Condensing and Fluidizing Effects of Ganglioside GM1 on Phospholipid Films
Shelli L Frey 1, Eva Y Chi 1, Cristobal Arratia 2, Jaroslaw Majewski 3, Kristian Kjaer 4 and Ka Yee C Lee 1*
1 University of Chicago
2 University of Chile
3 Los Alamos National Labs
4 Max-Planck Institute of Colloids and Interfaces
* To whom correspondence should be addressed. E-mail: kayeelee{at}uchicago.edu.
Submitted on August 17, 2007
Revised on September 10, 2007
Accepted on 26 October 2007
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Abstract |
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Mixed monolayers of the ganglioside GM1 and the lipid, dipalmitoylphosphatidlycholine (DPPC), at the air-water and solid-air interfaces were investigated using various biophysical techniques to ascertain the location and phase behavior of the ganglioside molecules in a mixed membrane. The effects induced by GM1 on the mean molecular area of the binary mixtures and the phase behavior of DPPC were followed for GM1 concentrations ranging from 5 - 60 mole %. Surface pressure isotherms and fluorescence microscopy imaging of domain formation indicate that at low concentrations of GM1 (< 25 mole %), the monolayer becomes continually more condensed than DPPC upon further addition of ganglioside. At higher GM1 concentrations (> 25 mole %), the mixed monolayer becomes more expanded or fluid-like. After deposition onto a solid substrate, atomic force microscopy imaging of these lipid monolayers showed that GM1 and DPPC pack cooperatively in the condensed phase domain to form geometrically packed complexes that are more ordered than either individual component as evidenced by a more extended total height of the complex arising from a well packed hydrocarbon tail region. Grazing incidence x-ray diffraction (GIXD) on the DPPC:GM1 binary mixture provides evidence that ordering can emerge when two otherwise fluid components are mixed together. The addition of GM1 to DPPC gives rise to a unit cell that differs from that of a pure DPPC monolayer. To determine the region of the GM1 molecule that interacts with the DPPC molecule and causes condensation and subsequent expansion of the monolayer, surface pressure isotherms were obtained with molecules modeling the backbone or headgroup portions of the GM1 molecule. The observed concentration-dependent condensing and fluidizing effects are specific to the rigid, sugar headgroup portion of the GM1 molecule.
Key Words:
GM1, Langmuir monolayer, atomic force microscopy, grazing incidence diffraction, phase behavior, phospholipids
