Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
in 3:1 mol ratio, and mixture of DMPC/DMPG (1,2-Dimyristoyl-sn-
Glycero-3-[Phospho-rac-(1-glycerol)]) also in 3:1 mol ratio with and without
the antibiotic peptide magainin 2. For the pure OPPC lipid, the
resulting bilayer electron density profile agrees well with previously published
data of a molecular dynamics (MD) simulation.
AKB 50.68 Fr 10:30 B
Investigation of Lipid Multilayers under Electric Field Using
X-Ray Reflectivity — •Doru Constantin, Christoph Ollinger,
and Tim Salditt — Institut fuer Roentgenphysik, Universitaet Goettingen
The influence of an external electric field upon lipid bilayers is of
paramount importance, both from a practical and a fundamental point
of view. Cell electroporation has been long employed for introducing exogenous
molecules into cells, while electroformation is the best method to
date for obtaining giant unilamellar vesicles. Although very important,
these phenomena are still in need of a clear-cut explanation.
We studied the behaviour of solid-supported multilayers of the zwitterionic
lipid DMPC under an applied transverse electric field, using specular
and non-specular X-ray scattering. As a first step, we determined the kinetics
of unbinding under field for various applied voltages, both for thin
(tens of bilayers) and thick (thousands of bilayers) samples. Surprising
similarities with the previously characterized thermal unbinding kinetics
were revealed.
No changes were detected in the diffuse scattering signal, hinting that
the unbinding could be a surface phenomenon, not related to instabilities
in the bulk, but rather affecting only a few layers at the top of the stack.
This hypothesis is reinforced by the fact that the time correlation function
of the reflectivity signal from thin samples exhibits a peak at the
frequency of the applied field, which does not appear for thick samples.
AKB 50.69 Fr 10:30 B
A cluster mode–coupling approach to weak gelation in colloids
and proteins — •Klaus Kroy — Hahn-Meitner Institut, Berlin
Mode–coupling theory (MCT) predicts arrest of colloids in terms of
their volume fraction, and the range and depth of the interparticle attraction.
We discuss how effective values of these parameters evolve under
cluster aggregation. We argue that weak gelation in colloids and proteins
can be idealized as a two–stage ergodicity breaking: first at short scales
(approximated by the bare MCT) and then at larger scales (governed
by MCT applied to clusters). The competition between arrest and phase
separation is considered in relation to recent experiments with colloids
and proteins. Among other things we predict a long–lived ‘semi–ergodic’
phase of mobile clusters, a ‘kinked’ gel line, and strong stretching of the
relaxation even at low volume fractions.
(Work done in collaboration with the soft condensed matter group of
the University of Edinburgh, see cond-mat/0310566,0309616)
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Strain hardening of biological tissue — •Klaus Kroy and
Thomas Franosch — Hahn-Meitner Institut, Berlin
It is an everyday experience that our tissue can easily be stretched to
a certain (substantial) extent before dramatic strain hardening painfully
prevents any further elongation. Our toy model for the non–linear elasticity
of stiff polymer networks rationalizes this general phenomenology
in terms of non–affine local deformations of the individual polymers that
are attributed to their highly anisotropic elastic response. We analyze
a particularly simple schematic ansatz for this non–affinity guided by
an analogy to dilatancy in granular media, which leads to analytic results
for the non–linear modulus and the yield strain as a function of the
concentration and stiffness of the polymers.
AKB 50.71 Fr 10:30 B
Small Angle X-ray Scattering (SAXS) of Novel Pd Bionanocatalysts
— •Barbara Aichmayer 1,2 , Ingomar Jäger 2 , Michael
Mertig 3 , Oskar Paris 4 , and Peter Fratzl 4 — 1 Erich Schmid Institute
of Material Science, Austrian Academy of Sciences, Leoben, Austria
— 2 Institute of Metal Physics, University of Leoben, Austria — 3 IfWW,
University of Technology, Dresden, Germany — 4 Max-Planck-Institute of
Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany
We investigate two types of new bionanocatalysts which are produced
using biotechnology: the cell surface of bacteria as well as twodimensional
S-layer protein crystals serve as templates for the deposition
of Pd crystals. Structural investigations of the Pd particles were carried
out in order to optimise the catalysts and to find out how the biological
templates affect the precipitation of Pd. By means of SAXS the specific
surface and size of the Pd particles were evaluated. Correlations between
the specific surface and the catalytic activity were proven. Indications
for a change of the S-layer protein structure caused by the metal loading
were observed. Furthermore, it was shown that the particle formation
is affected by synchrotron radiation (SR). Pd clusters with a diameter
of 2-3 nm were built in the synchrotron beam. Although the amount of
these particles increased with increasing exposure time to SR, no coarsening
of the particles occurred. From this behaviour we conclude that the
nanoclusters are stabilized by the protein which prevents further growth
of the Pd particles.
