CONTACT INFORMATION
Professor: Mary L. Kraft
University of Illinois
600 S. Mathews Ave,. MC-712
Urbana, IL 61801
phone: 217-333-2228
fax: 217-333-5052

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research

Phase-Separation in Model Lipid Membranes

Although the proteins and lipids in the cell membrane exhibit some degree of lateral fluidity, lateral organization in the cell membrane is believed to be required for biological function. Elucidation of the lateral distributions of lipids and cholesterol in both cellular and model membranes is currently a major challenge.

Self-assembled phospholipid bilayers are simplified models of cell membranes that provide clues to how lipids are organized in cellular membranes. Essentially, a lipid bilayer is a two dimensional fluid: the lipids that form the bilayer diffuse within the plane of the membrane.  Differences in lipid structure can lead to non-ideal mixing and phase-separation within model lipid membranes.  We apply fundamental principles of chemical engineering, biophysics, and physical chemistry to understand the driving forces for phase-separation in phospholipid membranes. Using a range of imaging technologies, including fluorescence microscopy, atomic force microscopy, and a new high lateral resolution imaging mass spectrometry technology, we can visualize the physical structure and the precise lipid composition within nanoscale domains in the lipid bilayer. This information provides insight into the molecular interactions that drive cell membrane organization, and it can be used to design new materials for biomedical applications.

The figure to the right shows one of the cutting-edge imaging technologies that are employed by the Kraft laboratory. This technology is called multiple isotope imaging mass spectrometry (MIMS), and it reveals the isotopic and elemental composition of a sample with a lateral resolution as high as 50 nm. During MIMS analysis, a focused cesium ion beam is scanned across the sample, extensively fragmenting the surface molecules that are within the beams focal area.  The resulting atomic and small molecular secondary ions are extracted and analyzed according to their mass-to-charge (m/z) ratios.  Secondary ions with up to five different m/z ratios are simultaneously detected at each beam position, and the ion intensities measured at each location are used to create maps of the sample’s elemental and isotopic distributions. To image the lipids in a membrane, we incorporate a distinct stable isotope into each lipid component of interest. Then the isotopically enriched secondary ions that are produced by each lipid species during MIMS analysis reveal which lipids are located within the beam's focal area.

Using MIMS, the lipid distributions within phase-separated supported lipid membranes can be imaged with 100 nm lateral resolution. In this experiment, we studied a 2-component, phase-separated lipid membrane. The lipid that formed the fluid phase contained nitrogen-15 (15-DLPC), and the lipid that formed the gel phase contained carbon-13 (13C-DSPC). The ¹²C¹⁵N^- (green) and ¹³C¹H^- (red) secondary ion signals that were detected at each 100-nm region on the sample revealed the locations of the nitrogen-15 and carbon-13 labeled lipids, respectively. The MIMS image of the lipid bilayer shown on the left below has a lateral resolution of 100 nm. The AFM image on the right below reveals the structure of the lipid membrane. Comparison of the images produced by each technique reveal that the taller domains (yellow regions in the AFM image) were mainly composed of 13C-DSPC and the neighboring regions primarily consisted of 15N-DLPC. The compositions within small regions of a phase-separated supported lipid membrane were also quantified with this approach (Kraft et al. Science 2007).

Current research in the Kraft laboratory involves using MIMS and other technologies to study more complex and biologically relavent model lipid membranes. This information provides insight into the molecular interactions that drive cell membrane organization, and it can be used to design new materials for biomedical applications.