Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
AKB 50.127 Fr 10:30 B
Induced changes in comformation and charge density of thin
polymer films at the interface - a neutron and x-ray reflectivity
study towards controlled protein-surface interaction —
•Florian Rehfeldt 1 , Roland Steitz 2 , Regine v. Klitzing 2 , and
Motomu Tanaka 1 — 1 Lehrstuhl für Biophysik E22, Technische Universität
München, James-Franck-Str. 1, 85748 Garching, Germany —
2 Stranski-Laboratorium, TU Berlin, Straße d. 17. Juni 112, D-10623
Berlin, Germany
The switching of physical properties of thin polymer films (charge density,
conformation, etc.) under physiological conditions draws increasing
attention to control cell-surface/protein-surface interaction by means of
external stimulation. For this purpose, one of the attractive candidates is
a relatively ”weak” polyelectrolyte, whose degree of ionisation (D.I.) can
be controlled by pH titration. We chose a diblock-copolymer that possesses
a hydrophobic MMA block and a hydrophilic DMAEMA block.
The D.I. of DMAEMA block is 0.6 at pH = 5.5, and 0.2 at pH=8.5,
respectively. When the block ratio is 1:1, this type of diblock forms an
insoluble monolayer at air/water interface. Recently, we demonstrated
the adsorption and desorption of weakly charged DMAEMA chains at
air/water interface by pH titration. Such insoluble monolayers can be
transferred from air/water interface onto hydrophobized silicon blocks,
where distinct changes in the layer thickness as well as in the scattering
length density were monitored with neutron reflectivity. Furthermore,
adsorption of water-soluble proteins on charged- and uncharged brushes
can be detected by ex-situ x-ray reflectivity and fluorescence microscopy.
AKB 50.128 Fr 10:30 B
Solid domains in fluid vesicles — •Erwin Gutlederer, Thomas
Gruhn und Reinhard Lipowsky — MPI Kolloid- und Grenzflaechenforschung
14424 Potsdam
Recent experiments have shown the existence of solid domains in multicomponent
fluid vesicles when they are cooled below a certain temperature.
In our theoretical approach we analyse thermodynamic properties
of this domain formation. With the help of free energy minimisation techniques
we estimate the curvature energy of vesicles with a solid domain
as a function of the domain size. This allows us to investigate the temperature
dependence of domain formation for various values of bending
rigidities and molecular composition of the vesicle membrane.
AKB 50.129 Fr 10:30 B
Functional Incorporation of Cell Receptors into Polymer
Tethered Membranes - Influence of Spacer Length and Density
— •Oliver Purrucker 1 , Stephanie Gönnenwein 1 , Monika
Rusp 1 , Anton Förtig 2 , Rainer Jordan 2 , and Motomu Tanaka 1
— 1 Technische Universität München, Lehrstuhl für Biophysik E22,
James-Franck-Str., D-85748 Garching — 2 Technische Universität
München, Lehrstuhl für Makromolekulare Stoffe, Lichtenbergstr. 4,
D-85748 Garching
To study physical principles of cell-cell interactions, supported lipid
membranes with cell surface receptors have been intensively and widely
investigated as a general model of plasma membranes. The close proximity
of the artificial membrane to the solid support (0.5-2 nm) does not
provide a sufficient water reservoir that even causes nonspecific adsorption
and denaturing of proteins. This problem can be overcome by introduction
of hydrophilic polymer spacers, which can provide more ”fluid”
environments for proteins. In our recent study, we designed a new type
of polymer-tethered membrane with defined poly(2-methyl-2-oxazoline)
lipopolymer spacers and could demonstrate the incorporation of transmembrane
cell receptors 1 .
Measurements of adhesion free energy of a ligand-doped lipid vesicle
on the supported membrane with integrin αIIbβ3 receptors, we found
that the functionality of incorporated integrin receptors systematically
depends on the density and length of the lipopolymer tethers.
[1] O. Purrucker, A. Förtig, R. Jordan, M. Tanaka, ChemPhysChem
in print
AKB 50.130 Fr 10:30 B
Dissipative Micro-Domain Formation in Transferred Lipid
/ Lipopolymer Monolayers — •Oliver Purrucker 1 , Anton
Förtig 2 , Rainer Jordan 2 , and Motomu Tanaka 1 — 1 Technische
Universität München, Lehrstuhl für Biophysik E22, James-Franck-Str.,
D-85748 Garching — 2 Technische Universität München, Lehrstuhl für
Makromolekulare Stoffe, Lichtenbergstr. 4, D-85748 Garching
Ultra-thin (d < 10 nm) liquid films that ”wet” solid surfaces can be
stabilized by surfactant films covering the air/liquid interface. E.g., single
chain surfactants can increase the thickness of a glucose solution by two
orders of magnitude 1 . Langmuir monolayers of lipid/lipopolymer mixtures
can be treated as models of cell surfaces, where hydrated polymer
films create high osmotic pressures to stabilize membrane structures 2 .
