Membrane Fusion at the Molecular Level
Research in my group is focused on measuring intermolecular and intersurface forces in complex fluid
systems with an emphasis on polymers, polyelectrolytes, biomembranes, and bio-mimetic materials in
order to develop new materials with useful properties. These materials are being studied since they
can be prepared from renewable resources, they can be biocompatible and biodegradable, and often
possess excellent physical properties. We utilize modern principles and techniques of surface
chemistry, physics, and biology as the means to achieve our goal of producing materials with superior
properties for biomedical and engineering applications. There are currently two main focus areas.
Neutron Scattering Measurements of Confined Complex Fluids
In this work, we use a novel apparatus, Neutron Confinement Cell, to measure the molecular
density and orientation of confined, ultra-thin complex fluids under static and dynamic flow
conditions. The device couples the utility of the Surface Forces Apparatus - ability to control
surface separation and alignment under applied loads - with in-situ structural characterization of
the intervening material utilizing neutron reflectivity measurements. The apparatus is thus suitable
for studying dynamic and time-dependent interactions, the conformations of molecules trapped
between surfaces, and the rheology of thin films. Are initial studies are focused on determining the
structure of adsorbed polymer diblock films -polymer brush layers- at the solid-solution interface
as a function of confinement and polymer brush overlap. Such polymer layers are used to impart
colloidal stabilization, they are used as protective coatings (including mechanical protection of
solids against friction and wear), they govern the interactions of biological cell surfaces, and
through judicious design they are used to modulate dispersion properties (such as rheology) under
a variety of processing conditions. The proposed research should resolve long-standing questions
of the anomalous properties of such films, determine their equilibrium structure and should also be
the first to measure their behavior at the molecular level under flow. In the area of biomaterials,
we will examine the rheology, thin-film viscosity, and structure of the biopolymer hyaluronic acid, a
biopolysaccharide. Hyaluronic acid is found in the extracellular matrix and synovial fluids, where it
protects and lubricates joints. The material is known to form intra and interchain associations and
gel at high concentrations and pressures but rapidly converts to an extremely low friction, good
wear interface under conditions of lower pressure and shear. Our aim is to determine the
essential properties of this material in bulk, at solid-solution interfaces, and under dynamic
conditions of confinement and flow in order to engineer a suitable replacement as an osteoarthritis
and sports medicine therapy and for tissue engineering applications.
Membrane Interactions
- Fusion
One pathway for molecules to enter or exit cells or organelles is through membrane fusion.
Because of this, membrane fusion plays a key role in a variety of important biological processes,
including exocytosis, endocytosis, synaptic transmission, fertilization, and viral infection. However,
membranes do not fuse easily under normal circumstances. As a result, membrane fusion
frequently requires special proteins and is subject to highly selective controls–a constraint that is
crucial, both for maintaining the identity of the cell itself and for maintaining the individuality of each
of the intracellular compartments. Research in our group is obtaining a quantitative handle on
membrane fusion at the molecular level, by bringing to bear a wide range of theoretical and
experimental methods that are not typically applied to biological systems. Experimentally, we
study two-dimensional lipid monolayers at the air-water interface, supported lipid bilayers at the
solid-solution interface, and vesicles in solution, since they are compositionally easy to alter for
studying membrane fusion in realistic environments. By elucidating membrane interactions in
model systems at the molecular level, we gain the ability to engineer and manipulate biomembranes
that mimic real life systems. This is the key to a better understanding of the fundamental cellular
process of fusion as well as the use and design of lipid based (liposomal) drug delivery systems,
where the critical concern is how to achieve significant delivery of the therapeutic across biological
membranes.
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