I am interested in macromolecules in suspension and is
studying the relation between the interparticle interactions
and the macroscopic properties of the suspensions. One area
of interest is electrorheological fluids which are composed
of dielectric spheres suspended in a solvent to which an
electric field is applied. Under certain conditions the
suspension rapidly solidifies. These materials are being
developed for applications in the automotive industry. The
microscopic structure is being studied using digital video
optical microscopy, confocal laser scanning microscopy and
light scattering. A second area of interest is macromolecules
of rod-like virus particles. These particles form liquid
crystals as well as other phases. The virus particles are
very interesting because Nature has engineered them to be
monodisperse -- which has not been achieved in the laboratory.
Because of the simplicity of interparticle interactions
there is hope that a microscopic theory of the phase behavior
and bulk properties can be undertaken. These viral colloidal
systems offer the possibility of serving as model systems
for understanding all liquids composed of anisotropic particles.
This latter project is a collaborative effort with Professor
Meyer.
A third area of interest, of direct biological relavence,
is exploring the phenomena of "Macromolecular Crowding".
Macromolecules occupy 30% of the volume of the cell, strongly
influencing inter-molecular interactions. Even in the absence
of any direct interactions between particles, such as electrostatic,
hydrophobic, or van der Waals forces, the macromolecules
feel each others presence simply because two molecules cannot
occupy the same place at the same time. This crowding of
molecules causes like species to phase separate into different
regions of the cell, leading to macromolecular compartmentalization
without the need for any intracellular membranes. Strong
partitioning and bundling is observed for rodlike biopolymers
such as actin or microtubules when mixed with globular proteins
or polymers.
The biochemistry of the cell has evolved in this crowded,
thermodynamically non-ideal environment, and it may be that
the cell has exploited this fact. One can imagine, for example,
that bundling of filaments is driven by macromolecular crowding
and that the role of specific bundling proteins is to fine
tune certain aspects of the bundling, like the relative
alignment of the biopolymers in a bundle. There are a number
of questions we are seeking to answer. How crowded do suspensions
have to be in order to partition? How strong is the degree
of partitioning of the species? What is the organization
of the macromolecules in the partitioned phases? How different
do the species have to be from each other for this to occur?
How does the interparticle potential affect the phenomena?
To what extant is all this relavent to cellular biology?
We are studying the partitioning of mixtures of globular
and filamentous proteins in vitro using suspensions of the
biopolymer fd bacteriophage and Tobacco Mosaic Virus mixed
with polymers such as polyethylene glycol or globular proteins
like BSA. Genetic engineering methods are used to systematically
alter the length of the biopolymers, an important thermodynamic
variable. The viruses are labeled with fluorescent dyes
and equilbrated samples are observed in the light microscope.
Electron microscopy, light and x-ray scattering, and optical
microscopy are used to determine the structure of the macromolecular
suspension. We compare our experimental measurements of
the phase behavior of these mixtures with several theoretical
statistical mechcanical models developed by ourselves, by
Prof. J. Herzfeld of the Brandeis
University Chemistry department and others, as well as with
molecular dynamics computer simulations.
Selected
Publications
Dogic Z, Fraden S. (2001) Development of model colloidal
liquid crystals and the kinetics of the isotropic -smectic
transition. Philosophical Transactions of the Royal Society
of London A 359:997-1015. [online
paper]
Dassanayake U, Fraden S, van Blaaderen A. (2000) Structure
of Electrorheological Fluids, J Chem Phys 112:385-8.
[online
paper]
Dogic Z, Fraden S. (2000) Cholesteric phase in virus suspensions,
Langmuir 16:7820-4. [online
paper]
Dogic Z, Philipse AP, Fraden S, et al. (2000). Concentration
Dependent Sedimentation of Colloidal Rods. J Chem Phys
113:8368-80. [online
paper]
Dogic Z, Frenkel D, Fraden S. (2000) Enhanced stability
of layered phases in parallel hard spherocylinders due to
addition of hard spheres. Phys Rev E 62:3925-33.
[online
paper] [abstract]
Fraden S, Kamien RD. (2000) Self-assembly in vivo. Biophys
J. 78:2189-90. [online
paper]
Adams M. and Fraden, S. (1998). Phase behavior of mixtures
of rods (Tobacco Mosaic Virus) and Spheres (Polyethylene
Oxide, Bovine Serum Albumin). Biophys. J 74:669-677.
[online
paper]
Adams M., Dogic Z., Keller S.L., and Fraden, S. (1998).
Entropically driven microphase transitions in mixtures of
colloidal rods and spheres. Nature 393:349-352.
[online
paper]
Ospeck M, Fraden S. (1998) Solving the Poisson-Boltzmann
equation to Obtain Interaction Energies Between Confined,
Like-charged Cylinders. J Chem Phys 109:9166-71.
[online
paper]
Weiss, P. and Fraden, S. (1998) Another Face of Entropy.
Science News. 154:108-109. [online
paper]
Last update: Thursday, October 4, 2001. E-mail comments or
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