Here, we observed that Langmuir-Blodgett lipid/lipopolymer monolayers
form stripe-like microscopic domains that align parallel to the
transfer direction. Systematic characterization before and after transfer
demonstrated that formation of such micro-domains is not caused
by thermodynamic phase separation. Width and spacing of stripes are
clearly dependent on transfer velocity, suggesting a formation due to hydrodynamic
forces operating within ultra-thin liquid films between lipids
and substrates.
(1) G. Elender, E. Sackmann, J. Phys. II France 4 (1994) 455
(2) E. Sackmann, M. Tanaka, Trends Biotechnol. 18 (2000) 58
AKB 50.131 Fr 10:30 B
Biophysical applications of chemically engineered GaAs-based
semiconductors — •Klaus Adlkofer 1 , Daniel Gassul 1 , Andrey
Shaporenko 2 , Michael Zharnikov 2 , Michael Grunze 2 , Abraham
Ulman 3 , and Motomu Tanaka 1 — 1 Lehrstuhl für Biophysik
E22, Technische Universität München — 2 Lehrstuhl für Angewandte
Physikalische Chemie, Universität Heidelberg — 3 Polytechnic University
Brooklyn, New York, USA
Stable chemical engineering of stoichiometric GaAs [100] surfaces was
achieved by deposition of 4-′-substituted-4-mercaptobiphenyl terminated
with −H, −CH3 and −OH. Topography of the engineered surface was
studied by atomic force microscopy (AFM). Orientation of the molecules
was evaluated by near edge x-ray fine structure (NEXAFS) spectroscopy
and Fourier transformed infrared spectroscopy (FTIR), and the covalent
binding between the thiolate and surface arsenide was confirmed
by high-resolution x-ray photoelectron spectroscopy (HRXPS). Surface
free energies of the engineered surfaces were calculated from contact angle
measurements. Current-voltage scans showed the drastic suppression
of oxidation and reduction by coating of GaAs. Electrochemical properties
of the engineered GaAs in aqueous electrolytes were measured by
impedance spectroscopy, demonstrating that all three types of mercaptobiphenyls
can form stable monolayers with high electric resistances,
R > 2MΩcm 2 . The surface engineering method established here allows
for control of surface free energies towards deposition of model biomembranes
on GaAs-based device surfaces.
AKB 50.132 Fr 10:30 B
Thermal fluctuation analysis of grafted microtubules —
•Francesco Pampaloni 1 , Gianluca Lattanzi 2 , A. Jonas 1 ,
Erwin Frey 2 , Ernst-Ludwig Florin 1 , and 3 for the collaboration
— 1 EMBL, Meyerhofstrasse 1, D-69117 Heidelberg — 2 Abteilung
Theorie, Hahn-Meitner Institut, Glienicker Strasse 100, D-14109 Berlin
— 3
Cytoskeletal filaments play a fundamental role in imparting polarity
to the cell, determining the plane of symmetry in cell division, and regulating
cell movements and shape. Unlike most of the synthetic organic
polymers, cytoskeletal filaments are semiflexible polymers, with a typical
stiffness that is intermediate between that of a random coil and that of a
rigid rod. A basic parameter for characterizing the stiffness of polymeric
fibres is the persistence length, which is defined as the typical length over
which correlations of the tangent vectors of the filament contour decay. A
persistence length comparable to the filaments contour length, defined as
the maximum end-to-end distance, is a characteristic property of semiflexible
polymers. Whereas an established theoretical framework already
exists for flexible polymers, the static and dynamic properties of single
semiflexible filaments are still the object of intense and challenging theoretical
investigations [1]. Semiflexible chains have many interesting features
that disappear in the flexible and stiff limits. For example, the distribution
function of the end-to-end distances has a typical non-Gaussian
shape with the weight of the distribution shifting toward full stretching
[2, 3]. In this work, we present a novel method to measure the predicted
end-to-end distributions for grafted microtubules in three-dimensions using
a recently developed method for high-resolution particle tracking [4]
and without any disturbance by surface effects that affects earlier work.
In our assay, one end of the microtubules is grafted to a gold substrate
while the other end is freely fluctuating in solution. The thermally-driven
motion of the microtubules is measured by tracking the positions of single
200 nm fluorescent beads attached to them. Since the supporting