Plenarvorträge - DPG-Tagungen

Arbeitskreis Biologische Physik Freitag
AKB 50.19 Fr 10:30 B
Far-infrared spectroscopy of α amino acids — •Adriana Matei,
Natalia Drichko, Bruno Gompf, and Martin Dressel —
1.Physikalisches Institut, Universität Stuttgart
In contrast to covalent bonds, which leads to intramolecular vibrations
in mid-infrared, low energy interactions such as hydrogen bonds and van
der Waals forces can lead to intermolecular vibrations in far-infrared
(THz-range).
We report on FTIR measurements of a large number of amino acids
prepared as pellets in a polyethylene matrix in the region 50-650 cm −1 .
The observed absorption peaks can be related to five regions: the region
around 600 cm −1 (CO2 wagging), around 540 cm −1 (NH3 torsion and
CO2 rocking/wagging/bending), the one at 320 cm −1 (CC α N deformation),
and the one at 200 cm −1 (CO2 torsion and CC α N deformation).
The features between 50 and 200 cm −1 are intermolecular vibrations due
to hydrogen bonds. Starting from previous normal mode calculations, an
assignment of the observed vibration frequencies is possible.
AKB 50.20 Fr 10:30 B
Transverse fluctuations of grafted polymers — •Gianluca Lattanzi
1 , Tobias Munk 2 , and Erwin Frey 1,2 — 1 Abteilung Theoretische
Physik SF5, Hahn-Meitner Institut Berlin — 2 Fachbereich Physik,
Freie Universität Berlin
We explore the mechanical properties of a single polymer filament with
arbitrary stiffness in a two dimensional embedding space with Monte
Carlo simulations. Clamping one end of the filament induces non trivial
effects on the distribution of the second end, supposed free. The reduced
distribution in the transverse direction shows an anomalous double
peak structure that hallmarks the semiflexible parameter range. This
phenomenon represents a challenge for analytical theories of semiflexible
polymers and should be easily detectable for a number of experimental
situations involving F-Actin, Microtubules, DNA or coiled-coils in the
dimerization domain of kinesin and in myosin V. In addition, we explore
the response of the filament to a transverse force applied at its free end.
AKB 50.21 Fr 10:30 B
Structure control in living bone — •Markus A. Hartmann 1 ,
Richard Weinkamer 1 , Yves Brechet 2 , and Peter Fratzl 1 —
1 Max-Planck-Institute of Colloids and Interfaces, Dept. of Biomaterials,
14476 Potsdam, Germany — 2 Laboratory of Thermodynamics and Metallurgical
Physico-Chemistry/ENSEEG, Grenoble, France
Living bone is an example of a biological system, where the structure is
reacting to mechanical load and controlled by a biological feedback loop.
Specialised cells form new bone where the mechanical stimulus is high
and others dissolve it, where the stimulus is low (Wolff-Roux law). We
use a lattice model and a simple mechanical procedure to evaluate local
stresses to study trabecular bone remodelling in a human vertebra. Starting
from a homogenous, isotropic configuration, a network-like structure
emerges, with struts mainly in vertical and horizontal directions resembling
natural bone. With time, the bone volume fraction reaches a steady
state value independent of initial conditions, while some struts thicken
at the expense of others (coarsening) favouring the vertical main loading
direction. Our simulations provide a mechanistic explanation for some
features typically observed in ageing and disease.
AKB 50.22 Fr 10:30 B
Stability of adhesion clusters under constant force: a stochastic
analysis — •Thorsten Erdmann and Ulrich S. Schwarz —
Max Planck Institute of Colloids and Interfaces, Theory Division, 14424
Potsdam
Cells in multicellular organisms adhere to the extracellular matrix and
to other cells through two-dimensional clusters of transmembrane adhesion
receptors, which usually have to operate under mechanical load.
We present a theoretical model which allows to study the stability of
multiple parallel bonds under shared constant loading. The dynamics of
thermally activated bond rupture and rebinding is modelled by a one-step
Master equation with an absorbing boundary for the completely dissociated
state. For small forces and weak rebinding, mean cluster lifetime
grows only logarithmically with bond number. For strong rebinding, this
dependence becomes exponential, thus rebinding is a very efficient way
to stabilise adhesion clusters. For intermediate forces, a small increase in
force leads to a large reduction in mean cluster lifetime. This allows cells
to switch the stability of cell-matrix adhesions by loading through the
cytoskeleton.