This work is supported by the European Union Grant number GRD1-
2001-40424.
AKB 50.72 Fr 10:30 B
NEURON-SEMICONDUCTOR CHIP WITH CHEMICAL
SYNAPSES BETWEEN PAIRS OF IDENTIFIED NEURONS
— •Alexander Kaul 1 , Naweed Syed 2 , and Peter Fromherz 1
— 1 MPI für Biochemie, Membran- und Neurophysik — 2 University of
Calgary, Respiratory and Neuroscience Research Group
We here report on a first integration of a basic biological element of
learning with a silicon chip. We interfaced the pre- and postsynaptic
neuron of an excitatory chemical synapse by non-invasive contacts for
capacitive stimulation and transistor recording. Using the identified respiratory
neurons visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1)
from the mollusk Lymnaea stagnalis, we successfully stimulated presynaptic
neuron VD4 by a chip capacitor and recorded postsynaptic action
potentials in LPeD1 by a transistor. With the soma-soma paired neurons
the strength of the cholinergic synapse was potentiated by tetanic
capacitive stimulation from the chip. The non-invasive interfacing of a
synapse by a semiconductor is a fundamental step towards the development
of hybrid neurocomputing devices and also for long-term studies of
synaptic plasticity in complex neuronal nets [1].
1. Kaul, R.A., Syed N.I., Fromherz P. PRL in press.
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Fast and Slow Transistor Records of Recombinant Voltage-
Gated K + Channels — •Matthias Brittinger and Peter
Fromherz — MPI für Biochemie, Abt. Membran und Neurophysik,
Martinsried
To understand transistor recording of neuronal excitation it is necessary
to study the response to defined voltage-gated ion channels under
voltage-clamp.
HEK 293 cells were stable transfected with Kv1.3 potassium channels
and cultured on field effect transistors with an exposed silica gate. We
observed two kinds of transistor signals on a time scale of one microsecond
and ten milliseconds when the Kv1.3 channels opened and closed.
The relative amplitudes depended on the extracellular concentrations of
sodium and potassium.
The fast signal is due to a change of the gate voltage as caused by
charge diffusion in a core-coat conductor. The slow signal is related with
a change of extracellular ion concentration (electrodiffusion) which affects
the transistor threshold. It is rationalized in all details by a Stern
model of the electrical double layer with binding dynamics of potassium
ions.
AKB 50.74 Fr 10:30 B
Capacitive Opening of Ion Channels in Cell Membranes on Silicon
Chips — •Maximilian H. Ulbrich und Peter Fromherz —
Max Planck Institute for Biochemistry, Dept. Membrane and Neurophysics,
Am Klopferspitz 18 a, 82152 Martinsried
In order to understand and optimize capacitive excitation of neurons
from silicon microstructures, it is necessary to study the response of defined
voltage-gated ion channels to voltage transients applied to a chip.
/par HEK 293 cells were stably transfected with Kv1.3 potassium channels.
The cells were cultured on silicon chips insulated with a thin dielectric
and coated with fibronectin. Voltage transients were applied to the
chip. They were chosen such that capacitive coupling gave rise to a stationary
voltage in the narrow extracellular space between chip and cell.
The resulting changes of potassium current through the attached membrane
were recorded at constant intracellular voltage using whole-cell
patch clamp. /par We succeeded in capacitive gating of Kv1.3 channels
through negative extracellular voltages elicited from silicon. Extracellular
voltage-clamp with sufficient amplitude and sufficient duration required,
however, an enhanced time constant of high pass coupling that was