Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
the molecule. Small cantilevers (length: < 30µm, width: < 10µm, thickness:
< 200nm) show all necessary properties for force spectroscopy:
small spring constants, low viscous damping and high resonance frequencies.
We present an AFM that is capable of using small cantilevers for force
spectroscopy experiments of single biomolecules and our current results.
AKB 50.82 Fr 10:30 B
X-ray scattering and microscopy on spider silk fibers — •Anja
Glisovic, Juergen Thieme, Peter Guttmann, and Tim Salditt
— Institut fuer Roentgenphysik, Universitaet Goettingen
We report on the structural characterization of different types of spider
silk. Spider silk is a high performance biomaterial with a unique combination
of elastic properties consisting of only one to two proteins. The
structural basis for these properties on the molecular and mesoscopic
scale of the silk fiber is a matter of intensive scientific debate [1]. We
have used synchrotron based x-ray diffraction as well as x-ray microscopy
to investigate single as well as bundle of fibers. Some technical aspects
(beam collimation, background to noise, analysis of crystalline domains)
and first results of these experiments will be discussed. [1] F. Vollrath,
Knight DP, Nature 410, 541 (2001), and references therein.
AKB 50.83 Fr 10:30 B
Transistor Array probes Release of Vesicles of Chromaffin Cells
— •Janosch Lichtenberger and Peter Fromherz — Membran
and Neurophysics,MPI for Biohemistry
We monitored the release of large dense core vesicles from bovine chromaffin
cells using a linear array of open field effect transistors with a pitch
of 3.6 µm. When secretion was induced by barium, brief spikes in the
transistor current were observed. The events were well localized on the
transistor array with an amplitude of effective gate voltage up to 17 mV.
We assign the events to the local drop of pH in the narrow cleft between
cell and chip that is caused by the release of ATP by individual vesicles.
The pH change affects the threshold of the transistor by proton binding
to the exposed gate oxide. We found good agreement for amplitude,
duration and localization of the signals with the change of the electrical
surface potential that is computed with a model that takes into account
(i) local release of ATP from a vesicle at pH 5.5 into the cleft with an extracellular
buffer at pH 7.2, (ii) diffusion of protons, buffer and ions along
the cleft and (iii) binding of protons to the negatively charged oxide. The
investigation establishes the first chemical neuron-silicon synapse.
AKB 50.84 Fr 10:30 B
Effective pair–potential approach to entangled stiff polymers —
•Sven van Teeffelen, Erwin Frey, and Klaus Kroy — Hahn-
Meitner Institut, Berlin
The entanglement of stiff polymers in solution is remarkably well described
by an effective tube model, in complete analogy to the well–known
blob model for flexible polymers. We extend the scope of this (so far homogeneous)
model to account for spatial density fluctuations by reformulating
it in terms of a microscopically motivated effective pair potential.
This allows us to straightforwardly include additional interactions (depletion,
van der Waals, electrostatic...), and thus turns the model into
a versatile tool for predicting the static structure factor and the equilibrium
phase behavior of stiff polymer solutions. Non–equilibrium (kinetic
arrest) scenarios are also considered. Finally we discuss applications to
biopolymer solutions.
AKB 50.85 Fr 10:30 B
Silicon chip with cultured rat hippocampus slice interfaced with
arrays of capacitors and transistors — •Michael Hutzler and
Peter Fromherz — Max Planck Institute of Biochemistry, Martinsried,
Germany
In the past, field potentials of cultured hippocampal slices evoked by
tungsten electrode stimulation could be recorded by electrolyte-oxidesilicon
field effect transistors. We developed a new silicon chip with a
TiO2-coated surface, containing capacitor arrays for eliciting as well as
transistor arrays for detecting neuronal activity. After cultivating the
brain slices on the silicon chips for one week, we were able to capacitively
stimulate the slices in CA3 by application of defined voltage pulses. The
resulting field potential in CA1 could be recorded with the transistors.
By combining a row of capacitors with a row of transistors we also determined
a simple transfer matrix from CA3 to CA1. This novel type
of purely capacitive interfacing allows a mechanically noninvasive and
electrically minimally interfering contact compared to traditional electrophysiological
methods.
