Plasmon-Waveguide Resonance Spectroscopy Studies of Lateral Segregation in Solid-Supported Proteolipid Bilayers

Plasmon-waveguide resonance (PWR) spectroscopy is a high-sensitivity optical method for characterizing thin films immobilizedonto the outer surface of a glass prism coated with thin films of a metal (e.g., silver) and a dielectric (e.g., silica). Resonance excitation by a polarized continuous wave (CW) laser above the critical angle for total internal reflection generatesplasmon and waveguide modes, whose evanescent electromagnetic fields are localized on the outer surface and interact withthe immobilized sample (in the present case a proteolipid bilayer). Plots of reflected light intensity vs the incident angleof the exciting light constitute a PWR spectrum, whose properties are determined by the refractive index (
n
), the thickness (
t
), and the optical extinction at the exciting wavelength (
k
) of the sample. Plasmon excitation can occur using light polarized both perpendicular (
p
) and parallel (
s
) to the plane of the resonator surface, allowing characterization of the structural properties of uniaxially oriented proteolipidfilms deposited on the surface. As will be demonstrated in what follows, PWR spectroscopy provides a powerful tool for directlyobserving in real-time microdomain formation (rafts) in such bilayers owing to lateral segregation of both lipids and proteins. In favorable cases, protein trafficking can also be monitored. Spectral simulation using Maxwell’s equations allows theseraft domains to be characterized in terms of their mass densities and thicknesses.