Phase transition in bilayers could affect their performance
Phospholipid bilayers that mimic cell membranes in living organisms are of interest as substrates for biosensors and for the controlled release of pharmaceuticals. To better understand how these materials behave with embedded proteins, a necessary first step is to understand how the bilayers respond by themselves.
As will be reported in the Dec. 9 issue of Physical Review Letters (published online Nov. 21), scientists at the University of Illinois at Urbana-Champaign have studied the phase transition in a supported bilayer and discovered some fundamental properties that could affect the material’s performance in various applications.
"Like water turning into ice, bilayers can exist in either a fluid phase or a solid (gel) phase, depending upon temperature," said Andrew Gewirth, a professor of chemistry. "Using a sensitive atomic force microscope, we studied how the microstructure of these bilayers changed during the transformation process."
First, the scientists supported a phospholipid bilayer on a piece of exceptionally smooth mica. Then they studied the properties of this bilayer as it changed phases from fluid to gel and back to fluid. Because touching the surface would destroy the delicate film, the researchers used a noncontact mode in which they oscillated the probe tip in close proximity to the surface, and measured the resulting change in amplitude.
"The atomic force microscope images showed that the fluid to gel phase transition produced substantial tearing of the bilayer, resulting in numerous big, foam-like defects," Gewirth said.
Because the mica substrate was molecularly smooth with no significant surface defects, the scientists concluded that the rips and tears were caused by an intrinsic property of the phase transition itself.
"The gel phase is more dense than the fluid phase," Gewirth said, "so the defects are likely caused by the change in density and, to a lesser extent, by thermal contraction."
As the material solidified, it became highly strained as a consequence of the large density difference between the two phases, Gewirth said. When the membrane was melted again, stress was released in places the scientists hadn’t expected: The melting began in areas other than the defects. In fact, the defects were the last to change back to the fluid phase, because the strain had been removed in the defects as a result of the tearing process.
"The bottom line is that history matters," said Steve Granick, a professor of materials science, chemistry and physics. "The method of preparing the gel phase strongly affects the resulting defect structure, and this in turn has considerable impact on the subsequent gel to fluid transition."
The presence of the defects poses a few problems, but also offers some opportunities, to making and using the bilayers. In biosensors, for example, the defects could affect both device performance and long-term storage characteristics.
"These biosensors would normally be used with the membrane in the fluid phase, but they would be stored in the gel phase," Granick said. "The defects that form as the material solidifies could cause the membrane to respond differently than was expected. As a result, the sensor might not detect the chemical it was designed for."
On the other hand, the defects could be useful as sites for modifying the properties of supported bilayers through the incorporation of additional constituents, the scientists said. In this case, the defects would serve as portals through the membrane, where proteins or other components could be introduced, and then encased by raising the temperature.
"Our experiments have shown that these phospholipid bilayers are a lot more complicated than most people realized," Granick said. "There are many complex materials processing issues that must be considered when making and using them."
Collaborators on the project were graduate student Anne Xie and postdoctoral researcher Ryo Yamada. The U.S. Department of Energy funded the work through a grant to the Frederick Seitz Materials Research Laboratory on the Illinois campus.
Jim Kloeppel, Physical Sciences Editor
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James E. Kloeppel | UIUC news bureau