Plenarvorträge - DPG-Tagungen

Symposium Life Sciences on the Nanometer Scale - Physics Meets Biology Mittwoch
SYLS 3.15 Mi 16:00 B
Acid-attack on Dental Enamel Investigated by Scanning Force
Microscopy — •N. Schwender 1 , F. Al Marrawi 2 , K. Huber 2 , M.
Hannig 2 , and C. Ziegler 1 — 1 Department of Physics, University of
Kaiserslautern — 2 Department of Operative Dentistry and Periodontology,
Saarland University
As food or drinks always contain acids knowledge about the interaction
of acids with dental enamel is important in preventive dentistry.
Using the possibilities of Scanning Force Microscopy (SFM), the influence
of acids on enamel surfaces was therefore investigated here. For
this purpose, bovine enamel samples were ground flat, polished or cut
by a diamond knife. Samples were treated with different acids such as
phosphoric, citric, or hydrochloride acid, systematically changing the exposure
time and the pH-value. Typical etch-patterns with a variation
of height from the nano- to the micrometer scale as a result of mineral
loss was observed. Distinct differences between the different acids could
be observed which will be discussed taking into account the different reactivity
of the components of enamel. Furthermore, the etched enamel
shows a loss in stability and hardness noted by the erosion induced by
the cantilever. In a further step the adsorption of saliva proteins such
as BSA and lysozym on the etched surface compared to the undestroyed
surface was investigated.
SYLS 3.16 Mi 16:00 B
Towards a near-field optical observation of single particle
transport through an unsupported cell membrane — •Simone
Johnas 1 , Christiane Höppener 2 , and Andreas Naber 1 —
1 Institut für Angewandte Physik, Wolfgang-Gaede-Str. 1, Universität
Karlsruhe (TH), 76131 Karlsruhe — 2 Physikalisches Institut,
Wilhelm-Klemm-Str. 10, Universität Münster, 48149 Münster
Highly differentiated macromolecular protein assemblies, so-called nuclear
pore complexes (NPC), are embedded in the nuclear envelope (NE)
of a eukaryotic cell and tightly control the exchange of all kinds of
molecules between cytoplasm and nucleus. Since the NPCs are densely
packed in the membrane, conventional optical microscopy is not able to
distinguish between neighboring NPCs. By means of scanning near-field
optical microscopy (SNOM) we have recently attained an optically resolved
fluorescence image of dye-labeled NPCs in a functionally intact
NE for the first time [1, 2]. Since SNOM enables us to address single
NPCs, we are now aiming at a time-resolved observation of single transport
events. A major obstacle towards this goal is the need of two compartments
below and above the NE that mimics its natural environment.
We will discuss possible preparation techniques and ways to image a freestanding
membrane in a buffer solution with a SNOM. [1] C. Höppener,
D. Molenda, H. Fuchs, and A. Naber, J. Microsc. 210, 288 (2002). [2]
Contribution of C. Höppener et al. in this symposium.
SYLS 3.17 Mi 16:00 B
Biological photonic crystals — •Thomas Fuhrmann, Melanie El
Rharbi-Kucki, and Stefan Landwehr — Makromolekulare Chemie
und Molekulare Materialien, Fachbereich Naturwissenschaften und Center
for Interdisciplinary Nanostructure Science and Technology, Universität
Kassel
Active and passive photonic crystals are interesting for a wide range
of applications. We show that photonic crystal structures for visible light
can be found in centric diatoms which exhibit a very efficient photosynthesis.
We present electromagnetic wave calculations addressing the
photonic structure of the algae cells and experimental results about the
light distribution in the cell.
SYLS 3.18 Mi 16:00 B
Density functional theory study of alpha-helical polypeptides
under tensile strain — •Joel Ireta 1 , Jörg Neugebauer 1,2 ,
and Matthias Scheffler 1 — 1 Fritz-Haber-Institut der Max-
Planck-Gesellschaft, Berlin — 2 Theoretische Physik, Universität
Padeborn
Recent experimental techniques employing e. g. optical tweezers or
atomic force microscopes make it possible to monitor the behavior of
biopolymers under tensile strain. Using this technique force-driven phase
transitions occurring at single molecule level have been observed. While
such studies give important information they are (presently) not able
to provide insight into the atomistic details of such processes. We have
therefore studied the mechanical response of infinite polyalanine and
polyglicine chains in alpha-helical conformation since these structures
represent the smallest but still realistic models of a protein. We employ
density functional theory, ab initio pseudopotentials and the Perdew-
Burke-Ernzerhof formulation for the exchange and correlation functional.
