Radial Compression

1
Radial Compression Buckling of Microtubules
under Osmotic Stress A New Mechanical Probe of
Bio-nanotubesC. R. SAFINYA, UC Santa Barbara,
DMR-0503347
Microtubules (MTs) are model nanotubes, involved
in a range of cellular functions including
intracellular trafficking and cell division (see
figure). We have developed a new biophysical
method of probing the mechanical properties of
individual microtubules. The actual experiments
are deceptively simple One adds a certain
concentration of a large enough polymer (yellow
in figure) that does not fit within the lumen of
MTs to a solution of MTs. The difference in the
polymer concentration between outside and inside
of the MT wall creates an osmotic pressure which
when large enough causes the MT to buckle (Fig.
2, 2nd from left). Conceptually this is similar
to subjecting a hollow tube with closed ends to
enormous hydrostatic pressure, like a submarine
in deep sea. Here, we are doing it to a
nano-scale tube. The critical osmotic pressure
for MT buckling, is a measure of the bond
strength between neighboring protein units within
the microtubule wall. (D. J. Needleman, et al.,
Biophys. J. 89, 3410, 2005)
Illustration of nanoscale microtubules (MTs)
under osmotic pressure due to added polymer
(yellow). Top and bottom are cross section and
side views respectively. For osmotic pressures lt
600 Pa, the MTs are undistorted (left). Above
this pressure the MTs buckle to a noncircular
cross-section and form bundles with rectangular
symmetry (2nd from left). The MTs distort
further as the osmotic pressure increases (3rd
from left). At very high pressures gt 25,000 Pa
the polymer is forced inside the lumen of the
MTs, and they are converted to hexagonal bundles
of undistorted MTs as the pressure inside and
outside the MT is equalized (right).
This new technique is broadly applicable in
measuring the wall strength of individual
bio-nanotubes, which form a large group of
intensively studied materials in
nanotechnological applications, including as
chemical carrying tubules and as templates for
nanowires. We are currently probing the
biomechanical properties of MTs coated with
microtubule-associated-proteins (MAPs) to
elucidate the role of MAPs in mechanically
stabilizing tracks responsible for the
trafficking of signaling molecules between
neurons.
2
Phase Behavior and Interactions of Biomolecular
MaterialsC. R. SAFINYA, UC Santa Barbara, DMR-
0503347
Education Multidisciplinary teams comprised of
physics, chemistry, biology, and materials
science students and postdocs are educated in
methods to discover natures rules for assembling
the molecular building blocks in distinct shapes
and sizes for particular functions. The learned
concepts enable development of advanced nanoscale
materials for broad applications in electronic,
chemical, and pharmaceutical industries.
Outreach Nate Bouxsein (materials science
student), Kai Ewert (Project scientist
synthetic chemist), Chris McAllister (biology
student), and Raha Shirazi (chemistry student)
form an interdisciplinary group working on the
physical, chemical, and biological aspects of the
phase behavior of lipid-DNA complexes and its
applications in gene delivery (Top photo, left to
right). Nate mentored Lisa Boyer, the Science
Department Head at Cuyama Valley High School (New
Cuyama, CA) as part of a summer 2006 internship
program in the PIs group. Kai and postdoctoral
MC Choi (from the Korean Advanced Institute of
Science Technology) are mentoring Heike
Schirmer, a physics exchange Diploma (Masters)
graduate student from the Technical University of
Munich, on the phase behavior of lipid-DNA
complexes (Middle photo). Kelsey Gorter (bottom
photo, right), a 2005 Summer intern from Allan
Hancock Community College (Internships in
Nanosystems Science and Engineering Technology)
joined UCSB as an undergraduate chemistry student
and continued her research in the PIs group
during the 2005-2006 school year. She and Mentor
Jayna Jones (materials science student, bottom
photo, left) studied the phase behavior of
neurofilaments derived from nerve cells. The PIs
group in collaboration with Dr. Youli Li of the
Materials Research Laboratories provides service
to other campus researchers requiring x-ray
diffraction.