Oscillations, Waves, and Interactions - GWDG

438 S. Lakämper and C. F. Schmidt
parameters. There have been several motivations for such developments. In many
cases, and especially in biological systems, samples only come in small sizes. Another
strong motivation for biological applications has been the prospect of being able to
study inhomogeneities, for instance inside of cells. Furthermore, such techniques have
provided the possibility to study viscoelasticity at frequencies far above 1 kHz. Finally,
the ability to study materials such as polymer solutions with probes spanning
some of the characteristic microscopic length scales, e. g. approaching the inter-chain
separation or mesh size of gels, has led to new insights into the microscopic basis of
viscoelasticity in these systems.
We use these techniques to measure the frequency dependence of the shear elastic
modulus of both technical polymers and colloids, biological filamentous networks
and even whole cells. We use several different experimental approaches: in passive
microrheology we merely monitor either the fluctuations of individual probe particles
(one-particle passive microrheology) or the correlated fluctuations of pairs of particles
(two-particle passive microrheology). In active microrheology we exert oscillating
forces on one bead with the help of the trap and AODs and monitor the response
of a second particle. Cytoskeletal networks, for example entangled or cross-linked
actin networks, have been a focus of interest. In vitro reconstituted networks are
a step towards the highly complex and multi-component cytoskeleton of cells. An
important step in the direction of the real systems is the addition of molecular motors
to such model networks. Myosin motors can interact cyclically with the actin filaments
under ATP consumption and create tension in the network. In this situation the
system is out of equilibrium. The understanding of such non-equilibrium systems is
of value to the understanding of cellular systems which are almost by definition out
of equilibrium. A next step in complexity is to couple such non-equilibrium networks
to uni-lamellar lipid vesicles. Such systems are also a step on the way to an artificial
cell. In complementary approaches we also optically manipulate particles attached to
or introduced into living cells.
2 Biomolecular shell mechanics probed with atomic force microscopy
2.1 Microtubules
Microtubules, one of the three major types of cytoskeletal protein-filaments are polarized
polymers of tubulin. The 25 nm-diameter hollow tubules not only provide a
mechanical scaffolding for eukaryotic cells, but also form tracks for motor proteins
(kinesins and dyneins) which move various cargoes in a preferential direction along
the microtubules. One of our recent studies aimed at high-resolution imaging of the
nm-spacing of the tubulin subunits in the microtubule lattice. As can be seen in Fig. 1
– imaging resolves the building blocks of the microtubules and shows a distinct difference
in the topography of the interior and exterior surfaces of microtubules: the
exterior shows a clear radial periodicity of about 5 nm, corresponding to the spacing
of the protofilaments, while the interior surface reveals also the axial spacing of
tubulin subunits, reflected in a distinct 4 nm repeat [1].
The AFM tip can readily image the subunit structure when the forces used for
imaging are well controlled and low enough (∼ 100 pN), given the limited stability