Calculating the response force along the helix axis as function of strain,
we have identified a plateau region at forces of about 25 pN. The presence
of a plateau is characteristic for a first order phase transition. An analysis
of the helical structures shows that sizeable changes in the covalent bonds
of the peptide unit (particularly carbon and nitrogen pyramidalization)
are the driving force causing the phase transition.
SYLS 3.19 Mi 16:00 B
Carbon nanotube-assisted microwave absorption of gold
nanoparticles across bacterial membranes — •Jose Rojas-
Chapana 1 , Miguel Correa-Duarte 1 , Neli Sobal 1 , Krzysztof
Kempa 2 , and Michael Giersig 1 — 1 Caesar Research Center,
Ludwig-Erhard Allee 2, 53175 Bonn — 2 Boston College, Boston, MA,
USA
Acidothiobacillus ferrooxidans an acid tolerant bacterium was used as
a tool to study intracellular incorporation and storage of gold nanoparticles
(GN) induced by carbon nanotubes (CNTs). The microwave (MW)
irradiation induced an electrostatic charge in the CNTs. Under this condition
all the single CNTs work as an electric dipole. The addition of a
small concentration of CNTs to a medium containing both bacteria and
GN, and subsequent MW-irradiation leads to intracellular transport of
gold without altering the viability of bacterial cells. As is shown by the
transmission electron microscopy (TEM) examination, CNT tip heads are
very close to the cell wall of bacteria. This CNTs-cell electrical contact
is a prerequisite for the onset of temporary channels. One of the possible
mechanisms during the MW-irradiation is a local electrical dipole
induced in CNTs which leads to cell-wall destabilization, having the dimension
of the CNTs tip heads (ca.30 nm diameter), which in turn act as
a channel for the transport of gold. The occurrence of this phenomenon,
as documented by TEM images, constitutes the most important factor
facilitating MW-induced transport of GN across the cells.
SYLS 3.20 Mi 16:00 B
Quantum Monte Carlo study of small molecules and hydrogen
bonded model systems - benchmarking density functionals —
•M. Fuchs 1 , A. Badinski 1 , J. Ireta 1 , P. Kratzer 1 , C. Filippi
2 und M. Scheffler 1 — 1 Fritz-Haber-Institut der MPG, Berlin —
2 Instituut Lorentz, Univ. Leiden
Diffusion Monte Carlo (DMC) calculations can be useful to validate
(and correct) results from density-functional theory (DFT) where manyelectron
correlations must be approximated. DMC is not yet a routine
tool however, partly because experience to corroborate its robustness is
still scarce. Here we apply the pseudopotential fixed-node DMC method
to (i) selected small molecules and (ii) model systems for hydrogen bonds,
the latter playing a key role for the structure and functionality of, e.g.,
biomolecules. We carefully monitor how DMC performs dependent on
(nonlocal) pseudopotentials and on the optimization of the initial trial
wavefunction. We find that both aspects can be nontrivial, even for the
simple N2 diatomic. For finite formamide chains our DMC data show
that the cooperative increase of the H-bond strength in longer chains is
best reproduced in DFT using the PBE gradient corrected functional.
Our findings support a recent DFT study of the stabilization of α helical
polyalanine which found an unusually large cooperativity of the H-bonds
[J. Ireta et al., J. Phys. Chem. B 107, 1432 (2003)].
SYLS 3.21 Mi 16:00 B
Conformational flexibility of pigment-protein complexes
studied by optical single-molecule spectroscopy — •Silke
Oellerich 1 , Martijn Ketelaars 1 , Jean-Manuel Segura 1 ,
Ward P.F. de Ruijter 1 , Richard J. Cogdell 2 , and Thijs J.
Aartsma 1 — 1 Dept. of Biophysics, Leiden University, Niels Bohrweg 2,
2333 CA Leiden, The Netherlands — 2 Dept. of Biochemistry, University
of Glasgow, Scotland
Low temperature, optical single-molecule spectroscopy was employed
to study the conformational flexibility and its effect on the electronic
properties of individual pigment-protein complexes. We studied the bacterial
light-harvesting complex 3 (LH3), which consists of 27 bacteriochlorophyll
a (BChl a) pigments and exhibits a heterogeneous spectral
behaviour at the single-molecule level. The applied technique provides
information about both the interaction between the pigments within a
complex as well as the effect of the local environment on an individual pigment.
We show that the spectral heterogeneity is caused by light-induced
conformational changes of individual BChl a pigments which affect the