AKB 50.86 Fr 10:30 B
Growth of the Mineral Particles in Bone - Combined Study of
Small Angle X-ray Scattering (SAXS) and Electron Backscattering
(qBEI) — •A. Valenta 1,2 , P. Roschger 2 , B.M. Misof 2 ,
O. Paris 3 , W. Tesch 1,2 , S. Bernstorff 4 , H. Amenitsch 5 , K.
Klaushofer 2 , and P. Fratzl 3 — 1 Erich Schmid Inst. of Material Science,
Austrian Academy of Sciences and Inst. of Metal Physics, University
of Leoben, Leoben, Austria — 2 L. Boltzmann Inst. of Osteology,
4th Med. Dept., Hanusch Hospital & UKH-Meidling, Vienna, Austria
— 3 Max Planck Inst. of Colloids and Interfaces, Dept. of Biomaterials,
Potsdam, Germany — 4 Sincrotrone Trieste S.C.p.A., Basovizza, Trieste,
Italy — 5 IBR, Austrian Academy of Sciences, Graz, Austria
Bone is a nanofiber composite formed by mineralized collagen fibrils.
In this study bone areas from human biopsies with different degree of
mineralization were investigated. The mineral volume fraction (φ) was
assessed by qBEI, and then the particle surface per volume (σ) was determined
by scanning-SAXS using a micro focus (20 micron) synchrotron
x-ray beam. A biphasic correlation between φ and σ was found: In the
φ-range of 0-27 vol% mineral σ showed a monotone increase, whereas
in the range of 27-40 vol% σ remained constant. This finding suggests,
that after nucleation, mineralization proceeds by a rapid predominant-
2-dimensional growth of the mineral particles, followed by a slow increase
in thickness.
AKB 50.87 Fr 10:30 B
SARS membrane protein E in model membranes: structural
— •Ziad Khattari 1 , Guillaume Brotons 1 , Tim Salditt 1 , and
Shy Arkin 2 — 1 Institut fuer Roentgenphysik, Universitaet Goettingen,
Goettingen — 2 Department of Biological Chemistry, Hebrew University,
Jerusalm
We present a structural investigation of the SARS membrane protein
E in model membranes by x-ray reflectivity. After the recent publication
of the SARS coronavirus genome [1], structural characterization of its
membrane active proteins is of great importance. The SARS membrane
protein E is believed to be a viral ion channel, but it may also exhibit
fusiogenic functions. The structure and interaction of the membrane active
part of the protein is therefore investigated in model membranes.
Using x-ray reflectivity on highly aligned stacks of membranes on silicon
surfaces in the fluid La phase [2,3], we can determine the electron
density profile of the lipid bilayer as a function of peptide-lipid (P/L)
ratio. Structural properties of the peptide can be determined, as well as
changes in lipid bilayer properties as a function of protein concentration
may be assessed, ranging from bilayer thickness to acyl chain ordering
and head-group hydration. In addition we use site-specific iodination as
a marker in the density profile. Measurements have been performed at
the D4 bending magnet station of HASYLAB/DESY. The results are
complemented by spatial restraints from FTIR spectroscopy on samples
containing site-specific isotopic labels (peptidic 13C=18O). [1] Marra et
al. Science 300, 1399 (2003). [2] T. Salditt et al, Eur. Phys. J. E 7, 105
(2002). [3] Li, C. et al, accepted in J. Phys. D.
AKB 50.88 Fr 10:30 B
Dynamics of Lipid and Protein Domains in Biomembranes
— •Karin John and Markus Bär — Max-Planck-Institute für die
Physik komplexer Systeme, Nöthnitzer Strasse 38, D-01187 Dresden
Acidic lipids such as PIP2 and PIP3 are thought to elicit localized responses,
e.g. for remodeling the cytoskeleton in response to external stimuli.
We consider a mechanism that accounts for a nonrandom distribution
of acidic lipids in the plasma membrane: electrostatic sequestration by
basic proteins such as GMC (MARCKS, CAP23, GAP43) proteins. Our
strategy is to incorporate the different properties of GMC proteins into
a reaction-diffusion model:
1. GMC proteins are cytosolic proteins. Membrane association depends
on a basic effector domain, which interacts with acidic lipids in the membrane
and can lead to the formation of domains enriched in acidic lipids
and GMC.
2. GMC proteins are probably integrators of PKC and Ca 2+ signalling
pathways. Upon phosphorylation of residues within the basic effector domain
by a protein kinase C or interaction with Ca ++ /calmodulin GMC
proteins translocate from the membrane into the cytosol. Upon dephosphorylation
or a decrease in cytosolic Ca ++ GMC proteins reassociates
with the membrane. This cycle is called myristoyl-electrostatic switch