bio-specific functions.
AKB 50.121 Fr 10:30 B
Hydration limits of synthetic glycolipids under controlled osmotic
pressure - a small angle neutron scattering (SANS)
study — •Florian Rehfeldt 1 , Bruno Demé 2 , Christian Gege 3 ,
Richard R. Schmidt 3 , and Motomu Tanaka 1 — 1 Lehrstuhl für
Biophysik E22, Technische Universität München, James-Franck-Str. 1,
85748 Garching, Germany — 2 Institut Laue-Langevin, BP 156, F-38042
Grenoble Cedex 09, France — 3 Fachbereich Chemie, Universität Konstanz,
Fach M-725, D-78457 Konstanz, Germany
In animal cells, oligo- and polysaccharide chains form complexes with
glycolipids, glycoproteins, and proteoglycans (called glycocalix), and are
mostly expressed on cellular surfaces. Complex interplays of various physical
forces (electrostatic interaction, hydrogen bonding, etc.) enable them
to keep distinct conformation to serve not only as a hydrophilic cushion to
maintain intercellular spacing but also as a selective ligand against complimentary
receptors. To date, however, only a few studies have been conducted
to clarify the impact of molecular structures on their in-plane and
inter-plane correlation. In this study we investigated the hydration limits
and morphology of synthetic glycolipids under controlled osmotic pressure.
Multilayers of synthetic deuterated and protonated glycolipids with
different sugar head groups were deposited on silicon wafers by solvent
casting, and the quantative force-distance relationships were measured
in a humidity chamber using small angle neutron scattering (SANS).
AKB 50.122 Fr 10:30 B
Deposition of Supported Membranes with F1F0-ATP-Synthase
on Cellulose Thin Films — •Murat Tutus 1 , Thomas Nawroth 2 ,
and Motomu Tanaka 1 — 1 Biophysics Lab, Tech. Univ. Munich —
2 Dept. Phys. E17, Tech. Univ. Munich
A new method for the reconstitution of proteins (F1F0-ATP-Synthase
of Micrococcus luteus) into lipid vesicles without causing membrane disruption
is developed. To verify non-disruptive, orientation selective reconstitution
of ATP-Synthase in lipid vesicles, the vesicle size before and
after insertion is quantitatively compared by dynamic light scattering
experiments. To determine the side selectivity the proteoliposomes were
spread onto biocompatible ultra thin (thickness of 5 to 10 nm) polymer
supports where the homogeneity and the lateral mobility are characterized
with fluorescence microscopy and fluorescence recovery after photobleaching.
The protein distribution is checked by covalent coupling of
synthetic dyes as well as by immunofluorescence labeling with antibodies.
The glass slides coated with ultra thin cellulose films serve as interlayer
to achieve homogeneous distribution over macroscopically large (cm2 order)
surfaces. Moreover, immuno-labeling of F1 head group verifies the
reconstitution of the entire protein into the supported membrane.
AKB 50.123 Fr 10:30 B
Structure and Elasticity of DNA and Chromatin — •Ralf Everaers
1 and Boris Mergell 2 — 1 Max-Planck-Institut für Physik komplexer
Systeme, Nöthnitzerstr. 38, 01187 Dresden, Germany — 2 Max-
Planck-Institut für Polymerforschung, Ackermannweg 10, 55128 Mainz,
Germany
We use computer simulations to study structure formation and response
to mechanical forces in generic linked rigid body models of DNA
and chromatin. In the case of DNA we use a variant of the Gay-Berne
potential to represent the stacking interactions between neighboring basepairs.
The sugar-phosphate backbones are taken into account by semirigid
harmonic springs with a non-zero spring length. The competition of
these two interactions and the introduction of a simple geometrical constraint
lead to a stacked right-handed B-DNA-like conformation. Beyond
a critical stretching force we observe a transition to an overstretched S-
DNA conformation with highly inclined bases that partially preserves the
stacking of successive base-pairs. The geometry of the chromatin fiber is
based on the two-angle crossed-linker model. In this case the Gay-Berne
potential is used to model the excluded volume and short-range attraction
between nucleosomes. We find that the elastic properties such as
the bending and stretching moduli of condensed fibers are dominated
by the internucleosomal interactions. They can lead to the formation of
hairpin conformations whose tension induced opening manifests itself in
a quasi-plateau in the force-extension curve.
