Towards Synthetic Molecular Motors Interfaced by AFM

Molecular machines are at the ultimate limit of miniaturization.
Living organisms provide a variety of examples for such molecular
machines, but in order to utilize and to control them, they need to
be interfaced with the macroscopic world. On the other h and, there
are synthetic molecular machines. Some have been interfaced already
but usually in high vacuum at very low temperatures, which is
clearly not desirable for technical applications. In this thesis,
AFM-based single molecule force spectroscopy (S M FS) was utilized
to investigate the mechanical change in single synthetic molecules
upon environmental changes (external stimuli) in liquid environment
at room temperature. The molecules are either from theory or from
bulk experiments supposed to be ab le to convert such an external
stimulus into mechanical work, which is a prerequisite for
molecular motors. Three different types of molecules and various
external energy inputs were investigated which led to the
realization of a light driven synthetic mo le cular machine: -
Polyelectrolytes should, by OSF-theory, change their persistence
length (and therefore the overall length at a constant force) with
the Debye screening length of the solvent (which is manipulated by
the salt concentration). Therefore, t he elasticity of the
polyelectrolyte polyvinylamine, which could be covalently attached
to the substrate and the AFM tip, was investigated in dependence on
the salt concentration. It was found that the dependence of
persistence length on salt concentr atio n is much smaller than
expected from OSF-theory, which made this system less attractive
for a molecular machine, but led to new theoretical insights. - The
adhesive properties of polyelectrolytes onto charged solid supports
in aqueous solution are a subj ect of current research in industry
and academia. A manipulation of polymer – substrate adhesion, e. g.
at an AFM tip, could lead to a molecular 'grab and release' device.
Therefore, the desorption force of single polyvinylamine-molecules
from solid suppo rts was investigated. Polyvinylamine was
physisorbed to a glass substrate and covalently attached to the
cantilever. Then, the charge-charge interaction was manipulated by
variation in salt concentration and polymer charge. While this has
not led t o a sin gle molecule device yet, it gave new insights
into the desorption of polyelectrolytes from charged substrates.
The measurements performed here revealed that van der Waals forces
and other non-covalent chemical interactions such as hydrogen and
coo rdinative bonds can by far outweigh the electrostatic coulomb
force (namely at short distances), and are therefore a more
promising candidate for the tuning of adhesion forces. -
Elastin-based polypeptides have proven various kinds of energy
conversion i n cross-linked bulk samples. The mechanism is based on
a hydrophobic folding transition, which can be manipulated by
temperature, salt, pH, electrochemistry, and/or by the composition
(hydrophobicity) of the polymers. The difference between the folded
and random state could be detected and investigated here at the
level of individual polymer chains and characterized by the
force-extension traces of the two polypeptides (GVGVP)nx251 and
(GVGIP)nx260. Because of their different hydophobicity their
folding t emperatu res lie above and below room temperature,
respectively. With the polypeptide (GVGIP)nx260 the folded state
was investigated extensively. All observations support the
conjecture, that intermolecular aggregation dominates
intramolecular aggregatio n. This i s further supported by the
finding that neither a change in temperature nor the treatment with
sodium dodecyl sulfate or guanidinium hydrochloride could force any
of the two polypeptides from the folded to the random state or vice
versa within a n experime nt, which in turn would be a prerequisite
for a polypeptide based molecular motor. - The most successful
approach to building an AFM-interfaced molecular machine was in
taking advantage of reversible configurational changes in
azobenzene poly mer molecul es upon irradiation with light.
Azobenzene can be driven from a shorter 'cis' to a longer 'trans'
configuration by illumination with l = 420 nm light and vice versa
by l = 365 nm. In order to utilize azobenzene, a setup had to be
developed and built, whic h allows for the coupling of light into
the AFM experiment. Total internal reflection geometry was
necessary to avoid any artifacts due to direct effects of the light
on the cantilever. A polypeptide chain with multiple functional
azobenzene units was cov alently fixed to both, a gold coated
cantilever and a flint glass substrate. In the force-extension
traces lengthening as well as shortening of the polyazopeptide was
observed even under an applied external force. This is not only a
proof of principle for the first single molecule motor interfaced
to the macroscopic world, but also generates discussion concerning
potential energy landscapes under external force.