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Plenarvorträge - DPG-Tagungen

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Arbeitskreis Biologische Physik Freitag<br />

the molecule. Small cantilevers (length: < 30µm, width: < 10µm, thickness:<br />

< 200nm) show all necessary properties for force spectroscopy:<br />

small spring constants, low viscous damping and high resonance frequencies.<br />

We present an AFM that is capable of using small cantilevers for force<br />

spectroscopy experiments of single biomolecules and our current results.<br />

AKB 50.82 Fr 10:30 B<br />

X-ray scattering and microscopy on spider silk fibers — •Anja<br />

Glisovic, Juergen Thieme, Peter Guttmann, and Tim Salditt<br />

— Institut fuer Roentgenphysik, Universitaet Goettingen<br />

We report on the structural characterization of different types of spider<br />

silk. Spider silk is a high performance biomaterial with a unique combination<br />

of elastic properties consisting of only one to two proteins. The<br />

structural basis for these properties on the molecular and mesoscopic<br />

scale of the silk fiber is a matter of intensive scientific debate [1]. We<br />

have used synchrotron based x-ray diffraction as well as x-ray microscopy<br />

to investigate single as well as bundle of fibers. Some technical aspects<br />

(beam collimation, background to noise, analysis of crystalline domains)<br />

and first results of these experiments will be discussed. [1] F. Vollrath,<br />

Knight DP, Nature 410, 541 (2001), and references therein.<br />

AKB 50.83 Fr 10:30 B<br />

Transistor Array probes Release of Vesicles of Chromaffin Cells<br />

— •Janosch Lichtenberger and Peter Fromherz — Membran<br />

and Neurophysics,MPI for Biohemistry<br />

We monitored the release of large dense core vesicles from bovine chromaffin<br />

cells using a linear array of open field effect transistors with a pitch<br />

of 3.6 µm. When secretion was induced by barium, brief spikes in the<br />

transistor current were observed. The events were well localized on the<br />

transistor array with an amplitude of effective gate voltage up to 17 mV.<br />

We assign the events to the local drop of pH in the narrow cleft between<br />

cell and chip that is caused by the release of ATP by individual vesicles.<br />

The pH change affects the threshold of the transistor by proton binding<br />

to the exposed gate oxide. We found good agreement for amplitude,<br />

duration and localization of the signals with the change of the electrical<br />

surface potential that is computed with a model that takes into account<br />

(i) local release of ATP from a vesicle at pH 5.5 into the cleft with an extracellular<br />

buffer at pH 7.2, (ii) diffusion of protons, buffer and ions along<br />

the cleft and (iii) binding of protons to the negatively charged oxide. The<br />

investigation establishes the first chemical neuron-silicon synapse.<br />

AKB 50.84 Fr 10:30 B<br />

Effective pair–potential approach to entangled stiff polymers —<br />

•Sven van Teeffelen, Erwin Frey, and Klaus Kroy — Hahn-<br />

Meitner Institut, Berlin<br />

The entanglement of stiff polymers in solution is remarkably well described<br />

by an effective tube model, in complete analogy to the well–known<br />

blob model for flexible polymers. We extend the scope of this (so far homogeneous)<br />

model to account for spatial density fluctuations by reformulating<br />

it in terms of a microscopically motivated effective pair potential.<br />

This allows us to straightforwardly include additional interactions (depletion,<br />

van der Waals, electrostatic...), and thus turns the model into<br />

a versatile tool for predicting the static structure factor and the equilibrium<br />

phase behavior of stiff polymer solutions. Non–equilibrium (kinetic<br />

arrest) scenarios are also considered. Finally we discuss applications to<br />

biopolymer solutions.<br />

AKB 50.85 Fr 10:30 B<br />

Silicon chip with cultured rat hippocampus slice interfaced with<br />

arrays of capacitors and transistors — •Michael Hutzler and<br />

Peter Fromherz — Max Planck Institute of Biochemistry, Martinsried,<br />

Germany<br />

In the past, field potentials of cultured hippocampal slices evoked by<br />

tungsten electrode stimulation could be recorded by electrolyte-oxidesilicon<br />

field effect transistors. We developed a new silicon chip with a<br />

TiO2-coated surface, containing capacitor arrays for eliciting as well as<br />

transistor arrays for detecting neuronal activity. After cultivating the<br />

brain slices on the silicon chips for one week, we were able to capacitively<br />

stimulate the slices in CA3 by application of defined voltage pulses. The<br />

resulting field potential in CA1 could be recorded with the transistors.<br />

By combining a row of capacitors with a row of transistors we also determined<br />

a simple transfer matrix from CA3 to CA1. This novel type<br />

of purely capacitive interfacing allows a mechanically noninvasive and<br />

electrically minimally interfering contact compared to traditional electrophysiological<br />

