SELF-ASSEMBLY OF PHOSPHOLIPIDS IN
NANOSCALE CONFINEMENTS
The main research objective of this project is to explore effects of nano-scale
confinement on the structure and dynamics of self-assembled phospholipid
membranes by means of advanced High Field (HF) EPR spectroscopy in
combination with novel spin labeling methods.
Our research objective is motivated by the needs of merging two main strategies
of making novel hybrid devices with unique capabilities provided by
nanotechnology and addressable machine architectures. These are so-called
"bottom-up" and "top-down" strategies. The "bottom-up" strategy for hybrid
devices involves molecular engineering and self-assembly of functional
biomolecules (proteins) and their complexes for highly specific nano-scale tasks
while the "top-down" is based on various forms of lithography and replication.
The lithographic techniques are now reaching ca. 100 nm spatial scale thus
creating an unprecedented opportunity for a seamless transition to a still smaller
protein scale of just 3-10 nm. However, this transition from rigid lithographic
structures to "soft" biomolecules is not without difficulties. The two major
problems are yet to be solved are:
1. compatibility of biomolecules and the solid support including development
of new attachment method(s), and
2. a scale mismatch between state-of-the-art lithographic technologies and
protein sizes.
One of the most promising direction to solve these problems is to mimic the
nature which utilizes phospholipid bilayers to support, protect, and organize
membrane proteins. Phospholipids represent a natural environment for the
membrane proteins and this solves the compatibility issue. Under normal
conditions, complex electrostatic interactions between proteins and
phospholipids ensure a proper protein attachment to the membrane as well as
its optimum transmembrane location and a correct folding conformation. In
addition, the phospholipids have a property of being self-assembled into bilayers,
micelles, and other structures. This self-assembly is an ideal feature for filling the
existing scale gap between the sizes of wells that could be created
lithographically (or by other means) and dimensions of biomolecules. Thus,
phospholipids could be viewed as an ideal self-organizing "packing" material for
fitting "soft" biomolecules into "hard" man-made "boxes".
Because the geometry of solid-state support materials, their complex surface
and electrostatic properties are very different from natural cellular environments,
these artificial solid-state substrates are expected to have a profound effect on
phospholipid bilayers. What is needed is a detailed fundamental understanding
of the confinement effects on self-assembly and properties of phospholipid
membranes. This puts forward the following specific goals for our research
project:
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To determine how the order parameter, viscosity, phase transition, and
other properties of self-organized phospholipid structures (monolayers,
bilayers, and vesicles) are affected by the nano-scale confinements and
increased surface area and how these confinement effects scale with the
size of the structure;
To investigate what is the minimum confinement size at which the
self-assembly is still possible;
To study how the properties of the surface and various surface treatments
affect the supported bilayer;
To examine our hypothesis that the hydrophobic/hydrophilic mismatch at
the borders of substrate-supported lipid corrals can be minimized by a
mixture of short- and long-chain phospholipids. Figure 1 shows one
possible arrangement of a lipid corral on a shallow substrate well. Clearly,
such kind of arrangement could create a hydrophobic/hydrophilic
mismatch region (shown by arrows). If so, this mismatch could affect
stability of the bilayer. We propose to eliminate/minimize this mismatch by
using a mixture of long- and short-chain phospholipids such as DMPC
and DHPC (and also other short detergents). These mixtures are known
to form bicelle structures and were previously developed for high field
NMR studies of membrane peptides and proteins (in solutions the bicelles
spontaneously align in high magnetic fields). We hypothesize that the
bicelle structure could also be useful for preparing substrate-supported
membranes for the needs of nanotechnology (Figure 2).
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To investigate the effects of incorporation of model membrane peptides
(melittin, gramicidin) and proteins (bacteriorhodopsin) on the structure
and dynamic of nano-confined lipids.
Figure 1. A lipid bilayer on a Figure 2. A bicelle on a solid
solid substrate support
substrate: filled circles - long
chain phospholipids, open
circles - short-chain
phospholipids.
This work represents a developing multidisciplinary collaboration between North
Carolina State University (NC State) and the Argonne National Laboratory
(Argonne). The project combines cross-cutting technologies in materials science
with recent advances in high resolution high field EPR complemented by an
array of modern spectroscopic methods. This work explores a new area of
science of discovering nano-scale phenomena on the interface of biophysics,
physical chemistry, and materials science.