Characterization of Physical Properties of Supported Phospholipid Membranes Using Imaging Ellipsometry at Optical Wavelengths

¹ University of California - Davis

^* To whom correspondence should be addressed. E-mail: anparikh{at}ucdavis.edu.

Submitted on September 8, 2006
Revised on October 19, 2006
Accepted on 7 November 2006

	Abstract

Sub-nanometer scale vertical z-resolution coupled with large lateral area imaging, label-free, non-contact, and in-situ advantages make the technique of optical imaging ellipsometry highly suitable for quantitative characterization of lipid bilayers supported on oxide substrates and submerged in aqueous phases. This paper demonstrates the versatility of imaging ellipsometry in quantitative characterization of structural and functional properties of supported phospholipid membranes using previously well-characterized examples. These include (1) a single-step determination of bilayer thickness to 0.2 nm accuracy and large-area lateral uniformity using photochemically patterned single DMPC bilayers; (2) hydration-induced spreading kinetics of a single fluid POPC bilayers to illustrate the in-situ capability and image acquisition speed; (3) a large-area morphological characterization of phase-separating binary mixtures of DPLC and galactosylceramide; and (4) binding of cholera-toxin B sub-units to GM₁ incorporating bilayers. Additional insights derived from the present ellipsometric measurements are also discussed for each of these applications. Agreement with previous studies confirms that imaging ellipsometry provides a simple and convenient tool for a routine, quantitative characterization of these membrane properties. Our results also suggest that imaging ellipsometry complements more widely used fluorescence and scanning probe microscopies by combining large-area measurements with high vertical resolution without the use of labeled lipids.

Key Words: Supported lipid bilayers, bilayer formation, label-free assays, phase separation, protein-receptor binding, thickness