Atomic force microscopy study of functional and dysfunctional pulmonary surfactant films. I. Micro- and nano- structures of functional pulmonary surfactant films and the effect of SP-A
Yi Y Zuo 1, Eleonora Keating 1, Lin Zhao 1, Seyed M. Tadayyon 1, Ruud A.W. Veldhuizen 1, Nils O. Petersen 2 and Fred Possmayer 1*
1 University of Western Ontario
2 National Institute for Nanotechnology, National Research Council Canada
* To whom correspondence should be addressed. E-mail: fpossmay{at}uwo.ca.
Submitted on September 21, 2007
Revised on November 11, 2007
Accepted on 21 December 2007
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Abstract |
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Monolayers of a functional pulmonary surfactant (PS) can reach very low surface tensions well below their equilibrium value. The mechanism by which PS monolayers reach such low surface tensions and maintain film stability is still unknown. As previously revealed by fluorescence microscopy (FM), phospholipid phase transition and separation appear to be important for the normal biophysical properties of PS. The present work studied phospholipid phase transitions and separations in monolayers of bovine lipid extract surfactant (BLES) using atomic force microscopy (AFM). AFM revealed phospholipid phase separation upon film compression and a monolayer-to-multilayer transition at surface pressure 40-50 mN/m. The tilted-condensed (TC) phase consisted of domains not only on the micrometer scale, as previously detected by FM, but also on the nanometer scale, which is below the resolution limits of conventional optical methods. The nanodomains were uniformly embedded within the liquid-expanded (LE) phase. Upon compression, the microdomains broke up into nanodomains, thereby appearing to contribute to TC and LE phase remixing. Addition of surfactant protein A (SP-A) altered primarily the nanodomains and promoted the formation of multilayers. We concluded that the nanodomains play a predominant role in affecting the biophysical properties of a PS monolayer and the monolayer-to-multilayer transition.
Key Words:
AFM, SP-A, multilayers, phase separation, phospholipids, pulmonary surfactant