methods.<br />

AKB 50.86 Fr 10:30 B<br />

Growth of the Mineral Particles in Bone - Combined Study of<br />

Small Angle X-ray Scattering (SAXS) and Electron Backscattering<br />

(qBEI) — •A. Valenta 1,2 , P. Roschger 2 , B.M. Misof 2 ,<br />

O. Paris 3 , W. Tesch 1,2 , S. Bernstorff 4 , H. Amenitsch 5 , K.<br />

Klaushofer 2 , and P. Fratzl 3 — 1 Erich Schmid Inst. of Material Science,<br />

Austrian Academy of Sciences and Inst. of Metal Physics, University<br />

of Leoben, Leoben, Austria — 2 L. Boltzmann Inst. of Osteology,<br />

4th Med. Dept., Hanusch Hospital & UKH-Meidling, Vienna, Austria<br />

— 3 Max Planck Inst. of Colloids and Interfaces, Dept. of Biomaterials,<br />

Potsdam, Germany — 4 Sincrotrone Trieste S.C.p.A., Basovizza, Trieste,<br />

Italy — 5 IBR, Austrian Academy of Sciences, Graz, Austria<br />

Bone is a nanofiber composite formed by mineralized collagen fibrils.<br />

In this study bone areas from human biopsies with different degree of<br />

mineralization were investigated. The mineral volume fraction (φ) was<br />

assessed by qBEI, and then the particle surface per volume (σ) was determined<br />

by scanning-SAXS using a micro focus (20 micron) synchrotron<br />

x-ray beam. A biphasic correlation between φ and σ was found: In the<br />

φ-range of 0-27 vol% mineral σ showed a monotone increase, whereas<br />

in the range of 27-40 vol% σ remained constant. This finding suggests,<br />

that after nucleation, mineralization proceeds by a rapid predominant-<br />

2-dimensional growth of the mineral particles, followed by a slow increase<br />

in thickness.<br />

AKB 50.87 Fr 10:30 B<br />

SARS membrane protein E in model membranes: structural<br />

— •Ziad Khattari 1 , Guillaume Brotons 1 , Tim Salditt 1 , and<br />

Shy Arkin 2 — 1 Institut fuer Roentgenphysik, Universitaet Goettingen,<br />

Goettingen — 2 Department of Biological Chemistry, Hebrew University,<br />

Jerusalm<br />

We present a structural investigation of the SARS membrane protein<br />

E in model membranes by x-ray reflectivity. After the recent publication<br />

of the SARS coronavirus genome [1], structural characterization of its<br />

membrane active proteins is of great importance. The SARS membrane<br />

protein E is believed to be a viral ion channel, but it may also exhibit<br />

fusiogenic functions. The structure and interaction of the membrane active<br />

part of the protein is therefore investigated in model membranes.<br />

Using x-ray reflectivity on highly aligned stacks of membranes on silicon<br />

surfaces in the fluid La phase [2,3], we can determine the electron<br />

density profile of the lipid bilayer as a function of peptide-lipid (P/L)<br />

ratio. Structural properties of the peptide can be determined, as well as<br />

changes in lipid bilayer properties as a function of protein concentration<br />

may be assessed, ranging from bilayer thickness to acyl chain ordering<br />

and head-group hydration. In addition we use site-specific iodination as<br />

a marker in the density profile. Measurements have been performed at<br />

the D4 bending magnet station of HASYLAB/DESY. The results are<br />

complemented by spatial restraints from FTIR spectroscopy on samples<br />

containing site-specific isotopic labels (peptidic 13C=18O). [1] Marra et<br />

al. Science 300, 1399 (2003). [2] T. Salditt et al, Eur. Phys. J. E 7, 105<br />

(2002). [3] Li, C. et al, accepted in J. Phys. D.<br />

AKB 50.88 Fr 10:30 B<br />

Dynamics of Lipid and Protein Domains in Biomembranes<br />

— •Karin John and Markus Bär — Max-Planck-Institute für die<br />

Physik komplexer Systeme, Nöthnitzer Strasse 38, D-01187 Dresden<br />

Acidic lipids such as PIP2 and PIP3 are thought to elicit localized responses,<br />

e.g. for remodeling the cytoskeleton in response to external stimuli.<br />

We consider a mechanism that accounts for a nonrandom distribution<br />

of acidic lipids in the plasma membrane: electrostatic sequestration by<br />

basic proteins such as GMC (MARCKS, CAP23, GAP43) proteins. Our<br />

strategy is to incorporate the different properties of GMC proteins into<br />

a reaction-diffusion model:<br />

1. GMC proteins are cytosolic proteins. Membrane association depends<br />

on a basic effector domain, which interacts with acidic lipids in the membrane<br />

and can lead to the formation of domains enriched in acidic lipids<br />

and GMC.<br />

2. GMC proteins are probably integrators of PKC and Ca 2+ signalling<br />

pathways. Upon phosphorylation of residues within the basic effector domain<br />

by a protein kinase C or interaction with Ca ++ /calmodulin GMC<br />

proteins translocate from the membrane into the cytosol. Upon dephosphorylation<br />

or a decrease in cytosolic Ca ++ GMC proteins reassociates<br />

with the membrane. This cycle is called myristoyl-electrostatic switch

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