Transition from Nanodomains to Microdomains Induced by
Exposure of Lipid Monolayers to Air
Oana Coban 1, Melanie Burger 1, Jesse Popov 1, Dusan Vobornik 1 and Linda Johnston 2*
1 National Research Council Canada
2 Steacie Inst. for Molecular Sciences
* To whom correspondence should be addressed. E-mail: linda.johnston{at}nrc.ca.
Submitted on May 3, 2006
Revised on June 26, 2006
Accepted on 3 January 2007
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Abstract |
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The morphology of monolayers prepared from ternary lipid mixtures that have coexisting fluid phases has been examined by atomic force microscopy (AFM) for samples transferred to mica before and after exposure to air. Mixtures of DOPC and cholesterol with either egg sphingomyelin or DPPC were studied at several surface pressures. Both lipid mixtures have a combination of small islands and large microdomains at low surface pressure (5-10 mN/m) for monolayers deposited in either air or nitrogen. By contrast, monolayers have small interconnected nanodomains when deposited under nitrogen at 30 mN/m but mixtures of large microdomains and small nanodomains when transferred after exposure to air. These results are consistent with an earlier report which concluded that the formation of large domains at high surface pressures (>30 mN/m) for monolayers exposed to air is caused by lipid oxidation. However, the higher spatial resolution available with AFM indicates that exposure of the monolayers to air leads to an increase in the size of pre-existing nanodomains, rather than a change in the miscibility pressure. Examination of changes in surface morphology as a function of surface pressure demonstrate a gradual evolution in size and surface coverage for both nano and microdomains, prior to formation of a network of interconnected nanodomains. Similar studies for binary mixtures in the absence of cholesterol indicate that lipid oxidation results in analogous changes in domain size for monolayers with coexisting gel and fluid phases. These results illustrate the importance of using techniques capable of probing the nanoscale organization of membranes.
Key Words:
atomic force microscopy, fluorescence, lipid monolayers, lipid rafts, oxidation