AKB 50.23 Fr 10:30 B
Probing molecular free energy landscapes by periodic loading
— •Oliver Braun 1 , Andreas Hanke 1,2 , and Udo Seifert 1 —
1 Institute for Theoretical Physics II, University of Stuttgart, Pfaffenwaldring
57, D-70550 Stuttgart, Germany — 2 Theoretical Physics Department,
University of Oxford, 1 Keble Road, OX1 3NP Oxford, United
Kingdom
Recent experimental developments of highly sensitive force probes such
as atomic force microscopy, optical or magnetic tweezers and biomembrane
force probes allow one to manipulate a single biological macromolecule.
The free energy landscape along its end-to-end distance, which
is governed by molecular forces and self-interactions, may be recovered by
applying time-dependent forces. Previous experiments have mostly used
a linear force ramp.
Using periodically modulated forces we show how to reconstruct this
free energy landscape more efficiently. A central tool is Jarzynski’s identity,
which allows us to recover equilibrium quantities from the statistics
of non-equilibrium experimental data. As a main advantage, our method
also yields the convex unstable part of the free energy surface, where
the linear ramp protocol provides poor results. The quality of the reconstruction
depends crucially on the frequency of the periodic forcing. Best
results were obtained at frequencies of the order of the transition rates
between adjacent meta-stable configurations.
AKB 50.24 Fr 10:30 B
Dynamics of Eukaryotic Flagella — •Andreas Hilfinger 1 ,
Ingmar Riedel 2 , Amit Chattopadhyay 1 , Karsten Kruse 1 ,
Jonathon Howard 2 , and Frank Jülicher 1 — 1 Max-Planck-
Institute for Physics of Complex Systems, D-01187 Dresden, Germany
— 2 Max-Planck-Institute of Molecular Cell Biology and Genetics,
D-01307 Dresden, Germany
Many types of sperm swim in a viscous environment by beating a
whiplike appendage called the flagellum. The flagellum contains a highly
conserved structure called the axoneme, whose characteristic architecture
is based on a cylindrical arrangement of long elastic filaments composed
of microtubules. The microtubules in adjacent filaments are coupled by
the molecular motor dynein. In the presence of ATP these motor proteins
exert shear forces on neighbouring microtubules. The shear forces lead
to bending of the axoneme because microtubule sliding near the sperm
head is constrained by additional crosslinking proteins. Here we discuss
how regular beating patterns can emerge as nonlinear waves due to internal
stresses that are generated by a large ensemble of molecular motors
working within such an elastic structure. We compare our results with
experimentally observed beating patterns and analyse dynamic force distributions
in active bull sperm flagella.
AKB 50.25 Fr 10:30 B
Ab initio - Berechnungen von elektrischen Feldgradienten in
biologischen Molekülen — •Frank Heinrich und Wolfgang
Tröger — Institut für Experimentelle Physik II, Universität Leipzig
Die Messung des elektrischen Feldgradienten (EFG) am Ort einer
nuklearen Sonde ist über die Kernquadrupolwechselwirkung
mit verschiedenen Verfahren möglich, wie z.B. NMR oder γ-γ-
Winkelkorrelationsspektroskopie (TDPAC). Die Interpretation von Daten
des elektrischen Feldgradienten erfolgt in der Regel durch die Verwendung
von bekannten EFGs einiger Modellverbindungen mit bekannter
atomarer Struktur. Ab initio-Berechnungen des EFG sind eine Alternative,
um mutmaßliche chemische Strukturen zu überprüfen. Moderne
Dichte-Funktional-Programme, wie ADF (Amsterdam Density Functional
Code), ermöglichen auch quantenmechanische Berechnungen von
großen biologischen Molekülen. Es wurden für verschiedene Klassen von
Molekülen (Mercaptide, Thiokronenether) mit bis zu etwa 100 Atomen
die mit TDPAC ermittelten EFGs reproduziert. Damit wurde die Grundlage
geschaffen, um EFGs in Makromolekülen zu berechnen und offene
Probleme der Ligandierung von Metallzentren in Proteinen zu klären.
AKB 50.26 Fr 10:30 B
204m Pb: Eine ” neue“ isomere TDPAC-Sonde — •Frank Heinrich
1 , Wolfgang Tröger 1 und Heinz Haas 2 — 1 Institut für Experimentelle
Physik II, Universität Leipzig — 2 Hahn-Meitner-Institut,
Berlin
Am Massenseparator ISOLDE/CERN steht seit kurzer Zeit das Bleiisotop
204m Pb (t1/2 = 67 min) für TDPAC-Messungen (zeitdifferentielle
gestörte γ-γ-Winkelkorrelation) zur Verfügung. In ersten Experimenten
konnten einige 204m Pb(II)-Komplexe aus wässrigen Lösungen präpariert