This abstract was not submitted electronically.
[A7.002] Theory of polymorphism in bacterial flagella
Thomas Powers (Division of Engineering, Brown University)
Escherichia coli and Salmonella swim using several flagella,
each of which consists of a rotary motor, a universal joint
known as the hook, and a helical filament which acts a
propeller. The filament is normally left-handed in the
absence of external stress, but undergoes mechanical phase
transitions to other helical states ("polymorphs") in
response to external torque. The filament is made of
identical flagellin protein subunits which are organized
into eleven protofilaments which wind around the filament.
We develop an effective theory in which the flagellin
subunits and their connections along the protofilaments are
modelled with a double-well potential. A helical spring
represents the other connections of the subunits, and
introduces a twist-stretch coupling and an element of
frustration in our model. We solve for the ground states and
the phase diagram for filament shapes.
[A7.003] Mechanics of microtubules and viral capsids
Christoph F. Schmidt (Vrije Universiteit Amsterdam, Dept. Physics)
Polymeric macromolecular assemblies play crucial roles in
biology, from DNA to the cytoskeleton or the cell membrane.
I will report on recent measurements of the elastic
properties of two types of 2D-crystalline protein shells
which we have probed at the nanometer scale by indentation
with a scanning force microscope (SFM) tip. Microtubules are
cylindrical shells and we find a linear elastic regime that
can be described by both thin-shell theory and finite
element methods, in which microtubules are modeled as
homogeneous hollow tubes. We also find a non-linear regime
and catastrophic collapse of the microtubules under large
loads. The main physics of protein shells at the nanometer
scale shows simultaneously aspects of continuum elasticity
in their linear response, as well as molecular graininess in
their non-linear behavior. Bacteriophages use highly ordered
proteinaceous shells to protect their genome from the
environment and, interestingly, also to store elastic energy
for the injection process. We have studied empty and filled
bacteriophage Phi29 shells, again by SFM indentation. These
shells are approximately ellipsoidal. We again find a regime
of linear elastic response, followed by non-linear response
and break-down. The linear regime can again be described by
thin shell theory, assuming a homogeneous material, but we
observe, already in the linear regime, signatures of the
substructure of the shells.
[A7.004] Mechanics of DNA Packing in Viruses
Rob Phillips (California Institute of Technology)
Viruses are amongst the most beautiful and fascinating of
self-assembled structures. Recently, as a result of the
confluence of techniques ranging from structural biology to
single molecule biophysics, it has become possible to obtain
quantitative insights into the physical processes that
attend the viral life cycle. In particular, the forces that
build up as a result of the DNA packing process have been
measured in optical tweezers experiments. The aim of this
talk is to describe such experiments on the mechanics of
viral DNA packing and ejection and to show how these
experiments can be greeted with simple, yet predictive
models.
[A7.005] Structure of self - assembled two-dimensional spherical crystals
Andreas R. Bausch (Lehrstuhl fuer Biophysik E22, TU Muenchen, Germany)
Dense spherical particles on a flat surface usually pack into a simple triangular lattice, similar to billiard balls at the start of a game. The minimum energy configuration for interacting particles on the curved surface of a sphere, however, presents special difficulties, as recognized already by J.J. Thomson. We describe experimental investigations of the structure of two-dimensional spherical crystals. The crystals, formed by beads self-assembled on water droplets in oil, serve as model systems for exploring very general theories about the minimum energy configurations of particles with arbitrary repulsive interactions on curved surfaces. Above a critical system size we find that crystals develop distinctive high-angle grain boundaries or “scars” not found in planar crystals. The number of excess defects in a scar is shown to grow linearly with the dimensionless system size. First experiments where the melting of the crystal structure was observable will be discussed. Dynamic triangulation methods allow the analysis of the dynamics of the defects. Possible modifications towards mechanically stabilized self assembly structures result in so called Colloidosomes, which are promising for many different encapsulation purposes.