Spectroscopic studies of membrane protein-lipid bilayer systems deposited on multilayer thin film coatings

Abstract

Although thin film coatings have been used for many years in optical investigations of biological systems (especially as narrow band light filters), more recent applications of such films in two types of spectroscopic devices, surface plasmon resonance (SPR) and optical waveguides, have provided new biophysical tools for the study of protein-protein and protein-lipid membrane interactions [1]. Although both of these techniques are based on different physical phenomena, the thin film coatings have the same function, i.e. coupling devices in which incident light, under the appropriate optical conditions, can generate an evanescent surface-bound electromagnetic field, which propagates along the interface between the thin film and the emergent dielectric medium in a manner which depends on the interface characteristics. The resulting electric field intensity is concentrated at the outer surface of the film, and diminishes exponentially on both sides of the interface. As a consequence of these properties, it is possible to use SPR and waveguide spectroscopy to probe a few nanometers from the coated surface, a distance well below the wavelength of the light used to generate the evanescent waves, and hence these phenomena have been utilized extensively in studies of surfaces and thin films [for references see 1,2]. Although numerous other optical techniques have also been applied to such systems (e.g. ellipsometry, interferometry, spectrophotometry, and various forms of microscopy), the SPR method has very recently regained its popularity, mainly because of its superior sensitivity, as well as some additional very important advantages over these other methodologies [1,2]. These latter advantages include the following. First, the complete system of measurement is located on the side of the apparatus which is remote from the sample, and thus there is no optical interference from the bulk medium. Second, the outer surface of the sample needs no treatment to increase reflectance, because the necessary high reflectivity is achieved by using total internal reflectance. Third, there are three principal parameters of the resonance that can readily be measured, thereby yielding much more information about the sample and changes within it than the simple interferometric step height used in other sensitive optical techniques.