AKB 50.124 Fr 10:30 B
Towards folding and assembly of single light-harvesting
complexes from plants — •Peter Schwaderer 1 , Sebastian
Schuler 1 , Carsten Tietz 1 , Ulrich Gerlach 2 , Harald
Paulsen 2 , and Jörg Wrachtrup 1 — 1 3. Physikalisches Institut,
Universität Stuttgart — 2 Institut für Allg. Botanik, Universität Mainz
Individual light-harvesting chlorophyll a b protein complexes (LHCII)
from higher plants are investigated in vitro at room temperature. The
LHCII apoprotein binds about 15 pigments, chlorophylls (Chl) a and b
and carotenoids, and probably is the most abundant membrane protein
on earth.
The advantages of TIR microscopy are applied to investigate the properties
of single LHCII proteins in detergent solution that have been immobilized
on a quartz coverslip by different techniques. TIR excitation
via a prism as well as total reflection within a high numerical aperture
microscope objective are tested to find the best signal to background ratio.
Thus, the time resolved Chl fluorescence of single LHCII molecules
can be observed.
This experiment is a first step towards immobilizing the LHCII apoprotein
which is known to fold in the presents of Chl a, Chl a, and carotenoid
pigments. As the fluorescence of unbound pigments is not observable on
single molecule level because the molecules are trapped in the long-lived
triplet states, the fluorescence of correctly folded complexes, where the
triplet states of the Chl molecules are quenched by the carotenoids, can
be used as a monitor for the folding process.
AKB 50.125 Fr 10:30 B
Optical Microheology on Biological Cells — •Falk Wottawah,
Stefan Schinkinger, Bryan Lincoln, Maren Romeyke, Revathi
Ananthakrishnan, Josef Käs, and Jochen Guck — Fakultät für
Physik und Geowissenschaften, Universität Leipzig
Biological cells, such as fibroblasts, can be described as a polymeric
compound material. Their passive rheological response to optically applied
step stresses on the time scale of seconds suggests a hybrid between
a crosslinked and an entangled actin network, a transiently crosslinked
actin cortex, as the main contributor. Their frequency dependent shear
modulus reveals elastic to viscous transitions, requiring stress relaxation
beyond reptation-based mechanisms. This viscoelastic response is defining
for cells within the same cell line, and distinguishes between different
cell lines. Strain on the time scale of a minute additionally triggers an
active response, exceeding the mere relaxation of applied stresses.
AKB 50.126 Fr 10:30 B
Molecular motors in cells: an analogue for thermotropic ordering
— •David Smith and Josef Käs — Linne Str 5, 04103, Leipzig
All eukaryotic cells rely on the self-assembly of protein filaments to
form an intracellular cytoskeleton. The need for motility and reaction additionally
requires pathways that restructure and disassemble cytoskeletal
structures. Temperature-driven increases in disorder are the most obvious
method, thermodynamically, for dissolving complex structures, yet
could denature cellular components. This is exemplified in the unfolding
of double-stranded DNA for duplication. While de-hybridization of
the strands by a temperature increase represents the simplest pathway,
molecular motors are present to perform the same function in the nucleus
without heat-induced damage to the cell. We report another fundamental
mechanism whereby changes in the activity of the molecular motor
myosin II induce order-disorder transitions in actin networks. In nearchemical-rest
states, aggregates of myosin II motors acting as crosslinkers
induce a compounded state of aligned actin filaments and motors. This
results in the self-assembly of various macro-molecular structures such as
asters, neuron-like networks, and condensed super-precipitates. However,
when the molecular motors are turned on and the system assumes an active
nonequilibrium state, the myosin II-induced filament motility maintains
a disordered high-entropy state. Experiments with photo-activated
motors show the rapid dynamics of disassembly of actomyosin structures
and the reversibility of the changes. This ability for rapid transitions of
the entropic state by motor activity indicates that molecular motors, in
general, may substantially contribute to dynamic cellular organization.